diff --git a/.gitignore b/.gitignore index 1a71fb7c..6f90fd19 100644 --- a/.gitignore +++ b/.gitignore @@ -21,4 +21,3 @@ sftp-config.json Thumbs.db -/svgs \ No newline at end of file diff --git a/1-js/01-getting-started/1-intro/article.md b/1-js/01-getting-started/1-intro/article.md index 2f4f518f..ca5c46de 100644 --- a/1-js/01-getting-started/1-intro/article.md +++ b/1-js/01-getting-started/1-intro/article.md @@ -1,6 +1,6 @@ # An Introduction to JavaScript -Let's see what's so special about JavaScript, what we can achieve with it, and what other technologies play well with it. +Let's see what's so special about JavaScript, what we can achieve with it, and which other technologies play well with it. ## What is JavaScript? @@ -24,18 +24,18 @@ The browser has an embedded engine sometimes called a "JavaScript virtual machin Different engines have different "codenames". For example: -- [V8](https://en.wikipedia.org/wiki/V8_(JavaScript_engine)) -- in Chrome, Opera and Edge. +- [V8](https://en.wikipedia.org/wiki/V8_(JavaScript_engine)) -- in Chrome and Opera. - [SpiderMonkey](https://en.wikipedia.org/wiki/SpiderMonkey) -- in Firefox. -- ...There are other codenames like "Chakra" for IE, "JavaScriptCore", "Nitro" and "SquirrelFish" for Safari, etc. +- ...There are other codenames like "Chakra" for IE, "ChakraCore" for Microsoft Edge, "Nitro" and "SquirrelFish" for Safari, etc. -The terms above are good to remember because they are used in developer articles on the internet. We'll use them too. For instance, if "a feature X is supported by V8", then it probably works in Chrome, Opera and Edge. +The terms above are good to remember because they are used in developer articles on the internet. We'll use them too. For instance, if "a feature X is supported by V8", then it probably works in Chrome and Opera. ```smart header="How do engines work?" Engines are complicated. But the basics are easy. 1. The engine (embedded if it's a browser) reads ("parses") the script. -2. Then it converts ("compiles") the script to machine code. +2. Then it converts ("compiles") the script to the machine language. 3. And then the machine code runs, pretty fast. The engine applies optimizations at each step of the process. It even watches the compiled script as it runs, analyzes the data that flows through it, and further optimizes the machine code based on that knowledge. @@ -43,7 +43,7 @@ The engine applies optimizations at each step of the process. It even watches th ## What can in-browser JavaScript do? -Modern JavaScript is a "safe" programming language. It does not provide low-level access to memory or the CPU, because it was initially created for browsers which do not require it. +Modern JavaScript is a "safe" programming language. It does not provide low-level access to memory or CPU, because it was initially created for browsers which do not require it. JavaScript's capabilities greatly depend on the environment it's running in. For instance, [Node.js](https://wikipedia.org/wiki/Node.js) supports functions that allow JavaScript to read/write arbitrary files, perform network requests, etc. @@ -59,7 +59,7 @@ For instance, in-browser JavaScript is able to: ## What CAN'T in-browser JavaScript do? -JavaScript's abilities in the browser are limited to protect the user's safety. The aim is to prevent an evil webpage from accessing private information or harming the user's data. +JavaScript's abilities in the browser are limited for the sake of the user's safety. The aim is to prevent an evil webpage from accessing private information or harming the user's data. Examples of such restrictions include: @@ -67,17 +67,17 @@ Examples of such restrictions include: Modern browsers allow it to work with files, but the access is limited and only provided if the user does certain actions, like "dropping" a file into a browser window or selecting it via an `` tag. - There are ways to interact with the camera/microphone and other devices, but they require a user's explicit permission. So a JavaScript-enabled page may not sneakily enable a web-camera, observe the surroundings and send the information to the [NSA](https://en.wikipedia.org/wiki/National_Security_Agency). -- Different tabs/windows generally do not know about each other. Sometimes they do, for example when one window uses JavaScript to open the other one. But even in this case, JavaScript from one page may not access the other page if they come from different sites (from a different domain, protocol or port). + There are ways to interact with camera/microphone and other devices, but they require a user's explicit permission. So a JavaScript-enabled page may not sneakily enable a web-camera, observe the surroundings and send the information to the [NSA](https://en.wikipedia.org/wiki/National_Security_Agency). +- Different tabs/windows generally do not know about each other. Sometimes they do; for example when one window uses JavaScript to open the other one. But even in this case, JavaScript from one page may not access the other if they come from different sites (from a different domain, protocol or port). - This is called the "Same Origin Policy". To work around that, *both pages* must agree for data exchange and must contain special JavaScript code that handles it. We'll cover that in the tutorial. + This is called the "Same Origin Policy". To work around that, *both pages* must agree for data exchange and contain a special JavaScript code that handles it. We'll cover that in the tutorial. - This limitation is, again, for the user's safety. A page from `http://anysite.com` which a user has opened must not be able to access another browser tab with the URL `http://gmail.com`, for example, and steal information from there. + This limitation is, again, for the user's safety. A page from `http://anysite.com` which a user has opened must not be able to access another browser tab with the URL `http://gmail.com` and steal information from there. - JavaScript can easily communicate over the net to the server where the current page came from. But its ability to receive data from other sites/domains is crippled. Though possible, it requires explicit agreement (expressed in HTTP headers) from the remote side. Once again, that's a safety limitation. ![](limitations.svg) -Such limitations do not exist if JavaScript is used outside of the browser, for example on a server. Modern browsers also allow plugins/extensions which may ask for extended permissions. +Such limits do not exist if JavaScript is used outside of the browser, for example on a server. Modern browsers also allow plugin/extensions which may ask for extended permissions. ## What makes JavaScript unique? @@ -86,13 +86,13 @@ There are at least *three* great things about JavaScript: ```compare + Full integration with HTML/CSS. + Simple things are done simply. -+ Supported by all major browsers and enabled by default. ++ Support by all major browsers and enabled by default. ``` JavaScript is the only browser technology that combines these three things. That's what makes JavaScript unique. That's why it's the most widespread tool for creating browser interfaces. -That said, JavaScript can be used to create servers, mobile applications, etc. +That said, JavaScript also allows to create servers, mobile applications, etc. ## Languages "over" JavaScript @@ -100,23 +100,22 @@ The syntax of JavaScript does not suit everyone's needs. Different people want d That's to be expected, because projects and requirements are different for everyone. -So, recently a plethora of new languages appeared, which are *transpiled* (converted) to JavaScript before they run in the browser. +So recently a plethora of new languages appeared, which are *transpiled* (converted) to JavaScript before they run in the browser. Modern tools make the transpilation very fast and transparent, actually allowing developers to code in another language and auto-converting it "under the hood". Examples of such languages: -- [CoffeeScript](https://coffeescript.org/) is "syntactic sugar" for JavaScript. It introduces shorter syntax, allowing us to write clearer and more precise code. Usually, Ruby devs like it. -- [TypeScript](https://www.typescriptlang.org/) is concentrated on adding "strict data typing" to simplify the development and support of complex systems. It is developed by Microsoft. -- [Flow](https://flow.org/) also adds data typing, but in a different way. Developed by Facebook. +- [CoffeeScript](http://coffeescript.org/) is a "syntactic sugar" for JavaScript. It introduces shorter syntax, allowing us to write clearer and more precise code. Usually, Ruby devs like it. +- [TypeScript](http://www.typescriptlang.org/) is concentrated on adding "strict data typing" to simplify the development and support of complex systems. It is developed by Microsoft. +- [Flow](http://flow.org/) also adds data typing, but in a different way. Developed by Facebook. - [Dart](https://www.dartlang.org/) is a standalone language that has its own engine that runs in non-browser environments (like mobile apps), but also can be transpiled to JavaScript. Developed by Google. -- [Brython](https://brython.info/) is a Python transpiler to JavaScript that enables the writing of applications in pure Python without JavaScript. -- [Kotlin](https://kotlinlang.org/docs/reference/js-overview.html) is a modern, concise and safe programming language that can target the browser or Node. +- [Brython](https://brython.info/) is a Python transpiler to JavaScript that allow to write application in pure Python without JavaScript. -There are more. Of course, even if we use one of these transpiled languages, we should also know JavaScript to really understand what we're doing. +There are more. Of course, even if we use one of transpiled languages, we should also know JavaScript to really understand what we're doing. ## Summary -- JavaScript was initially created as a browser-only language, but it is now used in many other environments as well. -- Today, JavaScript has a unique position as the most widely-adopted browser language, fully integrated with HTML/CSS. +- JavaScript was initially created as a browser-only language, but is now used in many other environments as well. +- Today, JavaScript has a unique position as the most widely-adopted browser language with full integration with HTML/CSS. - There are many languages that get "transpiled" to JavaScript and provide certain features. It is recommended to take a look at them, at least briefly, after mastering JavaScript. diff --git a/1-js/01-getting-started/1-intro/limitations.svg b/1-js/01-getting-started/1-intro/limitations.svg index 76ea43fd..a7863c63 100644 --- a/1-js/01-getting-started/1-intro/limitations.svg +++ b/1-js/01-getting-started/1-intro/limitations.svg @@ -1 +1 @@ -https://javascript.info<script> ... </script>https://gmail.comhttps://javascript.info \ No newline at end of file +https://javascript.info<script> ... </script>https://gmail.comhttps://javascript.info \ No newline at end of file diff --git a/1-js/01-getting-started/2-manuals-specifications/article.md b/1-js/01-getting-started/2-manuals-specifications/article.md index 3fa24333..85a7737c 100644 --- a/1-js/01-getting-started/2-manuals-specifications/article.md +++ b/1-js/01-getting-started/2-manuals-specifications/article.md @@ -1,7 +1,7 @@ # Manuals and specifications -This book is a *tutorial*. It aims to help you gradually learn the language. But once you're familiar with the basics, you'll need other resources. +This book is a *tutorial*. It aims to help you gradually learn the language. But once you're familiar with the basics, you'll need other sources. ## Specification @@ -9,19 +9,24 @@ This book is a *tutorial*. It aims to help you gradually learn the language. But But being that formalized, it's difficult to understand at first. So if you need the most trustworthy source of information about the language details, the specification is the right place. But it's not for everyday use. -A new specification version is released every year. Between these releases, the latest specification draft is at . +A new specification version is released every year. In-between these releases, the latest specification draft is at . To read about new bleeding-edge features, including those that are "almost standard" (so-called "stage 3"), see proposals at . -Also, if you're developing for the browser, then there are other specifications covered in the [second part](info:browser-environment) of the tutorial. +Also, if you're in developing for the browser, then there are other specs covered in the [second part](info:browser-environment) of the tutorial. ## Manuals -- **MDN (Mozilla) JavaScript Reference** is the main manual with examples and other information. It's great to get in-depth information about individual language functions, methods etc. +- **MDN (Mozilla) JavaScript Reference** is a manual with examples and other information. It's great to get in-depth information about individual language functions, methods etc. - You can find it at . + One can find it at . -Although, it's often best to use an internet search instead. Just use "MDN [term]" in the query, e.g. to search for the `parseInt` function. + Although, it's often best to use an internet search instead. Just use "MDN [term]" in the query, e.g. to search for `parseInt` function. + + +- **MSDN** – Microsoft manual with a lot of information, including JavaScript (often referred to as JScript). If one needs something specific to Internet Explorer, better go there: . + + Also, we can use an internet search with phrases such as "RegExp MSDN" or "RegExp MSDN jscript". ## Compatibility tables @@ -29,9 +34,9 @@ JavaScript is a developing language, new features get added regularly. To see their support among browser-based and other engines, see: -- - per-feature tables of support, e.g. to see which engines support modern cryptography functions: . +- - per-feature tables of support, e.g. to see which engines support modern cryptography functions: . - - a table with language features and engines that support those or don't support. -All these resources are useful in real-life development, as they contain valuable information about language details, their support, etc. +All these resources are useful in real-life development, as they contain valuable information about language details, their support etc. Please remember them (or this page) for the cases when you need in-depth information about a particular feature. diff --git a/1-js/01-getting-started/3-code-editors/article.md b/1-js/01-getting-started/3-code-editors/article.md index 5a86f2a7..d03f03de 100644 --- a/1-js/01-getting-started/3-code-editors/article.md +++ b/1-js/01-getting-started/3-code-editors/article.md @@ -13,7 +13,7 @@ An IDE loads the project (which can be many files), allows navigation between fi If you haven't selected an IDE yet, consider the following options: - [Visual Studio Code](https://code.visualstudio.com/) (cross-platform, free). -- [WebStorm](https://www.jetbrains.com/webstorm/) (cross-platform, paid). +- [WebStorm](http://www.jetbrains.com/webstorm/) (cross-platform, paid). For Windows, there's also "Visual Studio", not to be confused with "Visual Studio Code". "Visual Studio" is a paid and mighty Windows-only editor, well-suited for the .NET platform. It's also good at JavaScript. There's also a free version [Visual Studio Community](https://www.visualstudio.com/vs/community/). @@ -31,6 +31,8 @@ In practice, lightweight editors may have a lot of plugins including directory-l The following options deserve your attention: +- [Atom](https://atom.io/) (cross-platform, free). +- [Visual Studio Code](https://code.visualstudio.com/) (cross-platform, free). - [Sublime Text](http://www.sublimetext.com) (cross-platform, shareware). - [Notepad++](https://notepad-plus-plus.org/) (Windows, free). - [Vim](http://www.vim.org/) and [Emacs](https://www.gnu.org/software/emacs/) are also cool if you know how to use them. diff --git a/1-js/02-first-steps/01-hello-world/article.md b/1-js/02-first-steps/01-hello-world/article.md index 35f82bf5..b3149f11 100644 --- a/1-js/02-first-steps/01-hello-world/article.md +++ b/1-js/02-first-steps/01-hello-world/article.md @@ -9,7 +9,7 @@ So first, let's see how we attach a script to a webpage. For server-side environ ## The "script" tag -JavaScript programs can be inserted almost anywhere into an HTML document using the ` ``` -Here, `/path/to/script.js` is an absolute path to the script from the site root. One can also provide a relative path from the current page. For instance, `src="script.js"`, just like `src="./script.js"`, would mean a file `"script.js"` in the current folder. +Here, `/path/to/script.js` is an absolute path to the script from the site root. One can also provide a relative path from the current page. For instance, `src="script.js"` would mean a file `"script.js"` in the current folder. We can give a full URL as well. For instance: diff --git a/1-js/02-first-steps/02-structure/article.md b/1-js/02-first-steps/02-structure/article.md index e81fd343..cf1dd53d 100644 --- a/1-js/02-first-steps/02-structure/article.md +++ b/1-js/02-first-steps/02-structure/article.md @@ -46,7 +46,7 @@ alert(3 + + 2); ``` -The code outputs `6` because JavaScript does not insert semicolons here. It is intuitively obvious that if the line ends with a plus `"+"`, then it is an "incomplete expression", so a semicolon there would be incorrect. And in this case, that works as intended. +The code outputs `6` because JavaScript does not insert semicolons here. It is intuitively obvious that if the line ends with a plus `"+"`, then it is an "incomplete expression", so the semicolon is not required. And in this case that works as intended. **But there are situations where JavaScript "fails" to assume a semicolon where it is really needed.** @@ -56,36 +56,40 @@ Errors which occur in such cases are quite hard to find and fix. If you're curious to see a concrete example of such an error, check this code out: ```js run -alert("Hello"); - -[1, 2].forEach(alert); +[1, 2].forEach(alert) ``` -No need to think about the meaning of the brackets `[]` and `forEach` yet. We'll study them later. For now, just remember the result of running the code: it shows `Hello`, then `1`, then `2`. +No need to think about the meaning of the brackets `[]` and `forEach` yet. We'll study them later. For now, just remember the result of the code: it shows `1` then `2`. -Now let's remove the semicolon after the `alert`: +Now, let's add an `alert` before the code and *not* finish it with a semicolon: ```js run no-beautify -alert("Hello") +alert("There will be an error") -[1, 2].forEach(alert); +[1, 2].forEach(alert) ``` -The difference compared to the code above is only one character: the semicolon at the end of the first line is gone. +Now if we run the code, only the first `alert` is shown and then we have an error! -If we run this code, only the first `Hello` shows (and there's an error, you may need to open the console to see it). There are no numbers any more. +But everything is fine again if we add a semicolon after `alert`: +```js run +alert("All fine now"); -That's because JavaScript does not assume a semicolon before square brackets `[...]`. So, the code in the last example is treated as a single statement. +[1, 2].forEach(alert) +``` -Here's how the engine sees it: +Now we have the "All fine now" message followed by `1` and `2`. + + +The error in the no-semicolon variant occurs because JavaScript does not assume a semicolon before square brackets `[...]`. + +So, because the semicolon is not auto-inserted, the code in the first example is treated as a single statement. Here's how the engine sees it: ```js run no-beautify -alert("Hello")[1, 2].forEach(alert); +alert("There will be an error")[1, 2].forEach(alert) ``` -Looks weird, right? Such merging in this case is just wrong. We need to put a semicolon after `alert` for the code to work correctly. - -This can happen in other situations also. +But it should be two separate statements, not one. Such a merging in this case is just wrong, hence the error. This can happen in other situations. ```` We recommend putting semicolons between statements even if they are separated by newlines. This rule is widely adopted by the community. Let's note once again -- *it is possible* to leave out semicolons most of the time. But it's safer -- especially for a beginner -- to use them. diff --git a/1-js/02-first-steps/04-variables/2-declare-variables/solution.md b/1-js/02-first-steps/04-variables/2-declare-variables/solution.md index 392f4e26..d56e54d2 100644 --- a/1-js/02-first-steps/04-variables/2-declare-variables/solution.md +++ b/1-js/02-first-steps/04-variables/2-declare-variables/solution.md @@ -6,7 +6,7 @@ That's simple: let ourPlanetName = "Earth"; ``` -Note, we could use a shorter name `planet`, but it might not be obvious what planet it refers to. It's nice to be more verbose. At least until the variable isNotTooLong. +Note, we could use a shorter name `planet`, but it might be not obvious what planet it refers to. It's nice to be more verbose. At least until the variable isNotTooLong. ## The name of the current visitor diff --git a/1-js/02-first-steps/04-variables/3-uppercast-constant/task.md b/1-js/02-first-steps/04-variables/3-uppercast-constant/task.md index f3c208a7..5fd18f90 100644 --- a/1-js/02-first-steps/04-variables/3-uppercast-constant/task.md +++ b/1-js/02-first-steps/04-variables/3-uppercast-constant/task.md @@ -12,14 +12,13 @@ const birthday = '18.04.1982'; const age = someCode(birthday); ``` -Here we have a constant `birthday` for the date, and also the `age` constant. - -The `age` is calculated from `birthday` using `someCode()`, which means a function call that we didn't explain yet (we will soon!), but the details don't matter here, the point is that `age` is calculated somehow based on the `birthday`. +Here we have a constant `birthday` date and the `age` is calculated from `birthday` with the help of some code (it is not provided for shortness, and because details don't matter here). Would it be right to use upper case for `birthday`? For `age`? Or even for both? ```js -const BIRTHDAY = '18.04.1982'; // make birthday uppercase? +const BIRTHDAY = '18.04.1982'; // make uppercase? -const AGE = someCode(BIRTHDAY); // make age uppercase? +const AGE = someCode(BIRTHDAY); // make uppercase? ``` + diff --git a/1-js/02-first-steps/04-variables/article.md b/1-js/02-first-steps/04-variables/article.md index 4c2d09de..03eeaa6d 100644 --- a/1-js/02-first-steps/04-variables/article.md +++ b/1-js/02-first-steps/04-variables/article.md @@ -24,7 +24,7 @@ Now, we can put some data into it by using the assignment operator `=`: let message; *!* -message = 'Hello'; // store the string 'Hello' in the variable named message +message = 'Hello'; // store the string */!* ``` @@ -64,7 +64,6 @@ let message = 'Hello'; ``` Some people also define multiple variables in this multiline style: - ```js no-beautify let user = 'John', age = 25, @@ -104,7 +103,6 @@ For instance, the variable `message` can be imagined as a box labeled `"message" We can put any value in the box. We can also change it as many times as we want: - ```js run let message; @@ -194,7 +192,7 @@ let my-name; // hyphens '-' aren't allowed in the name ``` ```smart header="Case matters" -Variables named `apple` and `APPLE` are two different variables. +Variables named `apple` and `AppLE` are two different variables. ``` ````smart header="Non-Latin letters are allowed, but not recommended" @@ -262,6 +260,7 @@ myBirthday = '01.01.2001'; // error, can't reassign the constant! When a programmer is sure that a variable will never change, they can declare it with `const` to guarantee and clearly communicate that fact to everyone. + ### Uppercase constants There is a widespread practice to use constants as aliases for difficult-to-remember values that are known prior to execution. @@ -292,14 +291,13 @@ When should we use capitals for a constant and when should we name it normally? Being a "constant" just means that a variable's value never changes. But there are constants that are known prior to execution (like a hexadecimal value for red) and there are constants that are *calculated* in run-time, during the execution, but do not change after their initial assignment. For instance: - ```js const pageLoadTime = /* time taken by a webpage to load */; ``` The value of `pageLoadTime` is not known prior to the page load, so it's named normally. But it's still a constant because it doesn't change after assignment. -In other words, capital-named constants are only used as aliases for "hard-coded" values. +In other words, capital-named constants are only used as aliases for "hard-coded" values. ## Name things right diff --git a/1-js/02-first-steps/04-variables/variable-change.svg b/1-js/02-first-steps/04-variables/variable-change.svg index 1b267923..427a6388 100644 --- a/1-js/02-first-steps/04-variables/variable-change.svg +++ b/1-js/02-first-steps/04-variables/variable-change.svg @@ -1 +1 @@ -"World!""Hello!"message \ No newline at end of file +"World!""Hello!"message \ No newline at end of file diff --git a/1-js/02-first-steps/04-variables/variable.svg b/1-js/02-first-steps/04-variables/variable.svg index 1c3d8b0c..5d15c9e4 100644 --- a/1-js/02-first-steps/04-variables/variable.svg +++ b/1-js/02-first-steps/04-variables/variable.svg @@ -1 +1 @@ -"Hello!"message \ No newline at end of file +"Hello!"message \ No newline at end of file diff --git a/1-js/02-first-steps/05-types/article.md b/1-js/02-first-steps/05-types/article.md index a697548a..66176000 100644 --- a/1-js/02-first-steps/05-types/article.md +++ b/1-js/02-first-steps/05-types/article.md @@ -46,15 +46,13 @@ Besides regular numbers, there are so-called "special numeric values" which also alert( "not a number" / 2 ); // NaN, such division is erroneous ``` - `NaN` is sticky. Any further mathematical operation on `NaN` returns `NaN`: + `NaN` is sticky. Any further operation on `NaN` returns `NaN`: ```js run - alert( NaN + 1 ); // NaN - alert( 3 * NaN ); // NaN - alert( "not a number" / 2 - 1 ); // NaN + alert( "not a number" / 2 + 5 ); // NaN ``` - So, if there's a `NaN` somewhere in a mathematical expression, it propagates to the whole result (there's only one exception to that: `NaN ** 0` is `1`). + So, if there's a `NaN` somewhere in a mathematical expression, it propagates to the whole result. ```smart header="Mathematical operations are safe" Doing maths is "safe" in JavaScript. We can do anything: divide by zero, treat non-numeric strings as numbers, etc. @@ -66,22 +64,11 @@ Special numeric values formally belong to the "number" type. Of course they are We'll see more about working with numbers in the chapter . -## BigInt [#bigint-type] +## BigInt -In JavaScript, the "number" type cannot safely represent integer values larger than (253-1) (that's `9007199254740991`), or less than -(253-1) for negatives. +In JavaScript, the "number" type cannot represent integer values larger than (253-1) (that's `9007199254740991`), or less than -(253-1) for negatives. It's a technical limitation caused by their internal representation. -To be really precise, the "number" type can store larger integers (up to 1.7976931348623157 * 10308), but outside of the safe integer range ±(253-1) there'll be a precision error, because not all digits fit into the fixed 64-bit storage. So an "approximate" value may be stored. - -For example, these two numbers (right above the safe range) are the same: - -```js -console.log(9007199254740991 + 1); // 9007199254740992 -console.log(9007199254740991 + 2); // 9007199254740992 -``` - -So to say, all odd integers greater than (253-1) can't be stored at all in the "number" type. - -For most purposes ±(253-1) range is quite enough, but sometimes we need the entire range of really big integers, e.g. for cryptography or microsecond-precision timestamps. +For most purposes that's quite enough, but sometimes we need really big numbers, e.g. for cryptography or microsecond-precision timestamps. `BigInt` type was recently added to the language to represent integers of arbitrary length. @@ -226,7 +213,14 @@ The `symbol` type is used to create unique identifiers for objects. We have to m The `typeof` operator returns the type of the argument. It's useful when we want to process values of different types differently or just want to do a quick check. -A call to `typeof x` returns a string with the type name: +It supports two forms of syntax: + +1. As an operator: `typeof x`. +2. As a function: `typeof(x)`. + +In other words, it works with parentheses or without them. The result is the same. + +The call to `typeof x` returns a string with the type name: ```js typeof undefined // "undefined" @@ -257,37 +251,25 @@ typeof alert // "function" (3) The last three lines may need additional explanation: 1. `Math` is a built-in object that provides mathematical operations. We will learn it in the chapter . Here, it serves just as an example of an object. -2. The result of `typeof null` is `"object"`. That's an officially recognized error in `typeof`, coming from very early days of JavaScript and kept for compatibility. Definitely, `null` is not an object. It is a special value with a separate type of its own. The behavior of `typeof` is wrong here. +2. The result of `typeof null` is `"object"`. That's an officially recognized error in `typeof` behavior, coming from the early days of JavaScript and kept for compatibility. Definitely, `null` is not an object. It is a special value with a separate type of its own. 3. The result of `typeof alert` is `"function"`, because `alert` is a function. We'll study functions in the next chapters where we'll also see that there's no special "function" type in JavaScript. Functions belong to the object type. But `typeof` treats them differently, returning `"function"`. That also comes from the early days of JavaScript. Technically, such behavior isn't correct, but can be convenient in practice. -```smart header="The `typeof(x)` syntax" -You may also come across another syntax: `typeof(x)`. It's the same as `typeof x`. - -To put it clear: `typeof` is an operator, not a function. The parentheses here aren't a part of `typeof`. It's the kind of parentheses used for mathematical grouping. - -Usually, such parentheses contain a mathematical expression, such as `(2 + 2)`, but here they contain only one argument `(x)`. Syntactically, they allow to avoid a space between the `typeof` operator and its argument, and some people like it. - -Some people prefer `typeof(x)`, although the `typeof x` syntax is much more common. -``` - ## Summary There are 8 basic data types in JavaScript. -- Seven primitive data types: - - `number` for numbers of any kind: integer or floating-point, integers are limited by ±(253-1). - - `bigint` for integer numbers of arbitrary length. - - `string` for strings. A string may have zero or more characters, there's no separate single-character type. - - `boolean` for `true`/`false`. - - `null` for unknown values -- a standalone type that has a single value `null`. - - `undefined` for unassigned values -- a standalone type that has a single value `undefined`. - - `symbol` for unique identifiers. -- And one non-primitive data type: - - `object` for more complex data structures. +- `number` for numbers of any kind: integer or floating-point, integers are limited by ±(253-1). +- `bigint` is for integer numbers of arbitrary length. +- `string` for strings. A string may have zero or more characters, there's no separate single-character type. +- `boolean` for `true`/`false`. +- `null` for unknown values -- a standalone type that has a single value `null`. +- `undefined` for unassigned values -- a standalone type that has a single value `undefined`. +- `object` for more complex data structures. +- `symbol` for unique identifiers. The `typeof` operator allows us to see which type is stored in a variable. -- Usually used as `typeof x`, but `typeof(x)` is also possible. +- Two forms: `typeof x` or `typeof(x)`. - Returns a string with the name of the type, like `"string"`. - For `null` returns `"object"` -- this is an error in the language, it's not actually an object. diff --git a/1-js/02-first-steps/07-type-conversions/article.md b/1-js/02-first-steps/07-type-conversions/article.md index ea8e0f81..cf97b330 100644 --- a/1-js/02-first-steps/07-type-conversions/article.md +++ b/1-js/02-first-steps/07-type-conversions/article.md @@ -7,7 +7,7 @@ For example, `alert` automatically converts any value to a string to show it. Ma There are also cases when we need to explicitly convert a value to the expected type. ```smart header="Not talking about objects yet" -In this chapter, we won't cover objects. For now, we'll just be talking about primitives. +In this chapter, we won't cover objects. For now we'll just be talking about primitives. Later, after we learn about objects, in the chapter we'll see how objects fit in. ``` @@ -70,7 +70,7 @@ Numeric conversion rules: |`undefined`|`NaN`| |`null`|`0`| |true and false | `1` and `0` | -| `string` | Whitespaces (includes spaces, tabs `\t`, newlines `\n` etc.) from the start and end are removed. If the remaining string is empty, the result is `0`. Otherwise, the number is "read" from the string. An error gives `NaN`. | +| `string` | Whitespaces from the start and end are removed. If the remaining string is empty, the result is `0`. Otherwise, the number is "read" from the string. An error gives `NaN`. | Examples: @@ -130,7 +130,7 @@ The conversion follows the rules: |`undefined`|`NaN`| |`null`|`0`| |true / false | `1 / 0` | -| `string` | The string is read "as is", whitespaces (includes spaces, tabs `\t`, newlines `\n` etc.) from both sides are ignored. An empty string becomes `0`. An error gives `NaN`. | +| `string` | The string is read "as is", whitespaces from both sides are ignored. An empty string becomes `0`. An error gives `NaN`. | **`Boolean Conversion`** -- Occurs in logical operations. Can be performed with `Boolean(value)`. diff --git a/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/solution.md b/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/solution.md index 7370b66a..9a8411fb 100644 --- a/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/solution.md +++ b/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/solution.md @@ -9,6 +9,7 @@ true + false = 1 "$" + 4 + 5 = "$45" "4" - 2 = 2 "4px" - 2 = NaN +7 / 0 = Infinity " -9 " + 5 = " -9 5" // (3) " -9 " - 5 = -14 // (4) null + 1 = 1 // (5) @@ -22,4 +23,4 @@ undefined + 1 = NaN // (6) 4. The subtraction always converts to numbers, so it makes `" -9 "` a number `-9` (ignoring spaces around it). 5. `null` becomes `0` after the numeric conversion. 6. `undefined` becomes `NaN` after the numeric conversion. -7. Space characters are trimmed off string start and end when a string is converted to a number. Here the whole string consists of space characters, such as `\t`, `\n` and a "regular" space between them. So, similarly to an empty string, it becomes `0`. +7. Space characters, are trimmed off string start and end when a string is converted to a number. Here the whole string consists of space characters, such as `\t`, `\n` and a "regular" space between them. So, similarly to an empty string, it becomes `0`. diff --git a/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/task.md b/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/task.md index 068420c7..930c7151 100644 --- a/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/task.md +++ b/1-js/02-first-steps/08-operators/3-primitive-conversions-questions/task.md @@ -16,6 +16,7 @@ true + false "$" + 4 + 5 "4" - 2 "4px" - 2 +7 / 0 " -9 " + 5 " -9 " - 5 null + 1 diff --git a/1-js/02-first-steps/08-operators/article.md b/1-js/02-first-steps/08-operators/article.md index 882b6cdb..0aaaf512 100644 --- a/1-js/02-first-steps/08-operators/article.md +++ b/1-js/02-first-steps/08-operators/article.md @@ -56,21 +56,17 @@ alert( 8 % 3 ); // 2, a remainder of 8 divided by 3 ### Exponentiation ** -The exponentiation operator `a ** b` raises `a` to the power of `b`. - -In school maths, we write that as ab. +The exponentiation operator `a ** b` multiplies `a` by itself `b` times. For instance: ```js run -alert( 2 ** 2 ); // 2² = 4 -alert( 2 ** 3 ); // 2³ = 8 -alert( 2 ** 4 ); // 2⁴ = 16 +alert( 2 ** 2 ); // 4 (2 multiplied by itself 2 times) +alert( 2 ** 3 ); // 8 (2 * 2 * 2, 3 times) +alert( 2 ** 4 ); // 16 (2 * 2 * 2 * 2, 4 times) ``` -Just like in maths, the exponentiation operator is defined for non-integer numbers as well. - -For example, a square root is an exponentiation by ½: +Mathematically, the exponentiation is defined for non-integer numbers as well. For example, a square root is an exponentiation by `1/2`: ```js run alert( 4 ** (1/2) ); // 2 (power of 1/2 is the same as a square root) @@ -108,12 +104,7 @@ Here's a more complex example: alert(2 + 2 + '1' ); // "41" and not "221" ``` -Here, operators work one after another. The first `+` sums two numbers, so it returns `4`, then the next `+` adds the string `1` to it, so it's like `4 + '1' = '41'`. - -```js run -alert('1' + 2 + 2); // "122" and not "14" -``` -Here, the first operand is a string, the compiler treats the other two operands as strings too. The `2` gets concatenated to `'1'`, so it's like `'1' + 2 = "12"` and `"12" + 2 = "122"`. +Here, operators work one after another. The first `+` sums two numbers, so it returns `4`, then the next `+` adds the string `1` to it, so it's like `4 + '1' = 41`. The binary `+` is the only operator that supports strings in such a way. Other arithmetic operators work only with numbers and always convert their operands to numbers. @@ -194,22 +185,22 @@ Here's an extract from the [precedence table](https://developer.mozilla.org/en-U | Precedence | Name | Sign | |------------|------|------| | ... | ... | ... | -| 14 | unary plus | `+` | -| 14 | unary negation | `-` | -| 13 | exponentiation | `**` | -| 12 | multiplication | `*` | -| 12 | division | `/` | -| 11 | addition | `+` | -| 11 | subtraction | `-` | +| 17 | unary plus | `+` | +| 17 | unary negation | `-` | +| 16 | exponentiation | `**` | +| 15 | multiplication | `*` | +| 15 | division | `/` | +| 13 | addition | `+` | +| 13 | subtraction | `-` | | ... | ... | ... | -| 2 | assignment | `=` | +| 3 | assignment | `=` | | ... | ... | ... | -As we can see, the "unary plus" has a priority of `14` which is higher than the `11` of "addition" (binary plus). That's why, in the expression `"+apples + +oranges"`, unary pluses work before the addition. +As we can see, the "unary plus" has a priority of `17` which is higher than the `13` of "addition" (binary plus). That's why, in the expression `"+apples + +oranges"`, unary pluses work before the addition. ## Assignment -Let's note that an assignment `=` is also an operator. It is listed in the precedence table with the very low priority of `2`. +Let's note that an assignment `=` is also an operator. It is listed in the precedence table with the very low priority of `3`. That's why, when we assign a variable, like `x = 2 * 2 + 1`, the calculations are done first and then the `=` is evaluated, storing the result in `x`. @@ -223,7 +214,7 @@ alert( x ); // 5 The fact of `=` being an operator, not a "magical" language construct has an interesting implication. -All operators in JavaScript return a value. That's obvious for `+` and `-`, but also true for `=`. +Most operators in JavaScript return a value. That's obvious for `+` and `-`, but also true for `=`. The call `x = value` writes the `value` into `x` *and then returns it*. @@ -303,9 +294,9 @@ Such operators have the same precedence as a normal assignment, so they run afte ```js run let n = 2; -n *= 3 + 5; // right part evaluated first, same as n *= 8 +n *= 3 + 5; -alert( n ); // 16 +alert( n ); // 16 (right part evaluated first, same as n *= 8) ``` ## Increment/decrement @@ -437,7 +428,7 @@ The list of operators: - RIGHT SHIFT ( `>>` ) - ZERO-FILL RIGHT SHIFT ( `>>>` ) -These operators are used very rarely, when we need to fiddle with numbers on the very lowest (bitwise) level. We won't need these operators any time soon, as web development has little use of them, but in some special areas, such as cryptography, they are useful. You can read the [Bitwise Operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#bitwise_operators) chapter on MDN when a need arises. +These operators are used very rarely, when we need to fiddle with numbers on the very lowest (bitwise) level. We won't need these operators any time soon, as web development has little use of them, but in some special areas, such as cryptography, they are useful. You can read the [Bitwise Operators](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Expressions_and_Operators#Bitwise) chapter on MDN when a need arises. ## Comma diff --git a/1-js/02-first-steps/09-comparison/article.md b/1-js/02-first-steps/09-comparison/article.md index a69317fe..ead7922f 100644 --- a/1-js/02-first-steps/09-comparison/article.md +++ b/1-js/02-first-steps/09-comparison/article.md @@ -7,7 +7,7 @@ In JavaScript they are written like this: - Greater/less than: a > b, a < b. - Greater/less than or equals: a >= b, a <= b. - Equals: `a == b`, please note the double equality sign `==` means the equality test, while a single one `a = b` means an assignment. -- Not equals: In maths the notation is , but in JavaScript it's written as a != b. +- Not equals. In maths the notation is , but in JavaScript it's written as a != b. In this article we'll learn more about different types of comparisons, how JavaScript makes them, including important peculiarities. diff --git a/1-js/02-first-steps/10-ifelse/2-check-standard/ifelse_task2.svg b/1-js/02-first-steps/10-ifelse/2-check-standard/ifelse_task2.svg index 47b020aa..25dc2744 100644 --- a/1-js/02-first-steps/10-ifelse/2-check-standard/ifelse_task2.svg +++ b/1-js/02-first-steps/10-ifelse/2-check-standard/ifelse_task2.svg @@ -1 +1 @@ -BeginYou don't know? “ECMAScript”!Right!What's the “official” name of JavaScript?OtherECMAScript \ No newline at end of file +BeginYou don't know? “ECMAScript”!Right!What's the “official” name of JavaScript?OtherECMAScript \ No newline at end of file diff --git a/1-js/02-first-steps/10-ifelse/2-check-standard/task.md b/1-js/02-first-steps/10-ifelse/2-check-standard/task.md index 4305584f..a4d94324 100644 --- a/1-js/02-first-steps/10-ifelse/2-check-standard/task.md +++ b/1-js/02-first-steps/10-ifelse/2-check-standard/task.md @@ -6,7 +6,7 @@ importance: 2 Using the `if..else` construct, write the code which asks: 'What is the "official" name of JavaScript?' -If the visitor enters "ECMAScript", then output "Right!", otherwise -- output: "You don't know? ECMAScript!" +If the visitor enters "ECMAScript", then output "Right!", otherwise -- output: "Didn't know? ECMAScript!" ![](ifelse_task2.svg) diff --git a/1-js/02-first-steps/10-ifelse/article.md b/1-js/02-first-steps/10-ifelse/article.md index 51514062..7327243b 100644 --- a/1-js/02-first-steps/10-ifelse/article.md +++ b/1-js/02-first-steps/10-ifelse/article.md @@ -68,7 +68,7 @@ if (cond) { ## The "else" clause -The `if` statement may contain an optional "else" block. It executes when the condition is falsy. +The `if` statement may contain an optional "else" block. It executes when the condition is false. For example: ```js run diff --git a/1-js/02-first-steps/11-logical-operators/3-alert-1-null-2/solution.md b/1-js/02-first-steps/11-logical-operators/3-alert-1-null-2/solution.md index 368b5940..5c2455ef 100644 --- a/1-js/02-first-steps/11-logical-operators/3-alert-1-null-2/solution.md +++ b/1-js/02-first-steps/11-logical-operators/3-alert-1-null-2/solution.md @@ -1,6 +1,6 @@ The answer: `null`, because it's the first falsy value from the list. ```js run -alert(1 && null && 2); +alert( 1 && null && 2 ); ``` diff --git a/1-js/02-first-steps/11-logical-operators/9-check-login/ifelse_task.svg b/1-js/02-first-steps/11-logical-operators/9-check-login/ifelse_task.svg index d22b518a..ca3e0aea 100644 --- a/1-js/02-first-steps/11-logical-operators/9-check-login/ifelse_task.svg +++ b/1-js/02-first-steps/11-logical-operators/9-check-login/ifelse_task.svg @@ -1 +1 @@ -BeginCanceledCanceledWelcome!I don't know youWrong passwordWho's there?Password?CancelCancelAdminTheMasterOtherOther \ No newline at end of file +BeginCanceledCanceledWelcome!I don't know youWrong passwordWho's there?Password?CancelCancelAdminTheMasterOtherOther \ No newline at end of file diff --git a/1-js/02-first-steps/11-logical-operators/article.md b/1-js/02-first-steps/11-logical-operators/article.md index 78c4fd2f..caa8cdfa 100644 --- a/1-js/02-first-steps/11-logical-operators/article.md +++ b/1-js/02-first-steps/11-logical-operators/article.md @@ -1,6 +1,6 @@ # Logical operators -There are four logical operators in JavaScript: `||` (OR), `&&` (AND), `!` (NOT), `??` (Nullish Coalescing). Here we cover the first three, the `??` operator is in the next article. +There are three logical operators in JavaScript: `||` (OR), `&&` (AND), `!` (NOT). Although they are called "logical", they can be applied to values of any type, not only boolean. Their result can also be of any type. @@ -64,7 +64,7 @@ if (hour < 10 || hour > 18 || isWeekend) { } ``` -## OR "||" finds the first truthy value [#or-finds-the-first-truthy-value] +## OR "||" finds the first truthy value The logic described above is somewhat classical. Now, let's bring in the "extra" features of JavaScript. @@ -123,7 +123,7 @@ This leads to some interesting usage compared to a "pure, classical, boolean-onl It means that `||` processes its arguments until the first truthy value is reached, and then the value is returned immediately, without even touching the other argument. - The importance of this feature becomes obvious if an operand isn't just a value, but an expression with a side effect, such as a variable assignment or a function call. + That importance of this feature becomes obvious if an operand isn't just a value, but an expression with a side effect, such as a variable assignment or a function call. In the example below, only the second message is printed: diff --git a/1-js/02-first-steps/12-nullish-coalescing-operator/article.md b/1-js/02-first-steps/12-nullish-coalescing-operator/article.md index ec0ffe78..55e0c206 100644 --- a/1-js/02-first-steps/12-nullish-coalescing-operator/article.md +++ b/1-js/02-first-steps/12-nullish-coalescing-operator/article.md @@ -2,14 +2,15 @@ [recent browser="new"] -The nullish coalescing operator is written as two question marks `??`. +Here, in this article, we'll say that an expression is "defined" when it's neither `null` nor `undefined`. -As it treats `null` and `undefined` similarly, we'll use a special term here, in this article. For brevity, we'll say that a value is "defined" when it's neither `null` nor `undefined`. +The nullish coalescing operator is written as two question marks `??`. The result of `a ?? b` is: - if `a` is defined, then `a`, - if `a` isn't defined, then `b`. + In other words, `??` returns the first argument if it's not `null/undefined`. Otherwise, the second one. The nullish coalescing operator isn't anything completely new. It's just a nice syntax to get the first "defined" value of the two. @@ -20,31 +21,29 @@ We can rewrite `result = a ?? b` using the operators that we already know, like result = (a !== null && a !== undefined) ? a : b; ``` -Now it should be absolutely clear what `??` does. Let's see where it helps. +The common use case for `??` is to provide a default value for a potentially undefined variable. -The common use case for `??` is to provide a default value. - -For example, here we show `user` if its value isn't `null/undefined`, otherwise `Anonymous`: +For example, here we show `Anonymous` if `user` isn't defined: ```js run let user; -alert(user ?? "Anonymous"); // Anonymous (user not defined) +alert(user ?? "Anonymous"); // Anonymous ``` -Here's the example with `user` assigned to a name: +Of course, if `user` had any value except `null/undefined`, then we would see it instead: ```js run let user = "John"; -alert(user ?? "Anonymous"); // John (user defined) +alert(user ?? "Anonymous"); // John ``` We can also use a sequence of `??` to select the first value from a list that isn't `null/undefined`. -Let's say we have a user's data in variables `firstName`, `lastName` or `nickName`. All of them may be not defined, if the user decided not to fill in the corresponding values. +Let's say we have a user's data in variables `firstName`, `lastName` or `nickName`. All of them may be undefined, if the user decided not to enter a value. -We'd like to display the user name using one of these variables, or show "Anonymous" if all of them are `null/undefined`. +We'd like to display the user name using one of these variables, or show "Anonymous" if all of them are undefined. Let's use the `??` operator for that: @@ -76,7 +75,7 @@ alert(firstName || lastName || nickName || "Anonymous"); // Supercoder */!* ``` -Historically, the OR `||` operator was there first. It exists since the beginning of JavaScript, so developers were using it for such purposes for a long time. +The OR `||` operator exists since the beginning of JavaScript, so developers were using it for such purposes for a long time. On the other hand, the nullish coalescing operator `??` was added to JavaScript only recently, and the reason for that was that people weren't quite happy with `||`. @@ -97,20 +96,18 @@ alert(height || 100); // 100 alert(height ?? 100); // 0 ``` -- The `height || 100` checks `height` for being a falsy value, and it's `0`, falsy indeed. - - so the result of `||` is the second argument, `100`. +- The `height || 100` checks `height` for being a falsy value, and it really is. + - so the result is the second argument, `100`. - The `height ?? 100` checks `height` for being `null/undefined`, and it's not, - so the result is `height` "as is", that is `0`. -In practice, the zero height is often a valid value, that shouldn't be replaced with the default. So `??` does just the right thing. +If the zero height is a valid value, that shouldn't be replaced with the default, then `??` does just the right thing. ## Precedence -The precedence of the `??` operator is the same as `||`. They both equal `3` in the [MDN table](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence#Table). +The precedence of the `??` operator is rather low: `5` in the [MDN table](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence#Table). So `??` is evaluated before `=` and `?`, but after most other operations, such as `+`, `*`. -That means that, just like `||`, the nullish coalescing operator `??` is evaluated before `=` and `?`, but after most other operations, such as `+`, `*`. - -So we may need to add parentheses in expressions like this: +So if we'd like to choose a value with `??` in an expression with other operators, consider adding parentheses: ```js run let height = null; @@ -128,7 +125,7 @@ Otherwise, if we omit parentheses, then as `*` has the higher precedence than `? // without parentheses let area = height ?? 100 * width ?? 50; -// ...works this way (not what we want): +// ...works the same as this (probably not what we want): let area = height ?? (100 * width) ?? 50; ``` @@ -142,7 +139,7 @@ The code below triggers a syntax error: let x = 1 && 2 ?? 3; // Syntax error ``` -The limitation is surely debatable, it was added to the language specification with the purpose to avoid programming mistakes, when people start to switch from `||` to `??`. +The limitation is surely debatable, but it was added to the language specification with the purpose to avoid programming mistakes, when people start to switch to `??` from `||`. Use explicit parentheses to work around it: diff --git a/1-js/02-first-steps/13-while-for/article.md b/1-js/02-first-steps/13-while-for/article.md index d1b74988..b3e3953b 100644 --- a/1-js/02-first-steps/13-while-for/article.md +++ b/1-js/02-first-steps/13-while-for/article.md @@ -6,19 +6,6 @@ For example, outputting goods from a list one after another or just running the *Loops* are a way to repeat the same code multiple times. -```smart header="The for..of and for..in loops" -A small announcement for advanced readers. - -This article covers only basic loops: `while`, `do..while` and `for(..;..;..)`. - -If you came to this article searching for other types of loops, here are the pointers: - -- See [for..in](info:object#forin) to loop over object properties. -- See [for..of](info:array#loops) and [iterables](info:iterable) for looping over arrays and iterable objects. - -Otherwise, please read on. -``` - ## The "while" loop The `while` loop has the following syntax: @@ -119,7 +106,7 @@ Let's examine the `for` statement part-by-part: | part | | | |-------|----------|----------------------------------------------------------------------------| -| begin | `let i = 0` | Executes once upon entering the loop. | +| begin | `i = 0` | Executes once upon entering the loop. | | condition | `i < 3`| Checked before every loop iteration. If false, the loop stops. | | body | `alert(i)`| Runs again and again while the condition is truthy. | | step| `i++` | Executes after the body on each iteration. | @@ -175,8 +162,10 @@ for (i = 0; i < 3; i++) { // use an existing variable alert(i); // 3, visible, because declared outside of the loop ``` + ```` + ### Skipping parts Any part of `for` can be skipped. @@ -279,7 +268,7 @@ for (let i = 0; i < 10; i++) { From a technical point of view, this is identical to the example above. Surely, we can just wrap the code in an `if` block instead of using `continue`. -But as a side effect, this created one more level of nesting (the `alert` call inside the curly braces). If the code inside of `if` is longer than a few lines, that may decrease the overall readability. +But as a side-effect, this created one more level of nesting (the `alert` call inside the curly braces). If the code inside of `if` is longer than a few lines, that may decrease the overall readability. ```` ````warn header="No `break/continue` to the right side of '?'" @@ -297,6 +286,7 @@ if (i > 5) { ...and rewrite it using a question mark: + ```js no-beautify (i > 5) ? alert(i) : *!*continue*/!*; // continue isn't allowed here ``` @@ -328,10 +318,9 @@ alert('Done!'); We need a way to stop the process if the user cancels the input. -The ordinary `break` after `input` would only break the inner loop. That's not sufficient -- labels, come to the rescue! +The ordinary `break` after `input` would only break the inner loop. That's not sufficient--labels, come to the rescue! A *label* is an identifier with a colon before a loop: - ```js labelName: for (...) { ... @@ -353,7 +342,6 @@ The `break ` statement in the loop below breaks out to the label: // do something with the value... } } - alert('Done!'); ``` @@ -374,26 +362,13 @@ The `continue` directive can also be used with a label. In this case, code execu Labels do not allow us to jump into an arbitrary place in the code. For example, it is impossible to do this: - ```js -break label; // jump to the label below (doesn't work) +break label; // doesn't jumps to the label below label: for (...) ``` -A `break` directive must be inside a code block. Technically, any labelled code block will do, e.g.: - -```js -label: { - // ... - break label; // works - // ... -} -``` - -...Although, 99.9% of the time `break` is used inside loops, as we've seen in the examples above. - -A `continue` is only possible from inside a loop. +A call to `break/continue` is only possible from inside a loop and the label must be somewhere above the directive. ```` ## Summary diff --git a/1-js/02-first-steps/14-switch/article.md b/1-js/02-first-steps/14-switch/article.md index d86babce..314c6cef 100644 --- a/1-js/02-first-steps/14-switch/article.md +++ b/1-js/02-first-steps/14-switch/article.md @@ -47,7 +47,7 @@ switch (a) { break; */!* case 5: - alert( 'Too big' ); + alert( 'Too large' ); break; default: alert( "I don't know such values" ); @@ -139,7 +139,7 @@ switch (a) { Now both `3` and `5` show the same message. -The ability to "group" cases is a side effect of how `switch/case` works without `break`. Here the execution of `case 3` starts from the line `(*)` and goes through `case 5`, because there's no `break`. +The ability to "group" cases is a side-effect of how `switch/case` works without `break`. Here the execution of `case 3` starts from the line `(*)` and goes through `case 5`, because there's no `break`. ## Type matters diff --git a/1-js/02-first-steps/15-function-basics/1-if-else-required/solution.md b/1-js/02-first-steps/15-function-basics/1-if-else-required/solution.md index e3a0df77..e41c8041 100644 --- a/1-js/02-first-steps/15-function-basics/1-if-else-required/solution.md +++ b/1-js/02-first-steps/15-function-basics/1-if-else-required/solution.md @@ -1,3 +1 @@ -No difference! - -In both cases, `return confirm('Did parents allow you?')` executes exactly when the `if` condition is falsy. \ No newline at end of file +No difference. \ No newline at end of file diff --git a/1-js/02-first-steps/15-function-basics/2-rewrite-function-question-or/solution.md b/1-js/02-first-steps/15-function-basics/2-rewrite-function-question-or/solution.md index e4850264..c8ee9618 100644 --- a/1-js/02-first-steps/15-function-basics/2-rewrite-function-question-or/solution.md +++ b/1-js/02-first-steps/15-function-basics/2-rewrite-function-question-or/solution.md @@ -14,4 +14,4 @@ function checkAge(age) { } ``` -Note that the parentheses around `age > 18` are not required here. They exist for better readability. +Note that the parentheses around `age > 18` are not required here. They exist for better readabilty. diff --git a/1-js/02-first-steps/15-function-basics/article.md b/1-js/02-first-steps/15-function-basics/article.md index d82e8677..b12d0b9e 100644 --- a/1-js/02-first-steps/15-function-basics/article.md +++ b/1-js/02-first-steps/15-function-basics/article.md @@ -20,11 +20,11 @@ function showMessage() { } ``` -The `function` keyword goes first, then goes the *name of the function*, then a list of *parameters* between the parentheses (comma-separated, empty in the example above, we'll see examples later) and finally the code of the function, also named "the function body", between curly braces. +The `function` keyword goes first, then goes the *name of the function*, then a list of *parameters* between the parentheses (comma-separated, empty in the example above) and finally the code of the function, also named "the function body", between curly braces. ```js -function name(parameter1, parameter2, ... parameterN) { - // body +function name(parameters) { + ...body... } ``` @@ -137,23 +137,26 @@ It's a good practice to minimize the use of global variables. Modern code has fe ## Parameters -We can pass arbitrary data to functions using parameters. +We can pass arbitrary data to functions using parameters (also called *function arguments*) . In the example below, the function has two parameters: `from` and `text`. ```js run -function showMessage(*!*from, text*/!*) { // parameters: from, text +function showMessage(*!*from, text*/!*) { // arguments: from, text alert(from + ': ' + text); } -*!*showMessage('Ann', 'Hello!');*/!* // Ann: Hello! (*) -*!*showMessage('Ann', "What's up?");*/!* // Ann: What's up? (**) +*!* +showMessage('Ann', 'Hello!'); // Ann: Hello! (*) +showMessage('Ann', "What's up?"); // Ann: What's up? (**) +*/!* ``` When the function is called in lines `(*)` and `(**)`, the given values are copied to local variables `from` and `text`. Then the function uses them. Here's one more example: we have a variable `from` and pass it to the function. Please note: the function changes `from`, but the change is not seen outside, because a function always gets a copy of the value: + ```js run function showMessage(from, text) { @@ -172,21 +175,9 @@ showMessage(from, "Hello"); // *Ann*: Hello alert( from ); // Ann ``` -When a value is passed as a function parameter, it's also called an *argument*. - -In other words, to put these terms straight: - -- A parameter is the variable listed inside the parentheses in the function declaration (it's a declaration time term). -- An argument is the value that is passed to the function when it is called (it's a call time term). - -We declare functions listing their parameters, then call them passing arguments. - -In the example above, one might say: "the function `showMessage` is declared with two parameters, then called with two arguments: `from` and `"Hello"`". - - ## Default values -If a function is called, but an argument is not provided, then the corresponding value becomes `undefined`. +If a parameter is not provided, then its value becomes `undefined`. For instance, the aforementioned function `showMessage(from, text)` can be called with a single argument: @@ -194,9 +185,9 @@ For instance, the aforementioned function `showMessage(from, text)` can be calle showMessage("Ann"); ``` -That's not an error. Such a call would output `"*Ann*: undefined"`. As the value for `text` isn't passed, it becomes `undefined`. +That's not an error. Such a call would output `"*Ann*: undefined"`. There's no `text`, so it's assumed that `text === undefined`. -We can specify the so-called "default" (to use if omitted) value for a parameter in the function declaration, using `=`: +If we want to use a "default" `text` in this case, then we can specify it after `=`: ```js run function showMessage(from, *!*text = "no text given"*/!*) { @@ -206,13 +197,7 @@ function showMessage(from, *!*text = "no text given"*/!*) { showMessage("Ann"); // Ann: no text given ``` -Now if the `text` parameter is not passed, it will get the value `"no text given"`. - -The default value also jumps in if the parameter exists, but strictly equals `undefined`, like this: - -```js -showMessage("Ann", undefined); // Ann: no text given -``` +Now if the `text` parameter is not passed, it will get the value `"no text given"` Here `"no text given"` is a string, but it can be a more complex expression, which is only evaluated and assigned if the parameter is missing. So, this is also possible: @@ -226,55 +211,19 @@ function showMessage(from, text = anotherFunction()) { ```smart header="Evaluation of default parameters" In JavaScript, a default parameter is evaluated every time the function is called without the respective parameter. -In the example above, `anotherFunction()` isn't called at all, if the `text` parameter is provided. - -On the other hand, it's independently called every time when `text` is missing. +In the example above, `anotherFunction()` is called every time `showMessage()` is called without the `text` parameter. ``` -````smart header="Default parameters in old JavaScript code" -Several years ago, JavaScript didn't support the syntax for default parameters. So people used other ways to specify them. - -Nowadays, we can come across them in old scripts. - -For example, an explicit check for `undefined`: - -```js -function showMessage(from, text) { -*!* - if (text === undefined) { - text = 'no text given'; - } -*/!* - - alert( from + ": " + text ); -} -``` - -...Or using the `||` operator: - -```js -function showMessage(from, text) { - // If the value of text is falsy, assign the default value - // this assumes that text == "" is the same as no text at all - text = text || 'no text given'; - ... -} -``` -```` - - ### Alternative default parameters -Sometimes it makes sense to assign default values for parameters at a later stage after the function declaration. +Sometimes it makes sense to set default values for parameters not in the function declaration, but at a later stage, during its execution. -We can check if the parameter is passed during the function execution, by comparing it with `undefined`: +To check for an omitted parameter, we can compare it with `undefined`: ```js run function showMessage(text) { - // ... - *!* - if (text === undefined) { // if the parameter is missing + if (text === undefined) { text = 'empty message'; } */!* @@ -288,18 +237,18 @@ showMessage(); // empty message ...Or we could use the `||` operator: ```js +// if text parameter is omitted or "" is passed, set it to 'empty' function showMessage(text) { - // if text is undefined or otherwise falsy, set it to 'empty' text = text || 'empty'; ... } ``` -Modern JavaScript engines support the [nullish coalescing operator](info:nullish-coalescing-operator) `??`, it's better when most falsy values, such as `0`, should be considered "normal": +Modern JavaScript engines support the [nullish coalescing operator](info:nullish-coalescing-operator) `??`, it's better when falsy values, such as `0`, are considered regular: ```js run +// if there's no "count" parameter, show "unknown" function showCount(count) { - // if count is undefined or null, show "unknown" alert(count ?? "unknown"); } @@ -462,7 +411,7 @@ Functions that are used *very often* sometimes have ultrashort names. For example, the [jQuery](http://jquery.com) framework defines a function with `$`. The [Lodash](http://lodash.com/) library has its core function named `_`. -These are exceptions. Generally function names should be concise and descriptive. +These are exceptions. Generally functions names should be concise and descriptive. ``` ## Functions == Comments @@ -528,7 +477,7 @@ function name(parameters, delimited, by, comma) { To make the code clean and easy to understand, it's recommended to use mainly local variables and parameters in the function, not outer variables. -It is always easier to understand a function which gets parameters, works with them and returns a result than a function which gets no parameters, but modifies outer variables as a side effect. +It is always easier to understand a function which gets parameters, works with them and returns a result than a function which gets no parameters, but modifies outer variables as a side-effect. Function naming: diff --git a/1-js/02-first-steps/16-function-expressions/article.md b/1-js/02-first-steps/16-function-expressions/article.md index b952d594..a8ccd6c6 100644 --- a/1-js/02-first-steps/16-function-expressions/article.md +++ b/1-js/02-first-steps/16-function-expressions/article.md @@ -12,9 +12,7 @@ function sayHi() { There is another syntax for creating a function that is called a *Function Expression*. -It allows us to create a new function in the middle of any expression. - -For example: +It looks like this: ```js let sayHi = function() { @@ -22,19 +20,9 @@ let sayHi = function() { }; ``` -Here we can see a variable `sayHi` getting a value, the new function, created as `function() { alert("Hello"); }`. +Here, the function is created and assigned to the variable explicitly, like any other value. No matter how the function is defined, it's just a value stored in the variable `sayHi`. -As the function creation happens in the context of the assignment expression (to the right side of `=`), this is a *Function Expression*. - -Please note, there's no name after the `function` keyword. Omitting a name is allowed for Function Expressions. - -Here we immediately assign it to the variable, so the meaning of these code samples is the same: "create a function and put it into the variable `sayHi`". - -In more advanced situations, that we'll come across later, a function may be created and immediately called or scheduled for a later execution, not stored anywhere, thus remaining anonymous. - -## Function is a value - -Let's reiterate: no matter how the function is created, a function is a value. Both examples above store a function in the `sayHi` variable. +The meaning of these code samples is the same: "create a function and put it into the variable `sayHi`". We can even print out that value using `alert`: @@ -75,10 +63,10 @@ Here's what happens above in detail: 2. Line `(2)` copies it into the variable `func`. Please note again: there are no parentheses after `sayHi`. If there were, then `func = sayHi()` would write *the result of the call* `sayHi()` into `func`, not *the function* `sayHi` itself. 3. Now the function can be called as both `sayHi()` and `func()`. -We could also have used a Function Expression to declare `sayHi`, in the first line: +Note that we could also have used a Function Expression to declare `sayHi`, in the first line: ```js -let sayHi = function() { // (1) create +let sayHi = function() { alert( "Hello" ); }; @@ -90,7 +78,7 @@ Everything would work the same. ````smart header="Why is there a semicolon at the end?" -You might wonder, why do Function Expressions have a semicolon `;` at the end, but Function Declarations do not: +You might wonder, why does Function Expression have a semicolon `;` at the end, but Function Declaration does not: ```js function sayHi() { @@ -102,9 +90,9 @@ let sayHi = function() { }*!*;*/!* ``` -The answer is simple: a Function Expression is created here as `function(…) {…}` inside the assignment statement: `let sayHi = …;`. The semicolon `;` is recommended at the end of the statement, it's not a part of the function syntax. - -The semicolon would be there for a simpler assignment, such as `let sayHi = 5;`, and it's also there for a function assignment. +The answer is simple: +- There's no need for `;` at the end of code blocks and syntax structures that use them like `if { ... }`, `for { }`, `function f { }` etc. +- A Function Expression is used inside the statement: `let sayHi = ...;`, as a value. It's not a code block, but rather an assignment. The semicolon `;` is recommended at the end of statements, no matter what the value is. So the semicolon here is not related to the Function Expression itself, it just terminates the statement. ```` ## Callback functions @@ -144,13 +132,13 @@ function showCancel() { ask("Do you agree?", showOk, showCancel); ``` -In practice, such functions are quite useful. The major difference between a real-life `ask` and the example above is that real-life functions use more complex ways to interact with the user than a simple `confirm`. In the browser, such functions usually draw a nice-looking question window. But that's another story. +In practice, such functions are quite useful. The major difference between a real-life `ask` and the example above is that real-life functions use more complex ways to interact with the user than a simple `confirm`. In the browser, such function usually draws a nice-looking question window. But that's another story. **The arguments `showOk` and `showCancel` of `ask` are called *callback functions* or just *callbacks*.** The idea is that we pass a function and expect it to be "called back" later if necessary. In our case, `showOk` becomes the callback for "yes" answer, and `showCancel` for "no" answer. -We can use Function Expressions to write an equivalent, shorter function: +We can use Function Expressions to write the same function much shorter: ```js run no-beautify function ask(question, yes, no) { @@ -186,7 +174,7 @@ Let's formulate the key differences between Function Declarations and Expression First, the syntax: how to differentiate between them in the code. -- *Function Declaration:* a function, declared as a separate statement, in the main code flow: +- *Function Declaration:* a function, declared as a separate statement, in the main code flow. ```js // Function Declaration @@ -194,7 +182,7 @@ First, the syntax: how to differentiate between them in the code. return a + b; } ``` -- *Function Expression:* a function, created inside an expression or inside another syntax construct. Here, the function is created on the right side of the "assignment expression" `=`: +- *Function Expression:* a function, created inside an expression or inside another syntax construct. Here, the function is created at the right side of the "assignment expression" `=`: ```js // Function Expression @@ -291,7 +279,7 @@ if (age < 18) { welcome(); // \ (runs) */!* // | - function welcome() { // | + function welcome() { // | alert("Hello!"); // | Function Declaration is available } // | everywhere in the block where it's declared // | @@ -301,7 +289,7 @@ if (age < 18) { } else { - function welcome() { + function welcome() { alert("Greetings!"); } } @@ -360,7 +348,7 @@ welcome(); // ok now ```smart header="When to choose Function Declaration versus Function Expression?" -As a rule of thumb, when we need to declare a function, the first thing to consider is Function Declaration syntax. It gives more freedom in how to organize our code, because we can call such functions before they are declared. +As a rule of thumb, when we need to declare a function, the first to consider is Function Declaration syntax. It gives more freedom in how to organize our code, because we can call such functions before they are declared. That's also better for readability, as it's easier to look up `function f(…) {…}` in the code than `let f = function(…) {…};`. Function Declarations are more "eye-catching". diff --git a/1-js/02-first-steps/17-arrow-functions-basics/1-rewrite-arrow/solution.md b/1-js/02-first-steps/17-arrow-functions-basics/1-rewrite-arrow/solution.md index 041db18b..3ea11247 100644 --- a/1-js/02-first-steps/17-arrow-functions-basics/1-rewrite-arrow/solution.md +++ b/1-js/02-first-steps/17-arrow-functions-basics/1-rewrite-arrow/solution.md @@ -1,7 +1,7 @@ ```js run function ask(question, yes, no) { - if (confirm(question)) yes(); + if (confirm(question)) yes() else no(); } diff --git a/1-js/02-first-steps/17-arrow-functions-basics/1-rewrite-arrow/task.md b/1-js/02-first-steps/17-arrow-functions-basics/1-rewrite-arrow/task.md index e18c08a8..2f44db27 100644 --- a/1-js/02-first-steps/17-arrow-functions-basics/1-rewrite-arrow/task.md +++ b/1-js/02-first-steps/17-arrow-functions-basics/1-rewrite-arrow/task.md @@ -5,7 +5,7 @@ Replace Function Expressions with arrow functions in the code below: ```js run function ask(question, yes, no) { - if (confirm(question)) yes(); + if (confirm(question)) yes() else no(); } diff --git a/1-js/02-first-steps/17-arrow-functions-basics/article.md b/1-js/02-first-steps/17-arrow-functions-basics/article.md index 50c0d475..e0fb5bda 100644 --- a/1-js/02-first-steps/17-arrow-functions-basics/article.md +++ b/1-js/02-first-steps/17-arrow-functions-basics/article.md @@ -5,15 +5,15 @@ There's another very simple and concise syntax for creating functions, that's of It's called "arrow functions", because it looks like this: ```js -let func = (arg1, arg2, ..., argN) => expression; +let func = (arg1, arg2, ...argN) => expression ``` -This creates a function `func` that accepts arguments `arg1..argN`, then evaluates the `expression` on the right side with their use and returns its result. +...This creates a function `func` that accepts arguments `arg1..argN`, then evaluates the `expression` on the right side with their use and returns its result. In other words, it's the shorter version of: ```js -let func = function(arg1, arg2, ..., argN) { +let func = function(arg1, arg2, ...argN) { return expression; }; ``` @@ -33,7 +33,7 @@ let sum = function(a, b) { alert( sum(1, 2) ); // 3 ``` -As you can see, `(a, b) => a + b` means a function that accepts two arguments named `a` and `b`. Upon the execution, it evaluates the expression `a + b` and returns the result. +As you can, see `(a, b) => a + b` means a function that accepts two arguments named `a` and `b`. Upon the execution, it evaluates the expression `a + b` and returns the result. - If we have only one argument, then parentheses around parameters can be omitted, making that even shorter. @@ -48,7 +48,7 @@ As you can see, `(a, b) => a + b` means a function that accepts two arguments na alert( double(3) ); // 6 ``` -- If there are no arguments, parentheses are empty, but they must be present: +- If there are no arguments, parentheses will be empty (but they should be present): ```js run let sayHi = () => alert("Hello!"); @@ -64,7 +64,7 @@ For instance, to dynamically create a function: let age = prompt("What is your age?", 18); let welcome = (age < 18) ? - () => alert('Hello!') : + () => alert('Hello') : () => alert("Greetings!"); welcome(); @@ -76,9 +76,9 @@ They are very convenient for simple one-line actions, when we're just too lazy t ## Multiline arrow functions -The arrow functions that we've seen so far were very simple. They took arguments from the left of `=>`, evaluated and returned the right-side expression with them. +The examples above took arguments from the left of `=>` and evaluated the right-side expression with them. -Sometimes we need a more complex function, with multiple expressions and statements. In that case, we can enclose them in curly braces. The major difference is that curly braces require a `return` within them to return a value (just like a regular function does). +Sometimes we need something a little bit more complex, like multiple expressions or statements. It is also possible, but we should enclose them in curly braces. Then use a normal `return` within them. Like this: @@ -86,7 +86,7 @@ Like this: let sum = (a, b) => { // the curly brace opens a multiline function let result = a + b; *!* - return result; // if we use curly braces, then we need an explicit "return" + return result; // if we use curly braces, then we need an explicit "return" */!* }; @@ -105,7 +105,7 @@ For now, we can already use arrow functions for one-line actions and callbacks. ## Summary -Arrow functions are handy for simple actions, especially for one-liners. They come in two flavors: +Arrow functions are handy for one-liners. They come in two flavors: -1. Without curly braces: `(...args) => expression` -- the right side is an expression: the function evaluates it and returns the result. Parentheses can be omitted, if there's only a single argument, e.g. `n => n*2`. +1. Without curly braces: `(...args) => expression` -- the right side is an expression: the function evaluates it and returns the result. 2. With curly braces: `(...args) => { body }` -- brackets allow us to write multiple statements inside the function, but we need an explicit `return` to return something. diff --git a/1-js/02-first-steps/18-javascript-specials/article.md b/1-js/02-first-steps/18-javascript-specials/article.md index 016214e3..97d0457b 100644 --- a/1-js/02-first-steps/18-javascript-specials/article.md +++ b/1-js/02-first-steps/18-javascript-specials/article.md @@ -55,7 +55,7 @@ To fully enable all features of modern JavaScript, we should start scripts with The directive must be at the top of a script or at the beginning of a function body. -Without `"use strict"`, everything still works, but some features behave in the old-fashioned, "compatible" way. We'd generally prefer the modern behavior. +Without `"use strict"`, everything still works, but some features behave in the old-fashion, "compatible" way. We'd generally prefer the modern behavior. Some modern features of the language (like classes that we'll study in the future) enable strict mode implicitly. @@ -144,7 +144,7 @@ Assignments : There is a simple assignment: `a = b` and combined ones like `a *= 2`. Bitwise -: Bitwise operators work with 32-bit integers at the lowest, bit-level: see the [docs](mdn:/JavaScript/Guide/Expressions_and_Operators#bitwise_operators) when they are needed. +: Bitwise operators work with 32-bit integers at the lowest, bit-level: see the [docs](mdn:/JavaScript/Guide/Expressions_and_Operators#Bitwise) when they are needed. Conditional : The only operator with three parameters: `cond ? resultA : resultB`. If `cond` is truthy, returns `resultA`, otherwise `resultB`. @@ -256,7 +256,7 @@ We covered three ways to create a function in JavaScript: 3. Arrow functions: ```js - // expression on the right side + // expression at the right side let sum = (a, b) => a + b; // or multi-line syntax with { ... }, need return here: @@ -273,7 +273,7 @@ We covered three ways to create a function in JavaScript: ``` -- Functions may have local variables: those declared inside its body or its parameter list. Such variables are only visible inside the function. +- Functions may have local variables: those declared inside its body. Such variables are only visible inside the function. - Parameters can have default values: `function sum(a = 1, b = 2) {...}`. - Functions always return something. If there's no `return` statement, then the result is `undefined`. diff --git a/1-js/03-code-quality/01-debugging-chrome/article.md b/1-js/03-code-quality/01-debugging-chrome/article.md index 4f50fb42..ee7dea4c 100644 --- a/1-js/03-code-quality/01-debugging-chrome/article.md +++ b/1-js/03-code-quality/01-debugging-chrome/article.md @@ -1,4 +1,4 @@ -# Debugging in the browser +# Debugging in Chrome Before writing more complex code, let's talk about debugging. @@ -38,7 +38,7 @@ If we press `key:Esc`, then a console opens below. We can type commands there an After a statement is executed, its result is shown below. -For example, here `1+2` results in `3`, while the function call `hello("debugger")` returns nothing, so the result is `undefined`: +For example, here `1+2` results in `3`, and `hello("debugger")` returns nothing, so the result is `undefined`: ![](chrome-sources-console.svg) @@ -63,12 +63,12 @@ We can always find a list of breakpoints in the right panel. That's useful when - ...And so on. ```smart header="Conditional breakpoints" -*Right click* on the line number allows to create a *conditional* breakpoint. It only triggers when the given expression, that you should provide when you create it, is truthy. +*Right click* on the line number allows to create a *conditional* breakpoint. It only triggers when the given expression is truthy. That's handy when we need to stop only for a certain variable value or for certain function parameters. ``` -## The command "debugger" +## Debugger command We can also pause the code by using the `debugger` command in it, like this: @@ -84,7 +84,8 @@ function hello(name) { } ``` -Such command works only when the development tools are open, otherwise the browser ignores it. +That's very convenient when we are in a code editor and don't want to switch to the browser and look up the script in developer tools to set the breakpoint. + ## Pause and look around @@ -98,7 +99,7 @@ Please open the informational dropdowns to the right (labeled with arrows). They 1. **`Watch` -- shows current values for any expressions.** - You can click the plus `+` and input an expression. The debugger will show its value, automatically recalculating it in the process of execution. + You can click the plus `+` and input an expression. The debugger will show its value at any moment, automatically recalculating it in the process of execution. 2. **`Call Stack` -- shows the nested calls chain.** @@ -134,11 +135,11 @@ There are buttons for it at the top of the right panel. Let's engage them. Clicking this again and again will step through all script statements one by one. -- "Step over": run the next command, but *don't go into a function*, hotkey `key:F10`. -: Similar to the previous "Step" command, but behaves differently if the next statement is a function call (not a built-in, like `alert`, but a function of our own). +: Similar to the previous the "Step" command, but behaves differently if the next statement is a function call. That is: not a built-in, like `alert`, but a function of our own. - If we compare them, the "Step" command goes into a nested function call and pauses the execution at its first line, while "Step over" executes the nested function call invisibly to us, skipping the function internals. + The "Step" command goes into it and pauses the execution at its first line, while "Step over" executes the nested function call invisibly, skipping the function internals. - The execution is then paused immediately after that function call. + The execution is then paused immediately after that function. That's good if we're not interested to see what happens inside the function call. @@ -154,7 +155,7 @@ There are buttons for it at the top of the right panel. Let's engage them. : That button does not move the execution. Just a mass on/off for breakpoints. -- enable/disable automatic pause in case of an error. -: When enabled, if the developer tools is open, an error during the script execution automatically pauses it. Then we can analyze variables in the debugger to see what went wrong. So if our script dies with an error, we can open debugger, enable this option and reload the page to see where it dies and what's the context at that moment. +: When enabled, and the developer tools is open, a script error automatically pauses the execution. Then we can analyze variables to see what went wrong. So if our script dies with an error, we can open debugger, enable this option and reload the page to see where it dies and what's the context at that moment. ```smart header="Continue to here" Right click on a line of code opens the context menu with a great option called "Continue to here". @@ -186,7 +187,7 @@ As we can see, there are three main ways to pause a script: 2. The `debugger` statements. 3. An error (if dev tools are open and the button is "on"). -When paused, we can debug: examine variables and trace the code to see where the execution goes wrong. +When paused, we can debug - examine variables and trace the code to see where the execution goes wrong. There are many more options in developer tools than covered here. The full manual is at . diff --git a/1-js/03-code-quality/01-debugging-chrome/chrome-open-sources.svg b/1-js/03-code-quality/01-debugging-chrome/chrome-open-sources.svg index 5fc6dce3..1f7d2128 100644 --- a/1-js/03-code-quality/01-debugging-chrome/chrome-open-sources.svg +++ b/1-js/03-code-quality/01-debugging-chrome/chrome-open-sources.svg @@ -1 +1 @@ -open sources \ No newline at end of file +open sources \ No newline at end of file diff --git a/1-js/03-code-quality/01-debugging-chrome/chrome-sources-breakpoint.svg b/1-js/03-code-quality/01-debugging-chrome/chrome-sources-breakpoint.svg index 63bf4966..6fb4332f 100644 --- a/1-js/03-code-quality/01-debugging-chrome/chrome-sources-breakpoint.svg +++ b/1-js/03-code-quality/01-debugging-chrome/chrome-sources-breakpoint.svg @@ -1 +1 @@ -here's the listbreakpoints \ No newline at end of file +here's the listbreakpoints \ No newline at end of file diff --git a/1-js/03-code-quality/01-debugging-chrome/chrome-sources-console.svg b/1-js/03-code-quality/01-debugging-chrome/chrome-sources-console.svg index 3fe5f124..25284d05 100644 --- a/1-js/03-code-quality/01-debugging-chrome/chrome-sources-console.svg +++ b/1-js/03-code-quality/01-debugging-chrome/chrome-sources-console.svg @@ -1 +1 @@ - \ No newline at end of file + \ No newline at end of file diff --git a/1-js/03-code-quality/01-debugging-chrome/chrome-sources-debugger-pause.svg b/1-js/03-code-quality/01-debugging-chrome/chrome-sources-debugger-pause.svg index 0147c2e0..40d9515a 100644 --- a/1-js/03-code-quality/01-debugging-chrome/chrome-sources-debugger-pause.svg +++ b/1-js/03-code-quality/01-debugging-chrome/chrome-sources-debugger-pause.svg @@ -1 +1 @@ -213see the outer call detailswatch expressionscurrent variables \ No newline at end of file +213see the outer call detailswatch expressionscurrent variables \ No newline at end of file diff --git a/1-js/03-code-quality/01-debugging-chrome/chrome-sources-debugger-trace-1.svg b/1-js/03-code-quality/01-debugging-chrome/chrome-sources-debugger-trace-1.svg index 9fa1b3b8..0d5bde9c 100644 --- a/1-js/03-code-quality/01-debugging-chrome/chrome-sources-debugger-trace-1.svg +++ b/1-js/03-code-quality/01-debugging-chrome/chrome-sources-debugger-trace-1.svg @@ -1 +1 @@ -nested calls \ No newline at end of file +nested calls \ No newline at end of file diff --git a/1-js/03-code-quality/01-debugging-chrome/chrome-tabs.svg b/1-js/03-code-quality/01-debugging-chrome/chrome-tabs.svg index 01670825..352fbcb7 100644 --- a/1-js/03-code-quality/01-debugging-chrome/chrome-tabs.svg +++ b/1-js/03-code-quality/01-debugging-chrome/chrome-tabs.svg @@ -1 +1 @@ -213 \ No newline at end of file +213 \ No newline at end of file diff --git a/1-js/03-code-quality/02-coding-style/article.md b/1-js/03-code-quality/02-coding-style/article.md index 904f0a93..4cdb2380 100644 --- a/1-js/03-code-quality/02-coding-style/article.md +++ b/1-js/03-code-quality/02-coding-style/article.md @@ -116,7 +116,7 @@ There are two types of indents: One advantage of spaces over tabs is that spaces allow more flexible configurations of indents than the tab symbol. - For instance, we can align the parameters with the opening bracket, like this: + For instance, we can align the arguments with the opening bracket, like this: ```js no-beautify show(parameters, @@ -301,11 +301,11 @@ The great thing about them is that style-checking can also find some bugs, like Here are some well-known linting tools: -- [JSLint](https://www.jslint.com/) -- one of the first linters. -- [JSHint](https://jshint.com/) -- more settings than JSLint. -- [ESLint](https://eslint.org/) -- probably the newest one. +- [JSLint](http://www.jslint.com/) -- one of the first linters. +- [JSHint](http://www.jshint.com/) -- more settings than JSLint. +- [ESLint](http://eslint.org/) -- probably the newest one. -All of them can do the job. The author uses [ESLint](https://eslint.org/). +All of them can do the job. The author uses [ESLint](http://eslint.org/). Most linters are integrated with many popular editors: just enable the plugin in the editor and configure the style. @@ -328,14 +328,14 @@ Here's an example of an `.eslintrc` file: }, "rules": { "no-console": 0, - "indent": 2 + "indent": ["warning", 2] } } ``` Here the directive `"extends"` denotes that the configuration is based on the "eslint:recommended" set of settings. After that, we specify our own. -It is also possible to download style rule sets from the web and extend them instead. See for more details about installation. +It is also possible to download style rule sets from the web and extend them instead. See for more details about installation. Also certain IDEs have built-in linting, which is convenient but not as customizable as ESLint. diff --git a/1-js/03-code-quality/02-coding-style/code-style.svg b/1-js/03-code-quality/02-coding-style/code-style.svg index 739d9f1e..bd62691c 100644 --- a/1-js/03-code-quality/02-coding-style/code-style.svg +++ b/1-js/03-code-quality/02-coding-style/code-style.svg @@ -1 +1 @@ -2No space between the function name and parentheses between the parentheses and the parameterIndentation 2 spacesA space after for/if/while…} else { without a line breakSpaces around a nested callAn empty line between logical blocksLines are not very longA semicolon ; is mandatorySpaces around operatorsCurly brace { on the same line, after a spaceA space between argumentsA space between parameters \ No newline at end of file +2No space between the function name and parentheses between the parentheses and the parameterIndentation 2 spacesA space after for/if/while…} else { without a line breakSpaces around a nested callAn empty line between logical blocksLines are not very longA semicolon ; is mandatorySpaces around operatorsCurly brace { on the same line, after a spaceA space between parametersA space between parameters \ No newline at end of file diff --git a/1-js/03-code-quality/03-comments/article.md b/1-js/03-code-quality/03-comments/article.md index af3a06c8..0d11c6c5 100644 --- a/1-js/03-code-quality/03-comments/article.md +++ b/1-js/03-code-quality/03-comments/article.md @@ -143,7 +143,7 @@ Such comments allow us to understand the purpose of the function and use it the By the way, many editors like [WebStorm](https://www.jetbrains.com/webstorm/) can understand them as well and use them to provide autocomplete and some automatic code-checking. -Also, there are tools like [JSDoc 3](https://github.com/jsdoc/jsdoc) that can generate HTML-documentation from the comments. You can read more information about JSDoc at . +Also, there are tools like [JSDoc 3](https://github.com/jsdoc3/jsdoc) that can generate HTML-documentation from the comments. You can read more information about JSDoc at . Why is the task solved this way? : What's written is important. But what's *not* written may be even more important to understand what's going on. Why is the task solved exactly this way? The code gives no answer. diff --git a/1-js/03-code-quality/05-testing-mocha/article.md b/1-js/03-code-quality/05-testing-mocha/article.md index 55826337..68ffcae4 100644 --- a/1-js/03-code-quality/05-testing-mocha/article.md +++ b/1-js/03-code-quality/05-testing-mocha/article.md @@ -2,7 +2,7 @@ Automated testing will be used in further tasks, and it's also widely used in real projects. -## Why do we need tests? +## Why we need tests? When we write a function, we can usually imagine what it should do: which parameters give which results. @@ -51,7 +51,7 @@ describe("pow", function() { A spec has three main building blocks that you can see above: `describe("title", function() { ... })` -: What functionality we're describing? In our case we're describing the function `pow`. Used to group "workers" -- the `it` blocks. +: What functionality we're describing. In our case we're describing the function `pow`. Used to group "workers" -- the `it` blocks. `it("use case description", function() { ... })` : In the title of `it` we *in a human-readable way* describe the particular use case, and the second argument is a function that tests it. @@ -79,7 +79,7 @@ So, the development is *iterative*. We write the spec, implement it, make sure t Let's see this development flow in our practical case. -The first step is already complete: we have an initial spec for `pow`. Now, before making the implementation, let's use a few JavaScript libraries to run the tests, just to see that they are working (they will all fail). +The first step is already complete: we have an initial spec for `pow`. Now, before making the implementation, let's use few JavaScript libraries to run the tests, just to see that they are working (they will all fail). ## The spec in action diff --git a/1-js/03-code-quality/06-polyfills/article.md b/1-js/03-code-quality/06-polyfills/article.md index 83a12fb1..75db49d2 100644 --- a/1-js/03-code-quality/06-polyfills/article.md +++ b/1-js/03-code-quality/06-polyfills/article.md @@ -1,92 +1,54 @@ -# Polyfills and transpilers +# Polyfills The JavaScript language steadily evolves. New proposals to the language appear regularly, they are analyzed and, if considered worthy, are appended to the list at and then progress to the [specification](http://www.ecma-international.org/publications/standards/Ecma-262.htm). Teams behind JavaScript engines have their own ideas about what to implement first. They may decide to implement proposals that are in draft and postpone things that are already in the spec, because they are less interesting or just harder to do. -So it's quite common for an engine to implement only part of the standard. +So it's quite common for an engine to implement only the part of the standard. A good page to see the current state of support for language features is (it's big, we have a lot to study yet). -As programmers, we'd like to use most recent features. The more good stuff - the better! +## Babel -On the other hand, how to make our modern code work on older engines that don't understand recent features yet? +When we use modern features of the language, some engines may fail to support such code. Just as said, not all features are implemented everywhere. -There are two tools for that: +Here Babel comes to the rescue. -1. Transpilers. -2. Polyfills. +[Babel](https://babeljs.io) is a [transpiler](https://en.wikipedia.org/wiki/Source-to-source_compiler). It rewrites modern JavaScript code into the previous standard. -Here, in this chapter, our purpose is to get the gist of how they work, and their place in web development. +Actually, there are two parts in Babel: -## Transpilers +1. First, the transpiler program, which rewrites the code. The developer runs it on their own computer. It rewrites the code into the older standard. And then the code is delivered to the website for users. Modern project build systems like [webpack](http://webpack.github.io/) provide means to run transpiler automatically on every code change, so that it's very easy to integrate into development process. -A [transpiler](https://en.wikipedia.org/wiki/Source-to-source_compiler) is a special piece of software that translates source code to another source code. It can parse ("read and understand") modern code and rewrite it using older syntax constructs, so that it'll also work in outdated engines. +2. Second, the polyfill. -E.g. JavaScript before year 2020 didn't have the "nullish coalescing operator" `??`. So, if a visitor uses an outdated browser, it may fail to understand the code like `height = height ?? 100`. + New language features may include new built-in functions and syntax constructs. + The transpiler rewrites the code, transforming syntax constructs into older ones. But as for new built-in functions, we need to implement them. JavaScript is a highly dynamic language, scripts may add/modify any functions, so that they behave according to the modern standard. -A transpiler would analyze our code and rewrite `height ?? 100` into `(height !== undefined && height !== null) ? height : 100`. + A script that updates/adds new functions is called "polyfill". It "fills in" the gap and adds missing implementations. -```js -// before running the transpiler -height = height ?? 100; + Two interesting polyfills are: + - [core js](https://github.com/zloirock/core-js) that supports a lot, allows to include only needed features. + - [polyfill.io](http://polyfill.io) service that provides a script with polyfills, depending on the features and user's browser. -// after running the transpiler -height = (height !== undefined && height !== null) ? height : 100; +So, if we're going to use modern language features, a transpiler and a polyfill are necessary. + +## Examples in the tutorial + + +````online +Most examples are runnable at-place, like this: + +```js run +alert('Press the "Play" button in the upper-right corner to run'); ``` -Now the rewritten code is suitable for older JavaScript engines. +Examples that use modern JS will work only if your browser supports it. +```` -Usually, a developer runs the transpiler on their own computer, and then deploys the transpiled code to the server. - -Speaking of names, [Babel](https://babeljs.io) is one of the most prominent transpilers out there. - -Modern project build systems, such as [webpack](https://webpack.js.org/), provide a means to run a transpiler automatically on every code change, so it's very easy to integrate into the development process. - -## Polyfills - -New language features may include not only syntax constructs and operators, but also built-in functions. - -For example, `Math.trunc(n)` is a function that "cuts off" the decimal part of a number, e.g `Math.trunc(1.23)` returns `1`. - -In some (very outdated) JavaScript engines, there's no `Math.trunc`, so such code will fail. - -As we're talking about new functions, not syntax changes, there's no need to transpile anything here. We just need to declare the missing function. - -A script that updates/adds new functions is called "polyfill". It "fills in" the gap and adds missing implementations. - -For this particular case, the polyfill for `Math.trunc` is a script that implements it, like this: - -```js -if (!Math.trunc) { // if no such function - // implement it - Math.trunc = function(number) { - // Math.ceil and Math.floor exist even in ancient JavaScript engines - // they are covered later in the tutorial - return number < 0 ? Math.ceil(number) : Math.floor(number); - }; -} +```offline +As you're reading the offline version, in PDF examples are not runnable. In EPUB some of them can run. ``` -JavaScript is a highly dynamic language. Scripts may add/modify any function, even built-in ones. - -Two interesting polyfill libraries are: -- [core js](https://github.com/zloirock/core-js) that supports a lot, allows to include only needed features. -- [polyfill.io](http://polyfill.io) service that provides a script with polyfills, depending on the features and user's browser. - - -## Summary - -In this chapter we'd like to motivate you to study modern and even "bleeding-edge" language features, even if they aren't yet well-supported by JavaScript engines. - -Just don't forget to use a transpiler (if using modern syntax or operators) and polyfills (to add functions that may be missing). They'll ensure that the code works. - -For example, later when you're familiar with JavaScript, you can setup a code build system based on [webpack](https://webpack.js.org/) with the [babel-loader](https://github.com/babel/babel-loader) plugin. - -Good resources that show the current state of support for various features: -- - for pure JavaScript. -- - for browser-related functions. - -P.S. Google Chrome is usually the most up-to-date with language features, try it if a tutorial demo fails. Most tutorial demos work with any modern browser though. - +Google Chrome is usually the most up-to-date with language features, good to run bleeding-edge demos without any transpilers, but other modern browsers also work fine. diff --git a/1-js/04-object-basics/01-object/article.md b/1-js/04-object-basics/01-object/article.md index 0fe5979f..513f2f3e 100644 --- a/1-js/04-object-basics/01-object/article.md +++ b/1-js/04-object-basics/01-object/article.md @@ -44,7 +44,7 @@ The resulting `user` object can be imagined as a cabinet with two signed files l ![user object](object-user.svg) -We can add, remove and read files from it at any time. +We can add, remove and read files from it any time. Property values are accessible using the dot notation: @@ -62,7 +62,7 @@ user.isAdmin = true; ![user object 2](object-user-isadmin.svg) -To remove a property, we can use the `delete` operator: +To remove a property, we can use `delete` operator: ```js delete user.age; @@ -92,6 +92,30 @@ let user = { ``` That is called a "trailing" or "hanging" comma. Makes it easier to add/remove/move around properties, because all lines become alike. +````smart header="Object with const can be changed" +Please note: an object declared as `const` *can* be modified. + +For instance: + +```js run +const user = { + name: "John" +}; + +*!* +user.name = "Pete"; // (*) +*/!* + +alert(user.name); // Pete +``` + +It might seem that the line `(*)` would cause an error, but no. The `const` fixes the value of `user`, but not its contents. + +The `const` would give an error only if we try to set `user=...` as a whole. + +There's another way to make constant object properties, we'll cover it later in the chapter . +```` + ## Square brackets For multiword properties, the dot access doesn't work: @@ -201,13 +225,13 @@ let bag = { }; ``` -Square brackets are much more powerful than dot notation. They allow any property names and variables. But they are also more cumbersome to write. +Square brackets are much more powerful than the dot notation. They allow any property names and variables. But they are also more cumbersome to write. So most of the time, when property names are known and simple, the dot is used. And if we need something more complex, then we switch to square brackets. ## Property value shorthand -In real code, we often use existing variables as values for property names. +In real code we often use existing variables as values for property names. For instance: @@ -252,7 +276,7 @@ let user = { ## Property names limitations -As we already know, a variable cannot have a name equal to one of the language-reserved words like "for", "let", "return" etc. +As we already know, a variable cannot have a name equal to one of language-reserved words like "for", "let", "return" etc. But for an object property, there's no such restriction: @@ -325,7 +349,7 @@ alert( "blabla" in user ); // false, user.blabla doesn't exist Please note that on the left side of `in` there must be a *property name*. That's usually a quoted string. -If we omit quotes, that means a variable should contain the actual name to be tested. For instance: +If we omit quotes, that means a variable, it should contain the actual name to be tested. For instance: ```js run let user = { age: 30 }; @@ -355,7 +379,7 @@ In the code above, the property `obj.test` technically exists. So the `in` opera Situations like this happen very rarely, because `undefined` should not be explicitly assigned. We mostly use `null` for "unknown" or "empty" values. So the `in` operator is an exotic guest in the code. -## The "for..in" loop [#forin] +## The "for..in" loop To walk over all keys of an object, there exists a special form of the loop: `for..in`. This is a completely different thing from the `for(;;)` construct that we studied before. @@ -412,7 +436,7 @@ for (let code in codes) { */!* ``` -The object may be used to suggest a list of options to the user. If we're making a site mainly for a German audience then we probably want `49` to be the first. +The object may be used to suggest a list of options to the user. If we're making a site mainly for German audience then we probably want `49` to be the first. But if we run the code, we see a totally different picture: @@ -424,10 +448,9 @@ The phone codes go in the ascending sorted order, because they are integers. So ````smart header="Integer properties? What's that?" The "integer property" term here means a string that can be converted to-and-from an integer without a change. -So, `"49"` is an integer property name, because when it's transformed to an integer number and back, it's still the same. But `"+49"` and `"1.2"` are not: +So, "49" is an integer property name, because when it's transformed to an integer number and back, it's still the same. But "+49" and "1.2" are not: ```js run -// Number(...) explicitly converts to a number // Math.trunc is a built-in function that removes the decimal part alert( String(Math.trunc(Number("49"))) ); // "49", same, integer property alert( String(Math.trunc(Number("+49"))) ); // "49", not same "+49" ⇒ not integer property @@ -482,7 +505,7 @@ They store properties (key-value pairs), where: To access a property, we can use: - The dot notation: `obj.property`. -- Square brackets notation `obj["property"]`. Square brackets allow taking the key from a variable, like `obj[varWithKey]`. +- Square brackets notation `obj["property"]`. Square brackets allow to take the key from a variable, like `obj[varWithKey]`. Additional operators: - To delete a property: `delete obj.prop`. diff --git a/1-js/04-object-basics/01-object/object-user-delete.svg b/1-js/04-object-basics/01-object/object-user-delete.svg index c5af7e7a..4bbf324b 100644 --- a/1-js/04-object-basics/01-object/object-user-delete.svg +++ b/1-js/04-object-basics/01-object/object-user-delete.svg @@ -1 +1 @@ -nameisAdminuser \ No newline at end of file +nameisAdminuser \ No newline at end of file diff --git a/1-js/04-object-basics/01-object/object-user-empty.svg b/1-js/04-object-basics/01-object/object-user-empty.svg index 99edb026..5359c45c 100644 --- a/1-js/04-object-basics/01-object/object-user-empty.svg +++ b/1-js/04-object-basics/01-object/object-user-empty.svg @@ -1 +1 @@ -emptyuser \ No newline at end of file +emptyuser \ No newline at end of file diff --git a/1-js/04-object-basics/01-object/object-user-isadmin.svg b/1-js/04-object-basics/01-object/object-user-isadmin.svg index e2cc0eaf..f4e7b09a 100644 --- a/1-js/04-object-basics/01-object/object-user-isadmin.svg +++ b/1-js/04-object-basics/01-object/object-user-isadmin.svg @@ -1 +1 @@ -nameageisAdminuser \ No newline at end of file +nameageisAdminuser \ No newline at end of file diff --git a/1-js/04-object-basics/01-object/object-user-props.svg b/1-js/04-object-basics/01-object/object-user-props.svg index b3d5c9b7..92958cfb 100644 --- a/1-js/04-object-basics/01-object/object-user-props.svg +++ b/1-js/04-object-basics/01-object/object-user-props.svg @@ -1 +1 @@ -nameagelikes birdsuser \ No newline at end of file +nameagelikes birdsuser \ No newline at end of file diff --git a/1-js/04-object-basics/01-object/object-user.svg b/1-js/04-object-basics/01-object/object-user.svg index f499fbc1..f91e4814 100644 --- a/1-js/04-object-basics/01-object/object-user.svg +++ b/1-js/04-object-basics/01-object/object-user.svg @@ -1 +1 @@ -nameageuser \ No newline at end of file +nameageuser \ No newline at end of file diff --git a/1-js/04-object-basics/01-object/object.svg b/1-js/04-object-basics/01-object/object.svg index 47431a6e..28582b94 100644 --- a/1-js/04-object-basics/01-object/object.svg +++ b/1-js/04-object-basics/01-object/object.svg @@ -1 +1 @@ -key1key2key3 \ No newline at end of file +key1key2key3 \ No newline at end of file diff --git a/1-js/04-object-basics/02-object-copy/article.md b/1-js/04-object-basics/02-object-copy/article.md index e80f748a..d40eba2f 100644 --- a/1-js/04-object-basics/02-object-copy/article.md +++ b/1-js/04-object-basics/02-object-copy/article.md @@ -1,29 +1,25 @@ -# Object references and copying +# Object copying, references -One of the fundamental differences of objects versus primitives is that objects are stored and copied "by reference", whereas primitive values: strings, numbers, booleans, etc -- are always copied "as a whole value". +One of the fundamental differences of objects vs primitives is that they are stored and copied "by reference". -That's easy to understand if we look a bit under the hood of what happens when we copy a value. +Primitive values: strings, numbers, booleans -- are assigned/copied "as a whole value". -Let's start with a primitive, such as a string. - -Here we put a copy of `message` into `phrase`: +For instance: ```js let message = "Hello!"; let phrase = message; ``` -As a result we have two independent variables, each one storing the string `"Hello!"`. +As a result we have two independent variables, each one is storing the string `"Hello!"`. ![](variable-copy-value.svg) -Quite an obvious result, right? - Objects are not like that. -**A variable assigned to an object stores not the object itself, but its "address in memory" -- in other words "a reference" to it.** +**A variable stores not the object itself, but its "address in memory", in other words "a reference" to it.** -Let's look at an example of such a variable: +Here's the picture for the object: ```js let user = { @@ -31,19 +27,11 @@ let user = { }; ``` -And here's how it's actually stored in memory: - ![](variable-contains-reference.svg) -The object is stored somewhere in memory (at the right of the picture), while the `user` variable (at the left) has a "reference" to it. +Here, the object is stored somewhere in memory. And the variable `user` has a "reference" to it. -We may think of an object variable, such as `user`, like a sheet of paper with the address of the object on it. - -When we perform actions with the object, e.g. take a property `user.name`, the JavaScript engine looks at what's at that address and performs the operation on the actual object. - -Now here's why it's important. - -**When an object variable is copied, the reference is copied, but the object itself is not duplicated.** +**When an object variable is copied -- the reference is copied, the object is not duplicated.** For instance: @@ -53,13 +41,11 @@ let user = { name: "John" }; let admin = user; // copy the reference ``` -Now we have two variables, each storing a reference to the same object: +Now we have two variables, each one with the reference to the same object: ![](variable-copy-reference.svg) -As you can see, there's still one object, but now with two variables that reference it. - -We can use either variable to access the object and modify its contents: +We can use any variable to access the object and modify its contents: ```js run let user = { name: 'John' }; @@ -73,13 +59,15 @@ admin.name = 'Pete'; // changed by the "admin" reference alert(*!*user.name*/!*); // 'Pete', changes are seen from the "user" reference ``` -It's as if we had a cabinet with two keys and used one of them (`admin`) to get into it and make changes. Then, if we later use another key (`user`), we are still opening the same cabinet and can access the changed contents. +The example above demonstrates that there is only one object. As if we had a cabinet with two keys and used one of them (`admin`) to get into it. Then, if we later use another key (`user`) we can see changes. ## Comparison by reference -Two objects are equal only if they are the same object. +The equality `==` and strict equality `===` operators for objects work exactly the same. -For instance, here `a` and `b` reference the same object, thus they are equal: +**Two objects are equal only if they are the same object.** + +Here two variables reference the same object, thus they are equal: ```js run let a = {}; @@ -89,7 +77,7 @@ alert( a == b ); // true, both variables reference the same object alert( a === b ); // true ``` -And here two independent objects are not equal, even though they look alike (both are empty): +And here two independent objects are not equal, even though both are empty: ```js run let a = {}; @@ -98,39 +86,17 @@ let b = {}; // two independent objects alert( a == b ); // false ``` -For comparisons like `obj1 > obj2` or for a comparison against a primitive `obj == 5`, objects are converted to primitives. We'll study how object conversions work very soon, but to tell the truth, such comparisons are needed very rarely -- usually they appear as a result of a programming mistake. +For comparisons like `obj1 > obj2` or for a comparison against a primitive `obj == 5`, objects are converted to primitives. We'll study how object conversions work very soon, but to tell the truth, such comparisons occur very rarely, usually as a result of a coding mistake. -````smart header="Const objects can be modified" -An important side effect of storing objects as references is that an object declared as `const` *can* be modified. - -For instance: - -```js run -const user = { - name: "John" -}; - -*!* -user.name = "Pete"; // (*) -*/!* - -alert(user.name); // Pete -``` - -It might seem that the line `(*)` would cause an error, but it does not. The value of `user` is constant, it must always reference the same object, but properties of that object are free to change. - -In other words, the `const user` gives an error only if we try to set `user=...` as a whole. - -That said, if we really need to make constant object properties, it's also possible, but using totally different methods. We'll mention that in the chapter . -```` - -## Cloning and merging, Object.assign [#cloning-and-merging-object-assign] +## Cloning and merging, Object.assign So, copying an object variable creates one more reference to the same object. -But what if we need to duplicate an object? +But what if we need to duplicate an object? Create an independent copy, a clone? -We can create a new object and replicate the structure of the existing one, by iterating over its properties and copying them on the primitive level. +That's also doable, but a little bit more difficult, because there's no built-in method for that in JavaScript. Actually, that's rarely needed. Copying by reference is good most of the time. + +But if we really want that, then we need to create a new object and replicate the structure of the existing one by iterating over its properties and copying them on the primitive level. Like this: @@ -155,22 +121,21 @@ clone.name = "Pete"; // changed the data in it alert( user.name ); // still John in the original object ``` -We can also use the method [Object.assign](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/assign). +Also we can use the method [Object.assign](mdn:js/Object/assign) for that. The syntax is: ```js -Object.assign(dest, ...sources) +Object.assign(dest, [src1, src2, src3...]) ``` - The first argument `dest` is a target object. -- Further arguments is a list of source objects. +- Further arguments `src1, ..., srcN` (can be as many as needed) are source objects. +- It copies the properties of all source objects `src1, ..., srcN` into the target `dest`. In other words, properties of all arguments starting from the second are copied into the first object. +- The call returns `dest`. -It copies the properties of all source objects into the target `dest`, and then returns it as the result. - -For example, we have `user` object, let's add a couple of permissions to it: - -```js run +For instance, we can use it to merge several objects into one: +```js let user = { name: "John" }; let permissions1 = { canView: true }; @@ -182,9 +147,6 @@ Object.assign(user, permissions1, permissions2); */!* // now user = { name: "John", canView: true, canEdit: true } -alert(user.name); // John -alert(user.canView); // true -alert(user.canEdit); // true ``` If the copied property name already exists, it gets overwritten: @@ -197,9 +159,9 @@ Object.assign(user, { name: "Pete" }); alert(user.name); // now user = { name: "Pete" } ``` -We also can use `Object.assign` to perform a simple object cloning: +We also can use `Object.assign` to replace `for..in` loop for simple cloning: -```js run +```js let user = { name: "John", age: 30 @@ -208,18 +170,13 @@ let user = { *!* let clone = Object.assign({}, user); */!* - -alert(clone.name); // John -alert(clone.age); // 30 ``` -Here it copies all properties of `user` into the empty object and returns it. - -There are also other methods of cloning an object, e.g. using the [spread syntax](info:rest-parameters-spread) `clone = {...user}`, covered later in the tutorial. +It copies all properties of `user` into the empty object and returns it. ## Nested cloning -Until now we assumed that all properties of `user` are primitive. But properties can be references to other objects. +Until now we assumed that all properties of `user` are primitive. But properties can be references to other objects. What to do with them? Like this: ```js run @@ -234,7 +191,9 @@ let user = { alert( user.sizes.height ); // 182 ``` -Now it's not enough to copy `clone.sizes = user.sizes`, because `user.sizes` is an object, and will be copied by reference, so `clone` and `user` will share the same sizes: +Now it's not enough to copy `clone.sizes = user.sizes`, because the `user.sizes` is an object, it will be copied by reference. So `clone` and `user` will share the same sizes: + +Like this: ```js run let user = { @@ -250,76 +209,18 @@ let clone = Object.assign({}, user); alert( user.sizes === clone.sizes ); // true, same object // user and clone share sizes -user.sizes.width = 60; // change a property from one place -alert(clone.sizes.width); // 60, get the result from the other one +user.sizes.width++; // change a property from one place +alert(clone.sizes.width); // 51, see the result from the other one ``` -To fix that and make `user` and `clone` truly separate objects, we should use a cloning loop that examines each value of `user[key]` and, if it's an object, then replicate its structure as well. That is called a "deep cloning" or "structured cloning". There's [structuredClone](https://developer.mozilla.org/en-US/docs/Web/API/structuredClone) method that implements deep cloning. +To fix that, we should use the cloning loop that examines each value of `user[key]` and, if it's an object, then replicate its structure as well. That is called a "deep cloning". - -### structuredClone - -The call `structuredClone(object)` clones the `object` with all nested properties. - -Here's how we can use it in our example: - -```js run -let user = { - name: "John", - sizes: { - height: 182, - width: 50 - } -}; - -*!* -let clone = structuredClone(user); -*/!* - -alert( user.sizes === clone.sizes ); // false, different objects - -// user and clone are totally unrelated now -user.sizes.width = 60; // change a property from one place -alert(clone.sizes.width); // 50, not related -``` - -The `structuredClone` method can clone most data types, such as objects, arrays, primitive values. - -It also supports circular references, when an object property references the object itself (directly or via a chain or references). - -For instance: - -```js run -let user = {}; -// let's create a circular reference: -// user.me references the user itself -user.me = user; - -let clone = structuredClone(user); -alert(clone.me === clone); // true -``` - -As you can see, `clone.me` references the `clone`, not the `user`! So the circular reference was cloned correctly as well. - -Although, there are cases when `structuredClone` fails. - -For instance, when an object has a function property: - -```js run -// error -structuredClone({ - f: function() {} -}); -``` - -Function properties aren't supported. - -To handle such complex cases we may need to use a combination of cloning methods, write custom code or, to not reinvent the wheel, take an existing implementation, for instance [_.cloneDeep(obj)](https://lodash.com/docs#cloneDeep) from the JavaScript library [lodash](https://lodash.com). +We can use recursion to implement it. Or, not to reinvent the wheel, take an existing implementation, for instance [_.cloneDeep(obj)](https://lodash.com/docs#cloneDeep) from the JavaScript library [lodash](https://lodash.com). ## Summary -Objects are assigned and copied by reference. In other words, a variable stores not the "object value", but a "reference" (address in memory) for the value. So copying such a variable or passing it as a function argument copies that reference, not the object itself. +Objects are assigned and copied by reference. In other words, a variable stores not the "object value", but a "reference" (address in memory) for the value. So copying such a variable or passing it as a function argument copies that reference, not the object. All operations via copied references (like adding/removing properties) are performed on the same single object. -To make a "real copy" (a clone) we can use `Object.assign` for the so-called "shallow copy" (nested objects are copied by reference) or a "deep cloning" function `structuredClone` or use a custom cloning implementation, such as [_.cloneDeep(obj)](https://lodash.com/docs#cloneDeep). +To make a "real copy" (a clone) we can use `Object.assign` for the so-called "shallow copy" (nested objects are copied by reference) or a "deep cloning" function, such as [_.cloneDeep(obj)](https://lodash.com/docs#cloneDeep). diff --git a/1-js/04-object-basics/02-object-copy/variable-contains-reference.svg b/1-js/04-object-basics/02-object-copy/variable-contains-reference.svg index 267f0457..a59c9210 100644 --- a/1-js/04-object-basics/02-object-copy/variable-contains-reference.svg +++ b/1-js/04-object-basics/02-object-copy/variable-contains-reference.svg @@ -1 +1 @@ -username \ No newline at end of file +username \ No newline at end of file diff --git a/1-js/04-object-basics/02-object-copy/variable-copy-reference.svg b/1-js/04-object-basics/02-object-copy/variable-copy-reference.svg index a847fb20..5d0bc159 100644 --- a/1-js/04-object-basics/02-object-copy/variable-copy-reference.svg +++ b/1-js/04-object-basics/02-object-copy/variable-copy-reference.svg @@ -1 +1 @@ -useradminname \ No newline at end of file +useradminname \ No newline at end of file diff --git a/1-js/04-object-basics/02-object-copy/variable-copy-value.svg b/1-js/04-object-basics/02-object-copy/variable-copy-value.svg index 0d6ca67b..e09f521f 100644 --- a/1-js/04-object-basics/02-object-copy/variable-copy-value.svg +++ b/1-js/04-object-basics/02-object-copy/variable-copy-value.svg @@ -1 +1 @@ -"Hello!"message"Hello!"phrase \ No newline at end of file +"Hello!"message"Hello!"phrase \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/article.md b/1-js/04-object-basics/03-garbage-collection/article.md index 50ada671..e20e5a5d 100644 --- a/1-js/04-object-basics/03-garbage-collection/article.md +++ b/1-js/04-object-basics/03-garbage-collection/article.md @@ -14,8 +14,8 @@ Simply put, "reachable" values are those that are accessible or usable somehow. For instance: - - The currently executing function, its local variables and parameters. - - Other functions on the current chain of nested calls, their local variables and parameters. + - Local variables and parameters of the current function. + - Variables and parameters for other functions on the current chain of nested calls. - Global variables. - (there are some other, internal ones as well) @@ -23,7 +23,7 @@ Simply put, "reachable" values are those that are accessible or usable somehow. 2. Any other value is considered reachable if it's reachable from a root by a reference or by a chain of references. - For instance, if there's an object in a global variable, and that object has a property referencing another object, *that* object is considered reachable. And those that it references are also reachable. Detailed examples to follow. + For instance, if there's an object in a global variable, and that object has a property referencing another object, that object is considered reachable. And those that it references are also reachable. Detailed examples to follow. There's a background process in the JavaScript engine that is called [garbage collector](https://en.wikipedia.org/wiki/Garbage_collection_(computer_science)). It monitors all objects and removes those that have become unreachable. @@ -74,7 +74,7 @@ Now if we do the same: user = null; ``` -...Then the object is still reachable via `admin` global variable, so it must stay in memory. If we overwrite `admin` too, then it can be removed. +...Then the object is still reachable via `admin` global variable, so it's in memory. If we overwrite `admin` too, then it can be removed. ## Interlinked objects @@ -169,11 +169,11 @@ The first step marks the roots: ![](garbage-collection-2.svg) -Then we follow their references and mark referenced objects: +Then their references are marked: ![](garbage-collection-3.svg) -...And continue to follow further references, while possible: +...And their references, while possible: ![](garbage-collection-4.svg) @@ -183,12 +183,12 @@ Now the objects that could not be visited in the process are considered unreacha We can also imagine the process as spilling a huge bucket of paint from the roots, that flows through all references and marks all reachable objects. The unmarked ones are then removed. -That's the concept of how garbage collection works. JavaScript engines apply many optimizations to make it run faster and not introduce any delays into the code execution. +That's the concept of how garbage collection works. JavaScript engines apply many optimizations to make it run faster and not affect the execution. Some of the optimizations: -- **Generational collection** -- objects are split into two sets: "new ones" and "old ones". In typical code, many objects have a short life span: they appear, do their job and die fast, so it makes sense to track new objects and clear the memory from them if that's the case. Those that survive for long enough, become "old" and are examined less often. -- **Incremental collection** -- if there are many objects, and we try to walk and mark the whole object set at once, it may take some time and introduce visible delays in the execution. So the engine splits the whole set of existing objects into multiple parts. And then clear these parts one after another. There are many small garbage collections instead of a total one. That requires some extra bookkeeping between them to track changes, but we get many tiny delays instead of a big one. +- **Generational collection** -- objects are split into two sets: "new ones" and "old ones". Many objects appear, do their job and die fast, they can be cleaned up aggressively. Those that survive for long enough, become "old" and are examined less often. +- **Incremental collection** -- if there are many objects, and we try to walk and mark the whole object set at once, it may take some time and introduce visible delays in the execution. So the engine tries to split the garbage collection into pieces. Then the pieces are executed one by one, separately. That requires some extra bookkeeping between them to track changes, but we have many tiny delays instead of a big one. - **Idle-time collection** -- the garbage collector tries to run only while the CPU is idle, to reduce the possible effect on the execution. There exist other optimizations and flavours of garbage collection algorithms. As much as I'd like to describe them here, I have to hold off, because different engines implement different tweaks and techniques. And, what's even more important, things change as engines develop, so studying deeper "in advance", without a real need is probably not worth that. Unless, of course, it is a matter of pure interest, then there will be some links for you below. @@ -199,14 +199,14 @@ The main things to know: - Garbage collection is performed automatically. We cannot force or prevent it. - Objects are retained in memory while they are reachable. -- Being referenced is not the same as being reachable (from a root): a pack of interlinked objects can become unreachable as a whole, as we've seen in the example above. +- Being referenced is not the same as being reachable (from a root): a pack of interlinked objects can become unreachable as a whole. Modern engines implement advanced algorithms of garbage collection. A general book "The Garbage Collection Handbook: The Art of Automatic Memory Management" (R. Jones et al) covers some of them. -If you are familiar with low-level programming, more detailed information about V8's garbage collector is in the article [A tour of V8: Garbage Collection](http://jayconrod.com/posts/55/a-tour-of-v8-garbage-collection). +If you are familiar with low-level programming, the more detailed information about V8 garbage collector is in the article [A tour of V8: Garbage Collection](http://jayconrod.com/posts/55/a-tour-of-v8-garbage-collection). -The [V8 blog](https://v8.dev/) also publishes articles about changes in memory management from time to time. Naturally, to learn more about garbage collection, you'd better prepare by learning about V8 internals in general and read the blog of [Vyacheslav Egorov](http://mrale.ph) who worked as one of the V8 engineers. I'm saying: "V8", because it is best covered by articles on the internet. For other engines, many approaches are similar, but garbage collection differs in many aspects. +[V8 blog](https://v8.dev/) also publishes articles about changes in memory management from time to time. Naturally, to learn the garbage collection, you'd better prepare by learning about V8 internals in general and read the blog of [Vyacheslav Egorov](http://mrale.ph) who worked as one of V8 engineers. I'm saying: "V8", because it is best covered with articles in the internet. For other engines, many approaches are similar, but garbage collection differs in many aspects. -In-depth knowledge of engines is good when you need low-level optimizations. It would be wise to plan that as the next step after you're familiar with the language. +In-depth knowledge of engines is good when you need low-level optimizations. It would be wise to plan that as the next step after you're familiar with the language. diff --git a/1-js/04-object-basics/03-garbage-collection/family-delete-refs.svg b/1-js/04-object-basics/03-garbage-collection/family-delete-refs.svg index a582ca64..2ae1f664 100644 --- a/1-js/04-object-basics/03-garbage-collection/family-delete-refs.svg +++ b/1-js/04-object-basics/03-garbage-collection/family-delete-refs.svg @@ -1 +1 @@ -<global variable>ObjectObjectwifefamilyname: "John"name: "Ann"motherObjectfatherhusband \ No newline at end of file +<global variable>ObjectObjectwifefamilyname: "John"name: "Ann"motherObjectfatherhusband \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/family-no-family.svg b/1-js/04-object-basics/03-garbage-collection/family-no-family.svg index c73dd6a4..655d1982 100644 --- a/1-js/04-object-basics/03-garbage-collection/family-no-family.svg +++ b/1-js/04-object-basics/03-garbage-collection/family-no-family.svg @@ -1 +1 @@ -<global>ObjectObjectfatherwifename: "John"name: "Ann"motherObjecthusbandfamily: null \ No newline at end of file +<global>ObjectObjectfatherwifename: "John"name: "Ann"motherObjecthusbandfamily: null \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/family-no-father-2.svg b/1-js/04-object-basics/03-garbage-collection/family-no-father-2.svg index 6bd13c0e..11f4ada3 100644 --- a/1-js/04-object-basics/03-garbage-collection/family-no-father-2.svg +++ b/1-js/04-object-basics/03-garbage-collection/family-no-father-2.svg @@ -1 +1 @@ -Objectfamilyname: "Ann"motherObject<global> \ No newline at end of file +Objectfamilyname: "Ann"motherObject<global> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/family-no-father.svg b/1-js/04-object-basics/03-garbage-collection/family-no-father.svg index fd1f2060..b76c868e 100644 --- a/1-js/04-object-basics/03-garbage-collection/family-no-father.svg +++ b/1-js/04-object-basics/03-garbage-collection/family-no-father.svg @@ -1 +1 @@ -ObjectObjectwifefamilyname: "John"name: "Ann"motherObject<global> \ No newline at end of file +ObjectObjectwifefamilyname: "John"name: "Ann"motherObject<global> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/family.svg b/1-js/04-object-basics/03-garbage-collection/family.svg index fd053487..bec2f4dd 100644 --- a/1-js/04-object-basics/03-garbage-collection/family.svg +++ b/1-js/04-object-basics/03-garbage-collection/family.svg @@ -1 +1 @@ -ObjectObjectfatherwifefamilyname: "John"name: "Ann"motherObjecthusband<global variable> \ No newline at end of file +ObjectObjectfatherwifefamilyname: "John"name: "Ann"motherObjecthusband<global variable> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/garbage-collection-1.svg b/1-js/04-object-basics/03-garbage-collection/garbage-collection-1.svg index 5cac52e9..2563c818 100644 --- a/1-js/04-object-basics/03-garbage-collection/garbage-collection-1.svg +++ b/1-js/04-object-basics/03-garbage-collection/garbage-collection-1.svg @@ -1 +1 @@ -<global> \ No newline at end of file +<global> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/garbage-collection-2.svg b/1-js/04-object-basics/03-garbage-collection/garbage-collection-2.svg index 7dd3a693..acd5025e 100644 --- a/1-js/04-object-basics/03-garbage-collection/garbage-collection-2.svg +++ b/1-js/04-object-basics/03-garbage-collection/garbage-collection-2.svg @@ -1 +1 @@ -<global> \ No newline at end of file +<global> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/garbage-collection-3.svg b/1-js/04-object-basics/03-garbage-collection/garbage-collection-3.svg index 10605778..4421ec78 100644 --- a/1-js/04-object-basics/03-garbage-collection/garbage-collection-3.svg +++ b/1-js/04-object-basics/03-garbage-collection/garbage-collection-3.svg @@ -1 +1 @@ -<global> \ No newline at end of file +<global> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/garbage-collection-4.svg b/1-js/04-object-basics/03-garbage-collection/garbage-collection-4.svg index bd485ade..74adc813 100644 --- a/1-js/04-object-basics/03-garbage-collection/garbage-collection-4.svg +++ b/1-js/04-object-basics/03-garbage-collection/garbage-collection-4.svg @@ -1 +1 @@ -<global> \ No newline at end of file +<global> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/garbage-collection-5.svg b/1-js/04-object-basics/03-garbage-collection/garbage-collection-5.svg index 2d85432b..abb127ab 100644 --- a/1-js/04-object-basics/03-garbage-collection/garbage-collection-5.svg +++ b/1-js/04-object-basics/03-garbage-collection/garbage-collection-5.svg @@ -1 +1 @@ -<global>unreachables \ No newline at end of file +<global>unreachables \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/memory-user-john-admin.svg b/1-js/04-object-basics/03-garbage-collection/memory-user-john-admin.svg index 19132435..dc4cce1c 100644 --- a/1-js/04-object-basics/03-garbage-collection/memory-user-john-admin.svg +++ b/1-js/04-object-basics/03-garbage-collection/memory-user-john-admin.svg @@ -1 +1 @@ -username: "John"Objectadmin<global> \ No newline at end of file +username: "John"Objectadmin<global> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/memory-user-john-lost.svg b/1-js/04-object-basics/03-garbage-collection/memory-user-john-lost.svg index 07914a9c..e75b8d46 100644 --- a/1-js/04-object-basics/03-garbage-collection/memory-user-john-lost.svg +++ b/1-js/04-object-basics/03-garbage-collection/memory-user-john-lost.svg @@ -1 +1 @@ -name: "John"Objectuser: null<global> \ No newline at end of file +name: "John"Objectuser: null<global> \ No newline at end of file diff --git a/1-js/04-object-basics/03-garbage-collection/memory-user-john.svg b/1-js/04-object-basics/03-garbage-collection/memory-user-john.svg index 15bd51af..0191e3f0 100644 --- a/1-js/04-object-basics/03-garbage-collection/memory-user-john.svg +++ b/1-js/04-object-basics/03-garbage-collection/memory-user-john.svg @@ -1 +1 @@ -username: "John"Object<global> \ No newline at end of file +username: "John"Object<global> \ No newline at end of file diff --git a/1-js/04-object-basics/04-object-methods/7-calculator/_js.view/test.js b/1-js/04-object-basics/04-object-methods/7-calculator/_js.view/test.js index 4decb76d..1f71eda4 100644 --- a/1-js/04-object-basics/04-object-methods/7-calculator/_js.view/test.js +++ b/1-js/04-object-basics/04-object-methods/7-calculator/_js.view/test.js @@ -15,11 +15,6 @@ describe("calculator", function() { afterEach(function() { prompt.restore(); }); - - it('the read get two values and saves them as object properties', function () { - assert.equal(calculator.a, 2); - assert.equal(calculator.b, 3); - }); it("the sum is 5", function() { assert.equal(calculator.sum(), 5); diff --git a/1-js/04-object-basics/04-object-methods/7-calculator/task.md b/1-js/04-object-basics/04-object-methods/7-calculator/task.md index 82d0da03..aa22608e 100644 --- a/1-js/04-object-basics/04-object-methods/7-calculator/task.md +++ b/1-js/04-object-basics/04-object-methods/7-calculator/task.md @@ -6,7 +6,7 @@ importance: 5 Create an object `calculator` with three methods: -- `read()` prompts for two values and saves them as object properties with names `a` and `b` respectively. +- `read()` prompts for two values and saves them as object properties. - `sum()` returns the sum of saved values. - `mul()` multiplies saved values and returns the result. @@ -21,3 +21,4 @@ alert( calculator.mul() ); ``` [demo] + diff --git a/1-js/04-object-basics/04-object-methods/8-chain-calls/_js.view/solution.js b/1-js/04-object-basics/04-object-methods/8-chain-calls/_js.view/solution.js index a35c009c..e98fe641 100644 --- a/1-js/04-object-basics/04-object-methods/8-chain-calls/_js.view/solution.js +++ b/1-js/04-object-basics/04-object-methods/8-chain-calls/_js.view/solution.js @@ -11,6 +11,5 @@ let ladder = { }, showStep: function() { alert(this.step); - return this; } }; \ No newline at end of file diff --git a/1-js/04-object-basics/04-object-methods/8-chain-calls/_js.view/test.js b/1-js/04-object-basics/04-object-methods/8-chain-calls/_js.view/test.js index b4f2459b..a2b17fcc 100644 --- a/1-js/04-object-basics/04-object-methods/8-chain-calls/_js.view/test.js +++ b/1-js/04-object-basics/04-object-methods/8-chain-calls/_js.view/test.js @@ -32,14 +32,6 @@ describe('Ladder', function() { it('down().up().up().up() ', function() { assert.equal(ladder.down().up().up().up().step, 2); }); - - it('showStep() should return this', function() { - assert.equal(ladder.showStep(), ladder); - }); - - it('up().up().down().showStep().down().showStep()', function () { - assert.equal(ladder.up().up().down().showStep().down().showStep().step, 0) - }); after(function() { ladder.step = 0; diff --git a/1-js/04-object-basics/04-object-methods/8-chain-calls/solution.md b/1-js/04-object-basics/04-object-methods/8-chain-calls/solution.md index f215461d..2b47873f 100644 --- a/1-js/04-object-basics/04-object-methods/8-chain-calls/solution.md +++ b/1-js/04-object-basics/04-object-methods/8-chain-calls/solution.md @@ -21,9 +21,9 @@ let ladder = { return this; */!* } -}; +} -ladder.up().up().down().showStep().down().showStep(); // shows 1 then 0 +ladder.up().up().down().up().down().showStep(); // 1 ``` We also can write a single call per line. For long chains it's more readable: @@ -33,7 +33,7 @@ ladder .up() .up() .down() - .showStep() // 1 + .up() .down() - .showStep(); // 0 + .showStep(); // 1 ``` diff --git a/1-js/04-object-basics/04-object-methods/8-chain-calls/task.md b/1-js/04-object-basics/04-object-methods/8-chain-calls/task.md index a2a19c62..eca9f4e9 100644 --- a/1-js/04-object-basics/04-object-methods/8-chain-calls/task.md +++ b/1-js/04-object-basics/04-object-methods/8-chain-calls/task.md @@ -28,14 +28,12 @@ ladder.up(); ladder.up(); ladder.down(); ladder.showStep(); // 1 -ladder.down(); -ladder.showStep(); // 0 ``` Modify the code of `up`, `down` and `showStep` to make the calls chainable, like this: ```js -ladder.up().up().down().showStep().down().showStep(); // shows 1 then 0 +ladder.up().up().down().showStep(); // 1 ``` Such approach is widely used across JavaScript libraries. diff --git a/1-js/04-object-basics/04-object-methods/article.md b/1-js/04-object-basics/04-object-methods/article.md index cea2b6a7..75bd1856 100644 --- a/1-js/04-object-basics/04-object-methods/article.md +++ b/1-js/04-object-basics/04-object-methods/article.md @@ -32,11 +32,11 @@ user.sayHi = function() { user.sayHi(); // Hello! ``` -Here we've just used a Function Expression to create a function and assign it to the property `user.sayHi` of the object. +Here we've just used a Function Expression to create the function and assign it to the property `user.sayHi` of the object. -Then we can call it as `user.sayHi()`. The user can now speak! +Then we can call it. The user can now speak! -A function that is a property of an object is called its *method*. +A function that is the property of an object is called its *method*. So, here we've got a method `sayHi` of the object `user`. @@ -51,7 +51,7 @@ let user = { // first, declare function sayHi() { alert("Hello!"); -} +}; // then add as a method user.sayHi = sayHi; @@ -81,7 +81,7 @@ user = { // method shorthand looks better, right? user = { *!* - sayHi() { // same as "sayHi: function(){...}" + sayHi() { // same as "sayHi: function()" */!* alert("Hello"); } @@ -90,7 +90,7 @@ user = { As demonstrated, we can omit `"function"` and just write `sayHi()`. -To tell the truth, the notations are not fully identical. There are subtle differences related to object inheritance (to be covered later), but for now they do not matter. In almost all cases, the shorter syntax is preferred. +To tell the truth, the notations are not fully identical. There are subtle differences related to object inheritance (to be covered later), but for now they do not matter. In almost all cases the shorter syntax is preferred. ## "this" in methods @@ -160,16 +160,14 @@ let user = { let admin = user; user = null; // overwrite to make things obvious -*!* -admin.sayHi(); // TypeError: Cannot read property 'name' of null -*/!* +admin.sayHi(); // Whoops! inside sayHi(), the old name is used! error! ``` If we used `this.name` instead of `user.name` inside the `alert`, then the code would work. ## "this" is not bound -In JavaScript, keyword `this` behaves unlike most other programming languages. It can be used in any function, even if it's not a method of an object. +In JavaScript, keyword `this` behaves unlike most other programming languages. It can be used in any function. There's no syntax error in the following example: diff --git a/1-js/04-object-basics/06-constructor-new/1-two-functions-one-object/task.md b/1-js/04-object-basics/06-constructor-new/1-two-functions-one-object/task.md index e932a201..8c1fea8e 100644 --- a/1-js/04-object-basics/06-constructor-new/1-two-functions-one-object/task.md +++ b/1-js/04-object-basics/06-constructor-new/1-two-functions-one-object/task.md @@ -4,14 +4,14 @@ importance: 2 # Two functions – one object -Is it possible to create functions `A` and `B` so that `new A() == new B()`? +Is it possible to create functions `A` and `B` such as `new A()==new B()`? ```js no-beautify function A() { ... } function B() { ... } -let a = new A(); -let b = new B(); +let a = new A; +let b = new B; alert( a == b ); // true ``` diff --git a/1-js/04-object-basics/06-constructor-new/2-calculator-constructor/_js.view/test.js b/1-js/04-object-basics/06-constructor-new/2-calculator-constructor/_js.view/test.js index bba80e5c..03605392 100644 --- a/1-js/04-object-basics/06-constructor-new/2-calculator-constructor/_js.view/test.js +++ b/1-js/04-object-basics/06-constructor-new/2-calculator-constructor/_js.view/test.js @@ -10,11 +10,6 @@ describe("calculator", function() { calculator = new Calculator(); calculator.read(); }); - - it("the read method asks for two values using prompt and remembers them in object properties", function() { - assert.equal(calculator.a, 2); - assert.equal(calculator.b, 3); - }); it("when 2 and 3 are entered, the sum is 5", function() { assert.equal(calculator.sum(), 5); diff --git a/1-js/04-object-basics/06-constructor-new/2-calculator-constructor/task.md b/1-js/04-object-basics/06-constructor-new/2-calculator-constructor/task.md index c862bec4..60e7c373 100644 --- a/1-js/04-object-basics/06-constructor-new/2-calculator-constructor/task.md +++ b/1-js/04-object-basics/06-constructor-new/2-calculator-constructor/task.md @@ -6,7 +6,7 @@ importance: 5 Create a constructor function `Calculator` that creates objects with 3 methods: -- `read()` prompts for two values and saves them as object properties with names `a` and `b` respectively. +- `read()` asks for two values using `prompt` and remembers them in object properties. - `sum()` returns the sum of these properties. - `mul()` returns the multiplication product of these properties. diff --git a/1-js/04-object-basics/06-constructor-new/article.md b/1-js/04-object-basics/06-constructor-new/article.md index a335464f..a885e35f 100644 --- a/1-js/04-object-basics/06-constructor-new/article.md +++ b/1-js/04-object-basics/06-constructor-new/article.md @@ -1,6 +1,6 @@ # Constructor, operator "new" -The regular `{...}` syntax allows us to create one object. But often we need to create many similar objects, like multiple users or menu items and so on. +The regular `{...}` syntax allows to create one object. But often we need to create many similar objects, like multiple users or menu items and so on. That can be done using constructor functions and the `"new"` operator. @@ -64,14 +64,13 @@ Now if we want to create other users, we can call `new User("Ann")`, `new User(" That's the main purpose of constructors -- to implement reusable object creation code. -Let's note once again -- technically, any function (except arrow functions, as they don't have `this`) can be used as a constructor. It can be run with `new`, and it will execute the algorithm above. The "capital letter first" is a common agreement, to make it clear that a function is to be run with `new`. +Let's note once again -- technically, any function can be used as a constructor. That is: any function can be run with `new`, and it will execute the algorithm above. The "capital letter first" is a common agreement, to make it clear that a function is to be run with `new`. ````smart header="new function() { ... }" -If we have many lines of code all about creation of a single complex object, we can wrap them in an immediately called constructor function, like this: +If we have many lines of code all about creation of a single complex object, we can wrap them in constructor function, like this: ```js -// create a function and immediately call it with new -let user = new function() { +let user = new function() { this.name = "John"; this.isAdmin = false; @@ -81,7 +80,7 @@ let user = new function() { }; ``` -This constructor can't be called again, because it is not saved anywhere, just created and called. So this trick aims to encapsulate the code that constructs the single object, without future reuse. +The constructor can't be called again, because it is not saved anywhere, just created and called. So this trick aims to encapsulate the code that constructs the single object, without future reuse. ```` ## Constructor mode test: new.target @@ -92,7 +91,7 @@ The syntax from this section is rarely used, skip it unless you want to know eve Inside a function, we can check whether it was called with `new` or without it, using a special `new.target` property. -It is undefined for regular calls and equals the function if called with `new`: +It is empty for regular calls and equals the function if called with `new`: ```js run function User() { @@ -171,7 +170,7 @@ alert( new SmallUser().name ); // John Usually constructors don't have a `return` statement. Here we mention the special behavior with returning objects mainly for the sake of completeness. ````smart header="Omitting parentheses" -By the way, we can omit parentheses after `new`: +By the way, we can omit parentheses after `new`, if it has no arguments: ```js let user = new User; // <-- no parentheses diff --git a/1-js/04-object-basics/07-optional-chaining/article.md b/1-js/04-object-basics/07-optional-chaining/article.md index 4c602942..c4a04213 100644 --- a/1-js/04-object-basics/07-optional-chaining/article.md +++ b/1-js/04-object-basics/07-optional-chaining/article.md @@ -9,88 +9,44 @@ The optional chaining `?.` is a safe way to access nested object properties, eve If you've just started to read the tutorial and learn JavaScript, maybe the problem hasn't touched you yet, but it's quite common. -As an example, let's say we have `user` objects that hold the information about our users. +As an example, let's consider objects for user data. Most of our users have addresses in `user.address` property, with the street `user.address.street`, but some did not provide them. -Most of our users have addresses in `user.address` property, with the street `user.address.street`, but some did not provide them. - -In such case, when we attempt to get `user.address.street`, and the user happens to be without an address, we get an error: +In such case, when we attempt to get `user.address.street`, we'll get an error: ```js run -let user = {}; // a user without "address" property +let user = {}; // the user without "address" property alert(user.address.street); // Error! ``` -That's the expected result. JavaScript works like this. As `user.address` is `undefined`, an attempt to get `user.address.street` fails with an error. +That's the expected result, JavaScript works like this, but many practical cases we'd prefer to get `undefined` instead of an error (meaning "no street"). -In many practical cases we'd prefer to get `undefined` instead of an error here (meaning "no street"). - -...and another example. In Web development, we can get an object that corresponds to a web page element using a special method call, such as `document.querySelector('.elem')`, and it returns `null` when there's no such element. +...And another example. In the web development, we may need to get an information about an element on the page, that sometimes doesn't exist: ```js run -// document.querySelector('.elem') is null if there's no element -let html = document.querySelector('.elem').innerHTML; // error if it's null +// Error if the result of querySelector(...) is null +let html = document.querySelector('.my-element').innerHTML; ``` -Once again, if the element doesn't exist, we'll get an error accessing `.innerHTML` property of `null`. And in some cases, when the absence of the element is normal, we'd like to avoid the error and just accept `html = null` as the result. +Before `?.` appeared in the language, the `&&` operator was used to work around that. -How can we do this? - -The obvious solution would be to check the value using `if` or the conditional operator `?`, before accessing its property, like this: - -```js -let user = {}; - -alert(user.address ? user.address.street : undefined); -``` - -It works, there's no error... But it's quite inelegant. As you can see, the `"user.address"` appears twice in the code. - -Here's how the same would look for `document.querySelector`: - -```js run -let html = document.querySelector('.elem') ? document.querySelector('.elem').innerHTML : null; -``` - -We can see that the element search `document.querySelector('.elem')` is actually called twice here. Not good. - -For more deeply nested properties, it becomes even uglier, as more repetitions are required. - -E.g. let's get `user.address.street.name` in a similar fashion. - -```js -let user = {}; // user has no address - -alert(user.address ? user.address.street ? user.address.street.name : null : null); -``` - -That's just awful, one may even have problems understanding such code. - -There's a little better way to write it, using the `&&` operator: +For example: ```js run let user = {}; // user has no address -alert( user.address && user.address.street && user.address.street.name ); // undefined (no error) +alert( user && user.address && user.address.street ); // undefined (no error) ``` -AND'ing the whole path to the property ensures that all components exist (if not, the evaluation stops), but also isn't ideal. - -As you can see, property names are still duplicated in the code. E.g. in the code above, `user.address` appears three times. - -That's why the optional chaining `?.` was added to the language. To solve this problem once and for all! +AND'ing the whole path to the property ensures that all components exist (if not, the evaluation stops), but is cumbersome to write. ## Optional chaining -The optional chaining `?.` stops the evaluation if the value before `?.` is `undefined` or `null` and returns `undefined`. +The optional chaining `?.` stops the evaluation and returns `undefined` if the part before `?.` is `undefined` or `null`. **Further in this article, for brevity, we'll be saying that something "exists" if it's not `null` and not `undefined`.** -In other words, `value?.prop`: -- works as `value.prop`, if `value` exists, -- otherwise (when `value` is `undefined/null`) it returns `undefined`. - -Here's the safe way to access `user.address.street` using `?.`: +Here's the safe way to access `user.address.street`: ```js run let user = {}; // user has no address @@ -98,14 +54,6 @@ let user = {}; // user has no address alert( user?.address?.street ); // undefined (no error) ``` -The code is short and clean, there's no duplication at all. - -Here's an example with `document.querySelector`: - -```js run -let html = document.querySelector('.elem')?.innerHTML; // will be undefined, if there's no element -``` - Reading the address with `user?.address` works even if `user` object doesn't exist: ```js run @@ -117,14 +65,16 @@ alert( user?.address.street ); // undefined Please note: the `?.` syntax makes optional the value before it, but not any further. -E.g. in `user?.address.street.name` the `?.` allows `user` to safely be `null/undefined` (and returns `undefined` in that case), but that's only for `user`. Further properties are accessed in a regular way. If we want some of them to be optional, then we'll need to replace more `.` with `?.`. +In the example above, `user?.` allows only `user` to be `null/undefined`. + +On the other hand, if `user` does exist, then it must have `user.address` property, otherwise `user?.address.street` gives an error at the second dot. ```warn header="Don't overuse the optional chaining" We should use `?.` only where it's ok that something doesn't exist. -For example, if according to our code logic `user` object must exist, but `address` is optional, then we should write `user.address?.street`, but not `user?.address?.street`. +For example, if according to our coding logic `user` object must be there, but `address` is optional, then `user.address?.street` would be better. -Then, if `user` happens to be undefined, we'll see a programming error about it and fix it. Otherwise, if we overuse `?.`, coding errors can be silenced where not appropriate, and become more difficult to debug. +So, if `user` happens to be undefined due to a mistake, we'll see a programming error about it and fix it. Otherwise, coding errors can be silenced where not appropriate, and become more difficult to debug. ``` ````warn header="The variable before `?.` must be declared" @@ -134,14 +84,14 @@ If there's no variable `user` at all, then `user?.anything` triggers an error: // ReferenceError: user is not defined user?.address; ``` -The variable must be declared (e.g. `let/const/var user` or as a function parameter). The optional chaining works only for declared variables. +There must be a declaration (e.g. `let/const/var user`). The optional chaining works only for declared variables. ```` ## Short-circuiting As it was said before, the `?.` immediately stops ("short-circuits") the evaluation if the left part doesn't exist. -So, if there are any further function calls or operations to the right of `?.`, they won't be made. +So, if there are any further function calls or side effects, they don't occur. For instance: @@ -149,7 +99,7 @@ For instance: let user = null; let x = 0; -user?.sayHi(x++); // no "user", so the execution doesn't reach sayHi call and x++ +user?.sayHi(x++); // no "sayHi", so the execution doesn't reach x++ alert(x); // 0, value not incremented ``` @@ -163,40 +113,39 @@ For example, `?.()` is used to call a function that may not exist. In the code below, some of our users have `admin` method, and some don't: ```js run -let userAdmin = { +let user1 = { admin() { alert("I am admin"); } -}; +} -let userGuest = {}; +let user2 = {}; *!* -userAdmin.admin?.(); // I am admin -*/!* - -*!* -userGuest.admin?.(); // nothing happens (no such method) +user1.admin?.(); // I am admin +user2.admin?.(); */!* ``` -Here, in both lines we first use the dot (`userAdmin.admin`) to get `admin` property, because we assume that the `user` object exists, so it's safe read from it. +Here, in both lines we first use the dot (`user1.admin`) to get `admin` property, because the user object must exist, so it's safe read from it. -Then `?.()` checks the left part: if the `admin` function exists, then it runs (that's so for `userAdmin`). Otherwise (for `userGuest`) the evaluation stops without errors. +Then `?.()` checks the left part: if the admin function exists, then it runs (that's so for `user1`). Otherwise (for `user2`) the evaluation stops without errors. The `?.[]` syntax also works, if we'd like to use brackets `[]` to access properties instead of dot `.`. Similar to previous cases, it allows to safely read a property from an object that may not exist. ```js run -let key = "firstName"; - let user1 = { firstName: "John" }; -let user2 = null; +let user2 = null; // Imagine, we couldn't authorize the user + +let key = "firstName"; alert( user1?.[key] ); // John alert( user2?.[key] ); // undefined + +alert( user1?.[key]?.something?.not?.existing); // undefined ``` Also we can use `?.` with `delete`: @@ -206,16 +155,17 @@ delete user?.name; // delete user.name if user exists ``` ````warn header="We can use `?.` for safe reading and deleting, but not writing" -The optional chaining `?.` has no use on the left side of an assignment. +The optional chaining `?.` has no use at the left side of an assignment. For example: ```js run let user = null; user?.name = "John"; // Error, doesn't work -// because it evaluates to: undefined = "John" +// because it evaluates to undefined = "John" ``` +It's just not that smart. ```` ## Summary @@ -230,4 +180,4 @@ As we can see, all of them are straightforward and simple to use. The `?.` check A chain of `?.` allows to safely access nested properties. -Still, we should apply `?.` carefully, only where it's acceptable, according to our code logic, that the left part doesn't exist. So that it won't hide programming errors from us, if they occur. +Still, we should apply `?.` carefully, only where it's acceptable that the left part doesn't to exist. So that it won't hide programming errors from us, if they occur. diff --git a/1-js/04-object-basics/08-symbol/article.md b/1-js/04-object-basics/08-symbol/article.md index 10a98af0..e469bb0b 100644 --- a/1-js/04-object-basics/08-symbol/article.md +++ b/1-js/04-object-basics/08-symbol/article.md @@ -1,16 +1,9 @@ # Symbol type -By specification, only two primitive types may serve as object property keys: +By specification, object property keys may be either of string type, or of symbol type. Not numbers, not booleans, only strings or symbols, these two types. -- string type, or -- symbol type. - -Otherwise, if one uses another type, such as number, it's autoconverted to string. So that `obj[1]` is the same as `obj["1"]`, and `obj[true]` is the same as `obj["true"]`. - -Until now we've been using only strings. - -Now let's explore symbols, see what they can do for us. +Till now we've been using only strings. Now let's see the benefits that symbols can give us. ## Symbols @@ -19,17 +12,18 @@ A "symbol" represents a unique identifier. A value of this type can be created using `Symbol()`: ```js +// id is a new symbol let id = Symbol(); ``` -Upon creation, we can give symbols a description (also called a symbol name), mostly useful for debugging purposes: +Upon creation, we can give symbol a description (also called a symbol name), mostly useful for debugging purposes: ```js // id is a symbol with the description "id" let id = Symbol("id"); ``` -Symbols are guaranteed to be unique. Even if we create many symbols with exactly the same description, they are different values. The description is just a label that doesn't affect anything. +Symbols are guaranteed to be unique. Even if we create many symbols with the same description, they are different values. The description is just a label that doesn't affect anything. For instance, here are two symbols with the same description -- they are not equal: @@ -44,8 +38,6 @@ alert(id1 == id2); // false If you are familiar with Ruby or another language that also has some sort of "symbols" -- please don't be misguided. JavaScript symbols are different. -So, to summarize, a symbol is a "primitive unique value" with an optional description. Let's see where we can use them. - ````warn header="Symbols don't auto-convert to a string" Most values in JavaScript support implicit conversion to a string. For instance, we can `alert` almost any value, and it will work. Symbols are special. They don't auto-convert. @@ -61,7 +53,6 @@ alert(id); // TypeError: Cannot convert a Symbol value to a string That's a "language guard" against messing up, because strings and symbols are fundamentally different and should not accidentally convert one into another. If we really want to show a symbol, we need to explicitly call `.toString()` on it, like here: - ```js run let id = Symbol("id"); *!* @@ -70,7 +61,6 @@ alert(id.toString()); // Symbol(id), now it works ``` Or get `symbol.description` property to show the description only: - ```js run let id = Symbol("id"); *!* @@ -82,7 +72,6 @@ alert(id.description); // id ## "Hidden" properties - Symbols allow us to create "hidden" properties of an object, that no other part of code can accidentally access or overwrite. For instance, if we're working with `user` objects, that belong to a third-party code. We'd like to add identifiers to them. @@ -103,9 +92,9 @@ alert( user[id] ); // we can access the data using the symbol as the key What's the benefit of using `Symbol("id")` over a string `"id"`? -As `user` objects belong to another codebase, it's unsafe to add fields to them, since we might affect pre-defined behavior in that other codebase. However, symbols cannot be accessed accidentally. The third-party code won't be aware of newly defined symbols, so it's safe to add symbols to the `user` objects. +As `user` objects belongs to another code, and that code also works with them, we shouldn't just add any fields to it. That's unsafe. But a symbol cannot be accessed accidentally, the third-party code probably won't even see it, so it's probably all right to do. -Also, imagine that another script wants to have its own identifier inside `user`, for its own purposes. +Also, imagine that another script wants to have its own identifier inside `user`, for its own purposes. That may be another JavaScript library, so that the scripts are completely unaware of each other. Then that script can create its own `Symbol("id")`, like this: @@ -120,7 +109,7 @@ There will be no conflict between our and their identifiers, because symbols are ...But if we used a string `"id"` instead of a symbol for the same purpose, then there *would* be a conflict: -```js +```js run let user = { name: "John" }; // Our script uses "id" property @@ -169,10 +158,10 @@ for (let key in user) alert(key); // name, age (no symbols) */!* // the direct access by the symbol works -alert( "Direct: " + user[id] ); // Direct: 123 +alert( "Direct: " + user[id] ); ``` -[Object.keys(user)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys) also ignores them. That's a part of the general "hiding symbolic properties" principle. If another script or a library loops over our object, it won't unexpectedly access a symbolic property. +`Object.keys(user)` also ignores them. That's a part of the general "hiding symbolic properties" principle. If another script or a library loops over our object, it won't unexpectedly access a symbolic property. In contrast, [Object.assign](mdn:js/Object/assign) copies both string and symbol properties: @@ -217,12 +206,12 @@ Symbols inside the registry are called *global symbols*. If we want an applicati ```smart header="That sounds like Ruby" In some programming languages, like Ruby, there's a single symbol per name. -In JavaScript, as we can see, that's true for global symbols. +In JavaScript, as we can see, that's right for global symbols. ``` ### Symbol.keyFor -We have seen that for global symbols, `Symbol.for(key)` returns a symbol by name. To do the opposite -- return a name by global symbol -- we can use: `Symbol.keyFor(sym)`: +For global symbols, not only `Symbol.for(key)` returns a symbol by name, but there's a reverse call: `Symbol.keyFor(sym)`, that does the reverse: returns a name by a global symbol. For instance: @@ -238,7 +227,7 @@ alert( Symbol.keyFor(sym2) ); // id The `Symbol.keyFor` internally uses the global symbol registry to look up the key for the symbol. So it doesn't work for non-global symbols. If the symbol is not global, it won't be able to find it and returns `undefined`. -That said, all symbols have the `description` property. +That said, any symbols have `description` property. For instance: @@ -279,11 +268,10 @@ Symbols are always different values, even if they have the same name. If we want Symbols have two main use cases: 1. "Hidden" object properties. - If we want to add a property into an object that "belongs" to another script or a library, we can create a symbol and use it as a property key. A symbolic property does not appear in `for..in`, so it won't be accidentally processed together with other properties. Also it won't be accessed directly, because another script does not have our symbol. So the property will be protected from accidental use or overwrite. So we can "covertly" hide something into objects that we need, but others should not see, using symbolic properties. 2. There are many system symbols used by JavaScript which are accessible as `Symbol.*`. We can use them to alter some built-in behaviors. For instance, later in the tutorial we'll use `Symbol.iterator` for [iterables](info:iterable), `Symbol.toPrimitive` to setup [object-to-primitive conversion](info:object-toprimitive) and so on. -Technically, symbols are not 100% hidden. There is a built-in method [Object.getOwnPropertySymbols(obj)](mdn:js/Object/getOwnPropertySymbols) that allows us to get all symbols. Also there is a method named [Reflect.ownKeys(obj)](mdn:js/Reflect/ownKeys) that returns *all* keys of an object including symbolic ones. But most libraries, built-in functions and syntax constructs don't use these methods. +Technically, symbols are not 100% hidden. There is a built-in method [Object.getOwnPropertySymbols(obj)](mdn:js/Object/getOwnPropertySymbols) that allows us to get all symbols. Also there is a method named [Reflect.ownKeys(obj)](mdn:js/Reflect/ownKeys) that returns *all* keys of an object including symbolic ones. So they are not really hidden. But most libraries, built-in functions and syntax constructs don't use these methods. diff --git a/1-js/04-object-basics/09-object-toprimitive/article.md b/1-js/04-object-basics/09-object-toprimitive/article.md index 8b0008b1..36b6c646 100644 --- a/1-js/04-object-basics/09-object-toprimitive/article.md +++ b/1-js/04-object-basics/09-object-toprimitive/article.md @@ -3,40 +3,19 @@ What happens when objects are added `obj1 + obj2`, subtracted `obj1 - obj2` or printed using `alert(obj)`? -JavaScript doesn't allow you to customize how operators work on objects. Unlike some other programming languages, such as Ruby or C++, we can't implement a special object method to handle addition (or other operators). - -In case of such operations, objects are auto-converted to primitives, and then the operation is carried out over these primitives and results in a primitive value. - -That's an important limitation: the result of `obj1 + obj2` (or another math operation) can't be another object! - -E.g. we can't make objects representing vectors or matrices (or achievements or whatever), add them and expect a "summed" object as the result. Such architectural feats are automatically "off the board". - -So, because we can't technically do much here, there's no maths with objects in real projects. When it happens, with rare exceptions, it's because of a coding mistake. - -In this chapter we'll cover how an object converts to primitive and how to customize it. - -We have two purposes: - -1. It will allow us to understand what's going on in case of coding mistakes, when such an operation happened accidentally. -2. There are exceptions, where such operations are possible and look good. E.g. subtracting or comparing dates (`Date` objects). We'll come across them later. - -## Conversion rules +In that case, objects are auto-converted to primitives, and then the operation is carried out. In the chapter we've seen the rules for numeric, string and boolean conversions of primitives. But we left a gap for objects. Now, as we know about methods and symbols it becomes possible to fill it. -1. There's no conversion to boolean. All objects are `true` in a boolean context, as simple as that. There exist only numeric and string conversions. +1. All objects are `true` in a boolean context. There are only numeric and string conversions. 2. The numeric conversion happens when we subtract objects or apply mathematical functions. For instance, `Date` objects (to be covered in the chapter ) can be subtracted, and the result of `date1 - date2` is the time difference between two dates. -3. As for the string conversion -- it usually happens when we output an object with `alert(obj)` and in similar contexts. +3. As for the string conversion -- it usually happens when we output an object like `alert(obj)` and in similar contexts. -We can implement string and numeric conversion by ourselves, using special object methods. +## ToPrimitive -Now let's get into technical details, because it's the only way to cover the topic in-depth. +We can fine-tune string and numeric conversion, using special object methods. -## Hints - -How does JavaScript decide which conversion to apply? - -There are three variants of type conversion, that happen in various situations. They're called "hints", as described in the [specification](https://tc39.github.io/ecma262/#sec-toprimitive): +There are three variants of type conversion, so-called "hints", described in the [specification](https://tc39.github.io/ecma262/#sec-toprimitive): `"string"` : For an object-to-string conversion, when we're doing an operation on an object that expects a string, like `alert`: @@ -64,12 +43,10 @@ There are three variants of type conversion, that happen in various situations. let greater = user1 > user2; ``` - Most built-in mathematical functions also include such conversion. - `"default"` : Occurs in rare cases when the operator is "not sure" what type to expect. - For instance, binary plus `+` can work both with strings (concatenates them) and numbers (adds them). So if a binary plus gets an object as an argument, it uses the `"default"` hint to convert it. + For instance, binary plus `+` can work both with strings (concatenates them) and numbers (adds them), so both strings and numbers would do. So if a binary plus gets an object as an argument, it uses the `"default"` hint to convert it. Also, if an object is compared using `==` with a string, number or a symbol, it's also unclear which conversion should be done, so the `"default"` hint is used. @@ -83,19 +60,21 @@ There are three variants of type conversion, that happen in various situations. The greater and less comparison operators, such as `<` `>`, can work with both strings and numbers too. Still, they use the `"number"` hint, not `"default"`. That's for historical reasons. -In practice though, things are a bit simpler. + In practice though, we don't need to remember these peculiar details, because all built-in objects except for one case (`Date` object, we'll learn it later) implement `"default"` conversion the same way as `"number"`. And we can do the same. -All built-in objects except for one case (`Date` object, we'll learn it later) implement `"default"` conversion the same way as `"number"`. And we probably should do the same. +```smart header="No `\"boolean\"` hint" +Please note -- there are only three hints. It's that simple. -Still, it's important to know about all 3 hints, soon we'll see why. +There is no "boolean" hint (all objects are `true` in boolean context) or anything else. And if we treat `"default"` and `"number"` the same, like most built-ins do, then there are only two conversions. +``` **To do the conversion, JavaScript tries to find and call three object methods:** 1. Call `obj[Symbol.toPrimitive](hint)` - the method with the symbolic key `Symbol.toPrimitive` (system symbol), if such method exists, 2. Otherwise if hint is `"string"` - - try calling `obj.toString()` or `obj.valueOf()`, whatever exists. + - try `obj.toString()` and `obj.valueOf()`, whatever exists. 3. Otherwise if hint is `"number"` or `"default"` - - try calling `obj.valueOf()` or `obj.toString()`, whatever exists. + - try `obj.valueOf()` and `obj.toString()`, whatever exists. ## Symbol.toPrimitive @@ -103,14 +82,11 @@ Let's start from the first method. There's a built-in symbol named `Symbol.toPri ```js obj[Symbol.toPrimitive] = function(hint) { - // here goes the code to convert this object to a primitive - // it must return a primitive value + // must return a primitive value // hint = one of "string", "number", "default" }; ``` -If the method `Symbol.toPrimitive` exists, it's used for all hints, and no more methods are needed. - For instance, here `user` object implements it: ```js run @@ -130,17 +106,18 @@ alert(+user); // hint: number -> 1000 alert(user + 500); // hint: default -> 1500 ``` -As we can see from the code, `user` becomes a self-descriptive string or a money amount, depending on the conversion. The single method `user[Symbol.toPrimitive]` handles all conversion cases. +As we can see from the code, `user` becomes a self-descriptive string or a money amount depending on the conversion. The single method `user[Symbol.toPrimitive]` handles all conversion cases. + ## toString/valueOf -If there's no `Symbol.toPrimitive` then JavaScript tries to find methods `toString` and `valueOf`: - -- For the `"string"` hint: call `toString` method, and if it doesn't exist or if it returns an object instead of a primitive value, then call `valueOf` (so `toString` has the priority for string conversions). -- For other hints: call `valueOf`, and if it doesn't exist or if it returns an object instead of a primitive value, then call `toString` (so `valueOf` has the priority for maths). - Methods `toString` and `valueOf` come from ancient times. They are not symbols (symbols did not exist that long ago), but rather "regular" string-named methods. They provide an alternative "old-style" way to implement the conversion. +If there's no `Symbol.toPrimitive` then JavaScript tries to find them and try in the order: + +- `toString -> valueOf` for "string" hint. +- `valueOf -> toString` otherwise. + These methods must return a primitive value. If `toString` or `valueOf` returns an object, then it's ignored (same as if there were no method). By default, a plain object has following `toString` and `valueOf` methods: @@ -159,9 +136,9 @@ alert(user.valueOf() === user); // true So if we try to use an object as a string, like in an `alert` or so, then by default we see `[object Object]`. -The default `valueOf` is mentioned here only for the sake of completeness, to avoid any confusion. As you can see, it returns the object itself, and so is ignored. Don't ask me why, that's for historical reasons. So we can assume it doesn't exist. +And the default `valueOf` is mentioned here only for the sake of completeness, to avoid any confusion. As you can see, it returns the object itself, and so is ignored. Don't ask me why, that's for historical reasons. So we can assume it doesn't exist. -Let's implement these methods to customize the conversion. +Let's implement these methods. For instance, here `user` does the same as above using a combination of `toString` and `valueOf` instead of `Symbol.toPrimitive`: @@ -206,27 +183,27 @@ alert(user + 500); // toString -> John500 In the absence of `Symbol.toPrimitive` and `valueOf`, `toString` will handle all primitive conversions. -### A conversion can return any primitive type +## Return types The important thing to know about all primitive-conversion methods is that they do not necessarily return the "hinted" primitive. -There is no control whether `toString` returns exactly a string, or whether `Symbol.toPrimitive` method returns a number for the hint `"number"`. +There is no control whether `toString` returns exactly a string, or whether `Symbol.toPrimitive` method returns a number for a hint `"number"`. The only mandatory thing: these methods must return a primitive, not an object. ```smart header="Historical notes" For historical reasons, if `toString` or `valueOf` returns an object, there's no error, but such value is ignored (like if the method didn't exist). That's because in ancient times there was no good "error" concept in JavaScript. -In contrast, `Symbol.toPrimitive` is stricter, it *must* return a primitive, otherwise there will be an error. +In contrast, `Symbol.toPrimitive` *must* return a primitive, otherwise there will be an error. ``` ## Further conversions As we know already, many operators and functions perform type conversions, e.g. multiplication `*` converts operands to numbers. -If we pass an object as an argument, then there are two stages of calculations: +If we pass an object as an argument, then there are two stages: 1. The object is converted to a primitive (using the rules described above). -2. If the necessary for further calculations, the resulting primitive is also converted. +2. If the resulting primitive isn't of the right type, it's converted. For instance: @@ -263,18 +240,16 @@ The object-to-primitive conversion is called automatically by many built-in func There are 3 types (hints) of it: - `"string"` (for `alert` and other operations that need a string) - `"number"` (for maths) -- `"default"` (few operators, usually objects implement it the same way as `"number"`) +- `"default"` (few operators) -The specification describes explicitly which operator uses which hint. +The specification describes explicitly which operator uses which hint. There are very few operators that "don't know what to expect" and use the `"default"` hint. Usually for built-in objects `"default"` hint is handled the same way as `"number"`, so in practice the last two are often merged together. The conversion algorithm is: 1. Call `obj[Symbol.toPrimitive](hint)` if the method exists, 2. Otherwise if hint is `"string"` - - try calling `obj.toString()` or `obj.valueOf()`, whatever exists. + - try `obj.toString()` and `obj.valueOf()`, whatever exists. 3. Otherwise if hint is `"number"` or `"default"` - - try calling `obj.valueOf()` or `obj.toString()`, whatever exists. + - try `obj.valueOf()` and `obj.toString()`, whatever exists. -All these methods must return a primitive to work (if defined). - -In practice, it's often enough to implement only `obj.toString()` as a "catch-all" method for string conversions that should return a "human-readable" representation of an object, for logging or debugging purposes. +In practice, it's often enough to implement only `obj.toString()` as a "catch-all" method for all conversions that return a "human-readable" representation of an object, for logging or debugging purposes. diff --git a/1-js/05-data-types/01-primitives-methods/1-string-new-property/task.md b/1-js/05-data-types/01-primitives-methods/1-string-new-property/task.md index 208f84cc..50c781ea 100644 --- a/1-js/05-data-types/01-primitives-methods/1-string-new-property/task.md +++ b/1-js/05-data-types/01-primitives-methods/1-string-new-property/task.md @@ -15,4 +15,4 @@ str.test = 5; alert(str.test); ``` -What do you think, will it work? What will be shown? +How do you think, will it work? What will be shown? diff --git a/1-js/05-data-types/01-primitives-methods/article.md b/1-js/05-data-types/01-primitives-methods/article.md index 69e7196e..6c13acda 100644 --- a/1-js/05-data-types/01-primitives-methods/article.md +++ b/1-js/05-data-types/01-primitives-methods/article.md @@ -39,7 +39,7 @@ Objects are "heavier" than primitives. They require additional resources to supp Here's the paradox faced by the creator of JavaScript: -- There are many things one would want to do with a primitive, like a string or a number. It would be great to access them using methods. +- There are many things one would want to do with a primitive like a string or a number. It would be great to access them as methods. - Primitives must be as fast and lightweight as possible. The solution looks a little bit awkward, but here it is: @@ -48,7 +48,7 @@ The solution looks a little bit awkward, but here it is: 2. The language allows access to methods and properties of strings, numbers, booleans and symbols. 3. In order for that to work, a special "object wrapper" that provides the extra functionality is created, and then is destroyed. -The "object wrappers" are different for each primitive type and are called: `String`, `Number`, `Boolean`, `Symbol` and `BigInt`. Thus, they provide different sets of methods. +The "object wrappers" are different for each primitive type and are called: `String`, `Number`, `Boolean` and `Symbol`. Thus, they provide different sets of methods. For instance, there exists a string method [str.toUpperCase()](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/toUpperCase) that returns a capitalized `str`. @@ -104,10 +104,9 @@ if (zero) { // zero is true, because it's an object } ``` -On the other hand, using the same functions `String/Number/Boolean` without `new` is totally fine and useful thing. They convert a value to the corresponding type: to a string, a number, or a boolean (primitive). +On the other hand, using the same functions `String/Number/Boolean` without `new` is a totally sane and useful thing. They convert a value to the corresponding type: to a string, a number, or a boolean (primitive). For example, this is entirely valid: - ```js let num = Number("123"); // convert a string to number ``` diff --git a/1-js/05-data-types/02-number/2-why-rounded-down/solution.md b/1-js/05-data-types/02-number/2-why-rounded-down/solution.md index 4bcd7451..a17a4671 100644 --- a/1-js/05-data-types/02-number/2-why-rounded-down/solution.md +++ b/1-js/05-data-types/02-number/2-why-rounded-down/solution.md @@ -28,6 +28,6 @@ Note that `63.5` has no precision loss at all. That's because the decimal part ` ```js run -alert( Math.round(6.35 * 10) / 10 ); // 6.35 -> 63.5 -> 64(rounded) -> 6.4 +alert( Math.round(6.35 * 10) / 10); // 6.35 -> 63.5 -> 64(rounded) -> 6.4 ``` diff --git a/1-js/05-data-types/02-number/article.md b/1-js/05-data-types/02-number/article.md index c704bd98..e768f4d4 100644 --- a/1-js/05-data-types/02-number/article.md +++ b/1-js/05-data-types/02-number/article.md @@ -2,9 +2,9 @@ In modern JavaScript, there are two types of numbers: -1. Regular numbers in JavaScript are stored in 64-bit format [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754), also known as "double precision floating point numbers". These are numbers that we're using most of the time, and we'll talk about them in this chapter. +1. Regular numbers in JavaScript are stored in 64-bit format [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754-2008_revision), also known as "double precision floating point numbers". These are numbers that we're using most of the time, and we'll talk about them in this chapter. -2. BigInt numbers represent integers of arbitrary length. They are sometimes needed because a regular integer number can't safely exceed (253-1) or be less than -(253-1), as we mentioned earlier in the chapter . As bigints are used in few special areas, we devote them a special chapter . +2. BigInt numbers, to represent integers of arbitrary length. They are sometimes needed, because a regular number can't exceed 253 or be less than -253. As bigints are used in few special areas, we devote them a special chapter . So here we'll talk about regular numbers. Let's expand our knowledge of them. @@ -16,56 +16,45 @@ Imagine we need to write 1 billion. The obvious way is: let billion = 1000000000; ``` -We also can use underscore `_` as the separator: +But in real life, we usually avoid writing a long string of zeroes as it's easy to mistype. Also, we are lazy. We will usually write something like `"1bn"` for a billion or `"7.3bn"` for 7 billion 300 million. The same is true for most large numbers. -```js -let billion = 1_000_000_000; -``` - -Here the underscore `_` plays the role of the "[syntactic sugar](https://en.wikipedia.org/wiki/Syntactic_sugar)", it makes the number more readable. The JavaScript engine simply ignores `_` between digits, so it's exactly the same one billion as above. - -In real life though, we try to avoid writing long sequences of zeroes. We're too lazy for that. We'll try to write something like `"1bn"` for a billion or `"7.3bn"` for 7 billion 300 million. The same is true for most large numbers. - -In JavaScript, we can shorten a number by appending the letter `"e"` to it and specifying the zeroes count: +In JavaScript, we shorten a number by appending the letter `"e"` to the number and specifying the zeroes count: ```js run let billion = 1e9; // 1 billion, literally: 1 and 9 zeroes -alert( 7.3e9 ); // 7.3 billions (same as 7300000000 or 7_300_000_000) +alert( 7.3e9 ); // 7.3 billions (7,300,000,000) ``` -In other words, `e` multiplies the number by `1` with the given zeroes count. +In other words, `"e"` multiplies the number by `1` with the given zeroes count. ```js -1e3 === 1 * 1000; // e3 means *1000 -1.23e6 === 1.23 * 1000000; // e6 means *1000000 +1e3 = 1 * 1000 +1.23e6 = 1.23 * 1000000 ``` Now let's write something very small. Say, 1 microsecond (one millionth of a second): ```js -let mсs = 0.000001; +let ms = 0.000001; ``` -Just like before, using `"e"` can help. If we'd like to avoid writing the zeroes explicitly, we could write the same as: +Just like before, using `"e"` can help. If we'd like to avoid writing the zeroes explicitly, we could say the same as: ```js -let mcs = 1e-6; // five zeroes to the left from 1 +let ms = 1e-6; // six zeroes to the left from 1 ``` -If we count the zeroes in `0.000001`, there are 6 of them. So naturally it's `1e-6`. +If we count the zeroes in `0.000001`, there are 6 of them. So naturally it's `1e-6`. In other words, a negative number after `"e"` means a division by 1 with the given number of zeroes: ```js // -3 divides by 1 with 3 zeroes -1e-3 === 1 / 1000; // 0.001 +1e-3 = 1 / 1000 (=0.001) // -6 divides by 1 with 6 zeroes -1.23e-6 === 1.23 / 1000000; // 0.00000123 - -// an example with a bigger number -1234e-2 === 1234 / 100; // 12.34, decimal point moves 2 times +1.23e-6 = 1.23 / 1000000 (=0.00000123) ``` ### Hex, binary and octal numbers @@ -121,7 +110,6 @@ Please note that two dots in `123456..toString(36)` is not a typo. If we want to If we placed a single dot: `123456.toString(36)`, then there would be an error, because JavaScript syntax implies the decimal part after the first dot. And if we place one more dot, then JavaScript knows that the decimal part is empty and now goes the method. Also could write `(123456).toString(36)`. - ``` ## Rounding @@ -137,7 +125,7 @@ There are several built-in functions for rounding: : Rounds up: `3.1` becomes `4`, and `-1.1` becomes `-1`. `Math.round` -: Rounds to the nearest integer: `3.1` becomes `3`, `3.6` becomes `4`, the middle case: `3.5` rounds up to `4` too. +: Rounds to the nearest integer: `3.1` becomes `3`, `3.6` becomes `4` and `-1.1` becomes `-1`. `Math.trunc` (not supported by Internet Explorer) : Removes anything after the decimal point without rounding: `3.1` becomes `3`, `-1.1` becomes `-1`. @@ -160,11 +148,11 @@ There are two ways to do so: 1. Multiply-and-divide. - For example, to round the number to the 2nd digit after the decimal, we can multiply the number by `100`, call the rounding function and then divide it back. + For example, to round the number to the 2nd digit after the decimal, we can multiply the number by `100` (or a bigger power of 10), call the rounding function and then divide it back. ```js run let num = 1.23456; - alert( Math.round(num * 100) / 100 ); // 1.23456 -> 123.456 -> 123 -> 1.23 + alert( Math.floor(num * 100) / 100 ); // 1.23456 -> 123.456 -> 123 -> 1.23 ``` 2. The method [toFixed(n)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toFixed) rounds the number to `n` digits after the point and returns a string representation of the result. @@ -181,20 +169,20 @@ There are two ways to do so: alert( num.toFixed(1) ); // "12.4" ``` - Please note that the result of `toFixed` is a string. If the decimal part is shorter than required, zeroes are appended to the end: + Please note that result of `toFixed` is a string. If the decimal part is shorter than required, zeroes are appended to the end: ```js run let num = 12.34; alert( num.toFixed(5) ); // "12.34000", added zeroes to make exactly 5 digits ``` - We can convert it to a number using the unary plus or a `Number()` call, e.g write `+num.toFixed(5)`. + We can convert it to a number using the unary plus or a `Number()` call: `+num.toFixed(5)`. ## Imprecise calculations -Internally, a number is represented in 64-bit format [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754), so there are exactly 64 bits to store a number: 52 of them are used to store the digits, 11 of them store the position of the decimal point, and 1 bit is for the sign. +Internally, a number is represented in 64-bit format [IEEE-754](https://en.wikipedia.org/wiki/IEEE_754-2008_revision), so there are exactly 64 bits to store a number: 52 of them are used to store the digits, 11 of them store the position of the decimal point (they are zero for integer numbers), and 1 bit is for the sign. -If a number is really huge, it may overflow the 64-bit storage and become a special numeric value `Infinity`: +If a number is too big, it would overflow the 64-bit storage, potentially giving an infinity: ```js run alert( 1e500 ); // Infinity @@ -202,7 +190,7 @@ alert( 1e500 ); // Infinity What may be a little less obvious, but happens quite often, is the loss of precision. -Consider this (falsy!) equality test: +Consider this (falsy!) test: ```js run alert( 0.1 + 0.2 == 0.3 ); // *!*false*/!* @@ -216,13 +204,13 @@ Strange! What is it then if not `0.3`? alert( 0.1 + 0.2 ); // 0.30000000000000004 ``` -Ouch! Imagine you're making an e-shopping site and the visitor puts `$0.10` and `$0.20` goods into their cart. The order total will be `$0.30000000000000004`. That would surprise anyone. +Ouch! There are more consequences than an incorrect comparison here. Imagine you're making an e-shopping site and the visitor puts `$0.10` and `$0.20` goods into their cart. The order total will be `$0.30000000000000004`. That would surprise anyone. But why does this happen? A number is stored in memory in its binary form, a sequence of bits - ones and zeroes. But fractions like `0.1`, `0.2` that look simple in the decimal numeric system are actually unending fractions in their binary form. -What is `0.1`? It is one divided by ten `1/10`, one-tenth. In decimal numeral system such numbers are easily representable. Compare it to one-third: `1/3`. It becomes an endless fraction `0.33333(3)`. +In other words, what is `0.1`? It is one divided by ten `1/10`, one-tenth. In decimal numeral system such numbers are easily representable. Compare it to one-third: `1/3`. It becomes an endless fraction `0.33333(3)`. So, division by powers `10` is guaranteed to work well in the decimal system, but division by `3` is not. For the same reason, in the binary numeral system, the division by powers of `2` is guaranteed to work, but `1/10` becomes an endless binary fraction. @@ -249,7 +237,7 @@ Can we work around the problem? Sure, the most reliable method is to round the r ```js run let sum = 0.1 + 0.2; -alert( sum.toFixed(2) ); // "0.30" +alert( sum.toFixed(2) ); // 0.30 ``` Please note that `toFixed` always returns a string. It ensures that it has 2 digits after the decimal point. That's actually convenient if we have an e-shopping and need to show `$0.30`. For other cases, we can use the unary plus to coerce it into a number: @@ -308,7 +296,7 @@ They belong to the type `number`, but are not "normal" numbers, so there are spe alert( isNaN("str") ); // true ``` - But do we need this function? Can't we just use the comparison `=== NaN`? Unfortunately not. The value `NaN` is unique in that it does not equal anything, including itself: + But do we need this function? Can't we just use the comparison `=== NaN`? Sorry, but the answer is no. The value `NaN` is unique in that it does not equal anything, including itself: ```js run alert( NaN === NaN ); // false @@ -332,46 +320,18 @@ let num = +prompt("Enter a number", ''); alert( isFinite(num) ); ``` -Please note that an empty or a space-only string is treated as `0` in all numeric functions including `isFinite`. +Please note that an empty or a space-only string is treated as `0` in all numeric functions including `isFinite`. -````smart header="`Number.isNaN` and `Number.isFinite`" -[Number.isNaN](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isNaN) and [Number.isFinite](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/isFinite) methods are the more "strict" versions of `isNaN` and `isFinite` functions. They do not autoconvert their argument into a number, but check if it belongs to the `number` type instead. +```smart header="Compare with `Object.is`" -- `Number.isNaN(value)` returns `true` if the argument belongs to the `number` type and it is `NaN`. In any other case it returns `false`. - - ```js run - alert( Number.isNaN(NaN) ); // true - alert( Number.isNaN("str" / 2) ); // true - - // Note the difference: - alert( Number.isNaN("str") ); // false, because "str" belongs to the string type, not the number type - alert( isNaN("str") ); // true, because isNaN converts string "str" into a number and gets NaN as a result of this conversion - ``` - -- `Number.isFinite(value)` returns `true` if the argument belongs to the `number` type and it is not `NaN/Infinity/-Infinity`. In any other case it returns `false`. - - ```js run - alert( Number.isFinite(123) ); // true - alert( Number.isFinite(Infinity) ); // false - alert( Number.isFinite(2 / 0) ); // false - - // Note the difference: - alert( Number.isFinite("123") ); // false, because "123" belongs to the string type, not the number type - alert( isFinite("123") ); // true, because isFinite converts string "123" into a number 123 - ``` - -In a way, `Number.isNaN` and `Number.isFinite` are simpler and more straightforward than `isNaN` and `isFinite` functions. In practice though, `isNaN` and `isFinite` are mostly used, as they're shorter to write. -```` - -```smart header="Comparison with `Object.is`" -There is a special built-in method `Object.is` that compares values like `===`, but is more reliable for two edge cases: +There is a special built-in method [`Object.is`](mdn:js/Object/is) that compares values like `===`, but is more reliable for two edge cases: 1. It works with `NaN`: `Object.is(NaN, NaN) === true`, that's a good thing. -2. Values `0` and `-0` are different: `Object.is(0, -0) === false`, technically that's correct, because internally the number has a sign bit that may be different even if all other bits are zeroes. +2. Values `0` and `-0` are different: `Object.is(0, -0) === false`, technically that's true, because internally the number has a sign bit that may be different even if all other bits are zeroes. In all other cases, `Object.is(a, b)` is the same as `a === b`. -We mention `Object.is` here, because it's often used in JavaScript specification. When an internal algorithm needs to compare two values for being exactly the same, it uses `Object.is` (internally called [SameValue](https://tc39.github.io/ecma262/#sec-samevalue)). +This way of comparison is often used in JavaScript specification. When an internal algorithm needs to compare two values for being exactly the same, it uses `Object.is` (internally called [SameValue](https://tc39.github.io/ecma262/#sec-samevalue)). ``` @@ -423,7 +383,7 @@ JavaScript has a built-in [Math](https://developer.mozilla.org/en/docs/Web/JavaS A few examples: `Math.random()` -: Returns a random number from 0 to 1 (not including 1). +: Returns a random number from 0 to 1 (not including 1) ```js run alert( Math.random() ); // 0.1234567894322 @@ -431,8 +391,8 @@ A few examples: alert( Math.random() ); // ... (any random numbers) ``` -`Math.max(a, b, c...)` and `Math.min(a, b, c...)` -: Returns the greatest and smallest from the arbitrary number of arguments. +`Math.max(a, b, c...)` / `Math.min(a, b, c...)` +: Returns the greatest/smallest from the arbitrary number of arguments. ```js run alert( Math.max(3, 5, -10, 0, 1) ); // 5 @@ -440,13 +400,13 @@ A few examples: ``` `Math.pow(n, power)` -: Returns `n` raised to the given power. +: Returns `n` raised the given power ```js run alert( Math.pow(2, 10) ); // 2 in power 10 = 1024 ``` -There are more functions and constants in `Math` object, including trigonometry, which you can find in the [docs for the Math object](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Math). +There are more functions and constants in `Math` object, including trigonometry, which you can find in the [docs for the Math](https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Math) object. ## Summary @@ -461,13 +421,6 @@ For different numeral systems: - `parseInt(str, base)` parses the string `str` into an integer in numeral system with given `base`, `2 ≤ base ≤ 36`. - `num.toString(base)` converts a number to a string in the numeral system with the given `base`. -For regular number tests: - -- `isNaN(value)` converts its argument to a number and then tests it for being `NaN` -- `Number.isNaN(value)` checks whether its argument belongs to the `number` type, and if so, tests it for being `NaN` -- `isFinite(value)` converts its argument to a number and then tests it for not being `NaN/Infinity/-Infinity` -- `Number.isFinite(value)` checks whether its argument belongs to the `number` type, and if so, tests it for not being `NaN/Infinity/-Infinity` - For converting values like `12pt` and `100px` to a number: - Use `parseInt/parseFloat` for the "soft" conversion, which reads a number from a string and then returns the value they could read before the error. diff --git a/1-js/05-data-types/03-string/1-ucfirst/solution.md b/1-js/05-data-types/03-string/1-ucfirst/solution.md index be5dd2aa..f7a332d0 100644 --- a/1-js/05-data-types/03-string/1-ucfirst/solution.md +++ b/1-js/05-data-types/03-string/1-ucfirst/solution.md @@ -8,7 +8,12 @@ let newStr = str[0].toUpperCase() + str.slice(1); There's a small problem though. If `str` is empty, then `str[0]` is `undefined`, and as `undefined` doesn't have the `toUpperCase()` method, we'll get an error. -The easiest way out is to add a test for an empty string, like this: +There are two variants here: + +1. Use `str.charAt(0)`, as it always returns a string (maybe empty). +2. Add a test for an empty string. + +Here's the 2nd variant: ```js run demo function ucFirst(str) { @@ -19,3 +24,4 @@ function ucFirst(str) { alert( ucFirst("john") ); // John ``` + diff --git a/1-js/05-data-types/03-string/3-truncate/solution.md b/1-js/05-data-types/03-string/3-truncate/solution.md index d51672ae..5546c47e 100644 --- a/1-js/05-data-types/03-string/3-truncate/solution.md +++ b/1-js/05-data-types/03-string/3-truncate/solution.md @@ -1,6 +1,6 @@ The maximal length must be `maxlength`, so we need to cut it a little shorter, to give space for the ellipsis. -Note that there is actually a single Unicode character for an ellipsis. That's not three dots. +Note that there is actually a single unicode character for an ellipsis. That's not three dots. ```js run demo function truncate(str, maxlength) { diff --git a/1-js/05-data-types/03-string/article.md b/1-js/05-data-types/03-string/article.md index 618f8ef3..765823d7 100644 --- a/1-js/05-data-types/03-string/article.md +++ b/1-js/05-data-types/03-string/article.md @@ -48,9 +48,9 @@ let guestList = "Guests: // Error: Unexpected token ILLEGAL * John"; ``` -Single and double quotes come from ancient times of language creation, when the need for multiline strings was not taken into account. Backticks appeared much later and thus are more versatile. +Single and double quotes come from ancient times of language creation when the need for multiline strings was not taken into account. Backticks appeared much later and thus are more versatile. -Backticks also allow us to specify a "template function" before the first backtick. The syntax is: func`string`. The function `func` is called automatically, receives the string and embedded expressions and can process them. This feature is called "tagged templates", it's rarely seen, but you can read about it in the MDN: [Template literals](mdn:/JavaScript/Reference/Template_literals#Tagged_templates). +Backticks also allow us to specify a "template function" before the first backtick. The syntax is: func`string`. The function `func` is called automatically, receives the string and embedded expressions and can process them. This is called "tagged templates". This feature makes it easier to implement custom templating, but is rarely used in practice. You can read more about it in the [manual](mdn:/JavaScript/Reference/Template_literals#Tagged_templates). ## Special characters @@ -59,10 +59,10 @@ It is still possible to create multiline strings with single and double quotes b ```js run let guestList = "Guests:\n * John\n * Pete\n * Mary"; -alert(guestList); // a multiline list of guests, same as above +alert(guestList); // a multiline list of guests ``` -As a simpler example, these two lines are equal, just written differently: +For example, these two lines are equal, just written differently: ```js run let str1 = "Hello\nWorld"; // two lines using a "newline symbol" @@ -74,26 +74,33 @@ World`; alert(str1 == str2); // true ``` -There are other, less common special characters: +There are other, less common "special" characters. + +Here's the full list: | Character | Description | |-----------|-------------| |`\n`|New line| -|`\r`|In Windows text files a combination of two characters `\r\n` represents a new break, while on non-Windows OS it's just `\n`. That's for historical reasons, most Windows software also understands `\n`. | -|`\'`, `\"`, \\`|Quotes| +|`\r`|Carriage return: not used alone. Windows text files use a combination of two characters `\r\n` to represent a line break. | +|`\'`, `\"`|Quotes| |`\\`|Backslash| |`\t`|Tab| -|`\b`, `\f`, `\v`| Backspace, Form Feed, Vertical Tab -- mentioned for completeness, coming from old times, not used nowadays (you can forget them right now). | +|`\b`, `\f`, `\v`| Backspace, Form Feed, Vertical Tab -- kept for compatibility, not used nowadays. | +|`\xXX`|Unicode character with the given hexadecimal unicode `XX`, e.g. `'\x7A'` is the same as `'z'`.| +|`\uXXXX`|A unicode symbol with the hex code `XXXX` in UTF-16 encoding, for instance `\u00A9` -- is a unicode for the copyright symbol `©`. It must be exactly 4 hex digits. | +|`\u{X…XXXXXX}` (1 to 6 hex characters)|A unicode symbol with the given UTF-32 encoding. Some rare characters are encoded with two unicode symbols, taking 4 bytes. This way we can insert long codes. | -As you can see, all special characters start with a backslash character `\`. It is also called an "escape character". - -Because it's so special, if we need to show an actual backslash `\` within the string, we need to double it: +Examples with unicode: ```js run -alert( `The backslash: \\` ); // The backslash: \ +alert( "\u00A9" ); // © +alert( "\u{20331}" ); // 佫, a rare Chinese hieroglyph (long unicode) +alert( "\u{1F60D}" ); // 😍, a smiling face symbol (another long unicode) ``` -So-called "escaped" quotes `\'`, `\"`, \\` are used to insert a quote into the same-quoted string. +All special characters start with a backslash character `\`. It is also called an "escape character". + +We might also use it if we wanted to insert a quote into the string. For instance: @@ -103,13 +110,21 @@ alert( 'I*!*\'*/!*m the Walrus!' ); // *!*I'm*/!* the Walrus! As you can see, we have to prepend the inner quote by the backslash `\'`, because otherwise it would indicate the string end. -Of course, only the quotes that are the same as the enclosing ones need to be escaped. So, as a more elegant solution, we could switch to double quotes or backticks instead: +Of course, only to the quotes that are the same as the enclosing ones need to be escaped. So, as a more elegant solution, we could switch to double quotes or backticks instead: ```js run -alert( "I'm the Walrus!" ); // I'm the Walrus! +alert( `I'm the Walrus!` ); // I'm the Walrus! ``` -Besides these special characters, there's also a special notation for Unicode codes `\u…`, it's rarely used and is covered in the optional chapter about [Unicode](info:unicode). +Note that the backslash `\` serves for the correct reading of the string by JavaScript, then disappears. The in-memory string has no `\`. You can clearly see that in `alert` from the examples above. + +But what if we need to show an actual backslash `\` within the string? + +That's possible, but we need to double it like `\\`: + +```js run +alert( `The backslash: \\` ); // The backslash: \ +``` ## String length @@ -124,36 +139,33 @@ Note that `\n` is a single "special" character, so the length is indeed `3`. ```warn header="`length` is a property" People with a background in some other languages sometimes mistype by calling `str.length()` instead of just `str.length`. That doesn't work. -Please note that `str.length` is a numeric property, not a function. There is no need to add parenthesis after it. Not `.length()`, but `.length`. +Please note that `str.length` is a numeric property, not a function. There is no need to add parenthesis after it. ``` ## Accessing characters -To get a character at position `pos`, use square brackets `[pos]` or call the method [str.at(pos)](mdn:js/String/at). The first character starts from the zero position: +To get a character at position `pos`, use square brackets `[pos]` or call the method [str.charAt(pos)](mdn:js/String/charAt). The first character starts from the zero position: ```js run let str = `Hello`; // the first character alert( str[0] ); // H -alert( str.at(0) ); // H +alert( str.charAt(0) ); // H // the last character alert( str[str.length - 1] ); // o -alert( str.at(-1) ); ``` -As you can see, the `.at(pos)` method has a benefit of allowing negative position. If `pos` is negative, then it's counted from the end of the string. +The square brackets are a modern way of getting a character, while `charAt` exists mostly for historical reasons. -So `.at(-1)` means the last character, and `.at(-2)` is the one before it, etc. - -The square brackets always return `undefined` for negative indexes, for instance: +The only difference between them is that if no character is found, `[]` returns `undefined`, and `charAt` returns an empty string: ```js run let str = `Hello`; -alert( str[-2] ); // undefined -alert( str.at(-2) ); // l +alert( str[1000] ); // undefined +alert( str.charAt(1000) ); // '' (an empty string) ``` We can also iterate over characters using `for..of`: @@ -202,7 +214,7 @@ alert( 'Interface'.toLowerCase() ); // interface Or, if we want a single character lowercased: -```js run +```js alert( 'Interface'[0].toLowerCase() ); // 'i' ``` @@ -227,7 +239,7 @@ alert( str.indexOf('widget') ); // -1, not found, the search is case-sensitive alert( str.indexOf("id") ); // 1, "id" is found at the position 1 (..idget with id) ``` -The optional second parameter allows us to start searching from a given position. +The optional second parameter allows us to search starting from the given position. For instance, the first occurrence of `"id"` is at position `1`. To look for the next occurrence, let's start the search from position `2`: @@ -298,6 +310,45 @@ if (str.indexOf("Widget") != -1) { } ``` +#### The bitwise NOT trick + +One of the old tricks used here is the [bitwise NOT](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Bitwise_Operators#Bitwise_NOT) `~` operator. It converts the number to a 32-bit integer (removes the decimal part if exists) and then reverses all bits in its binary representation. + +In practice, that means a simple thing: for 32-bit integers `~n` equals `-(n+1)`. + +For instance: + +```js run +alert( ~2 ); // -3, the same as -(2+1) +alert( ~1 ); // -2, the same as -(1+1) +alert( ~0 ); // -1, the same as -(0+1) +*!* +alert( ~-1 ); // 0, the same as -(-1+1) +*/!* +``` + +As we can see, `~n` is zero only if `n == -1` (that's for any 32-bit signed integer `n`). + +So, the test `if ( ~str.indexOf("...") )` is truthy only if the result of `indexOf` is not `-1`. In other words, when there is a match. + +People use it to shorten `indexOf` checks: + +```js run +let str = "Widget"; + +if (~str.indexOf("Widget")) { + alert( 'Found it!' ); // works +} +``` + +It is usually not recommended to use language features in a non-obvious way, but this particular trick is widely used in old code, so we should understand it. + +Just remember: `if (~str.indexOf(...))` reads as "if found". + +To be precise though, as big numbers are truncated to 32 bits by `~` operator, there exist other numbers that give `0`, the smallest is `~4294967295=0`. That makes such check is correct only if a string is not that long. + +Right now we can see this trick only in the old code, as modern JavaScript provides `.includes` method (see below). + ### includes, startsWith, endsWith The more modern method [str.includes(substr, pos)](mdn:js/String/includes) returns `true/false` depending on whether `str` contains `substr` within. @@ -320,8 +371,8 @@ alert( "Widget".includes("id", 3) ); // false, from position 3 there is no "id" The methods [str.startsWith](mdn:js/String/startsWith) and [str.endsWith](mdn:js/String/endsWith) do exactly what they say: ```js run -alert( "*!*Wid*/!*get".startsWith("Wid") ); // true, "Widget" starts with "Wid" -alert( "Wid*!*get*/!*".endsWith("get") ); // true, "Widget" ends with "get" +alert( "Widget".startsWith("Wid") ); // true, "Widget" starts with "Wid" +alert( "Widget".endsWith("get") ); // true, "Widget" ends with "get" ``` ## Getting a substring @@ -356,9 +407,9 @@ There are 3 methods in JavaScript to get a substring: `substring`, `substr` and ``` `str.substring(start [, end])` -: Returns the part of the string *between* `start` and `end` (not including `end`). +: Returns the part of the string *between* `start` and `end`. - This is almost the same as `slice`, but it allows `start` to be greater than `end` (in this case it simply swaps `start` and `end` values). + This is almost the same as `slice`, but it allows `start` to be greater than `end`. For instance: @@ -394,22 +445,18 @@ There are 3 methods in JavaScript to get a substring: `substring`, `substr` and alert( str.substr(-4, 2) ); // 'gi', from the 4th position get 2 characters ``` - This method resides in the [Annex B](https://tc39.es/ecma262/#sec-string.prototype.substr) of the language specification. It means that only browser-hosted Javascript engines should support it, and it's not recommended to use it. In practice, it's supported everywhere. - Let's recap these methods to avoid any confusion: | method | selects... | negatives | |--------|-----------|-----------| | `slice(start, end)` | from `start` to `end` (not including `end`) | allows negatives | -| `substring(start, end)` | between `start` and `end` (not including `end`)| negative values mean `0` | +| `substring(start, end)` | between `start` and `end` | negative values mean `0` | | `substr(start, length)` | from `start` get `length` characters | allows negative `start` | ```smart header="Which one to choose?" All of them can do the job. Formally, `substr` has a minor drawback: it is described not in the core JavaScript specification, but in Annex B, which covers browser-only features that exist mainly for historical reasons. So, non-browser environments may fail to support it. But in practice it works everywhere. -Of the other two variants, `slice` is a little bit more flexible, it allows negative arguments and shorter to write. - -So, for practical use it's enough to remember only `slice`. +Of the other two variants, `slice` is a little bit more flexible, it allows negative arguments and shorter to write. So, it's enough to remember solely `slice` of these three methods. ``` ## Comparing strings @@ -432,18 +479,17 @@ Although, there are some oddities. This may lead to strange results if we sort these country names. Usually people would expect `Zealand` to come after `Österreich` in the list. -To understand what happens, we should be aware that strings in Javascript are encoded using [UTF-16](https://en.wikipedia.org/wiki/UTF-16). That is: each character has a corresponding numeric code. +To understand what happens, let's review the internal representation of strings in JavaScript. -There are special methods that allow to get the character for the code and back: +All strings are encoded using [UTF-16](https://en.wikipedia.org/wiki/UTF-16). That is: each character has a corresponding numeric code. There are special methods that allow to get the character for the code and back. `str.codePointAt(pos)` -: Returns a decimal number representing the code for the character at position `pos`: +: Returns the code for the character at position `pos`: ```js run // different case letters have different codes - alert( "Z".codePointAt(0) ); // 90 alert( "z".codePointAt(0) ); // 122 - alert( "z".codePointAt(0).toString(16) ); // 7a (if we need a hexadecimal value) + alert( "Z".codePointAt(0) ); // 90 ``` `String.fromCodePoint(code)` @@ -451,7 +497,13 @@ There are special methods that allow to get the character for the code and back: ```js run alert( String.fromCodePoint(90) ); // Z - alert( String.fromCodePoint(0x5a) ); // Z (we can also use a hex value as an argument) + ``` + + We can also add unicode characters by their codes using `\u` followed by the hex code: + + ```js run + // 90 is 5a in hexadecimal system + alert( '\u005a' ); // Z ``` Now let's see the characters with codes `65..220` (the latin alphabet and a little bit extra) by making a string of them: @@ -463,7 +515,6 @@ for (let i = 65; i <= 220; i++) { str += String.fromCodePoint(i); } alert( str ); -// Output: // ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~€‚ƒ„ // ¡¢£¤¥¦§¨©ª«¬­®¯°±²³´µ¶·¸¹º»¼½¾¿ÀÁÂÃÄÅÆÇÈÉÊËÌÍÎÏÐÑÒÓÔÕÖרÙÚÛÜ ``` @@ -475,15 +526,15 @@ Now it becomes obvious why `a > Z`. The characters are compared by their numeric code. The greater code means that the character is greater. The code for `a` (97) is greater than the code for `Z` (90). - All lowercase letters go after uppercase letters because their codes are greater. -- Some letters like `Ö` stand apart from the main alphabet. Here, its code is greater than anything from `a` to `z`. +- Some letters like `Ö` stand apart from the main alphabet. Here, it's code is greater than anything from `a` to `z`. -### Correct comparisons [#correct-comparisons] +### Correct comparisons The "right" algorithm to do string comparisons is more complex than it may seem, because alphabets are different for different languages. So, the browser needs to know the language to compare. -Luckily, modern browsers support the internationalization standard [ECMA-402](https://www.ecma-international.org/publications-and-standards/standards/ecma-402/). +Luckily, all modern browsers (IE10- requires the additional library [Intl.js](https://github.com/andyearnshaw/Intl.js/)) support the internationalization standard [ECMA-402](http://www.ecma-international.org/ecma-402/1.0/ECMA-402.pdf). It provides a special method to compare strings in different languages, following their rules. @@ -501,10 +552,118 @@ alert( 'Österreich'.localeCompare('Zealand') ); // -1 This method actually has two additional arguments specified in [the documentation](mdn:js/String/localeCompare), which allows it to specify the language (by default taken from the environment, letter order depends on the language) and setup additional rules like case sensitivity or should `"a"` and `"á"` be treated as the same etc. +## Internals, Unicode + +```warn header="Advanced knowledge" +The section goes deeper into string internals. This knowledge will be useful for you if you plan to deal with emoji, rare mathematical or hieroglyphic characters or other rare symbols. + +You can skip the section if you don't plan to support them. +``` + +### Surrogate pairs + +All frequently used characters have 2-byte codes. Letters in most european languages, numbers, and even most hieroglyphs, have a 2-byte representation. + +But 2 bytes only allow 65536 combinations and that's not enough for every possible symbol. So rare symbols are encoded with a pair of 2-byte characters called "a surrogate pair". + +The length of such symbols is `2`: + +```js run +alert( '𝒳'.length ); // 2, MATHEMATICAL SCRIPT CAPITAL X +alert( '😂'.length ); // 2, FACE WITH TEARS OF JOY +alert( '𩷶'.length ); // 2, a rare Chinese hieroglyph +``` + +Note that surrogate pairs did not exist at the time when JavaScript was created, and thus are not correctly processed by the language! + +We actually have a single symbol in each of the strings above, but the `length` shows a length of `2`. + +`String.fromCodePoint` and `str.codePointAt` are few rare methods that deal with surrogate pairs right. They recently appeared in the language. Before them, there were only [String.fromCharCode](mdn:js/String/fromCharCode) and [str.charCodeAt](mdn:js/String/charCodeAt). These methods are actually the same as `fromCodePoint/codePointAt`, but don't work with surrogate pairs. + +Getting a symbol can be tricky, because surrogate pairs are treated as two characters: + +```js run +alert( '𝒳'[0] ); // strange symbols... +alert( '𝒳'[1] ); // ...pieces of the surrogate pair +``` + +Note that pieces of the surrogate pair have no meaning without each other. So the alerts in the example above actually display garbage. + +Technically, surrogate pairs are also detectable by their codes: if a character has the code in the interval of `0xd800..0xdbff`, then it is the first part of the surrogate pair. The next character (second part) must have the code in interval `0xdc00..0xdfff`. These intervals are reserved exclusively for surrogate pairs by the standard. + +In the case above: + +```js run +// charCodeAt is not surrogate-pair aware, so it gives codes for parts + +alert( '𝒳'.charCodeAt(0).toString(16) ); // d835, between 0xd800 and 0xdbff +alert( '𝒳'.charCodeAt(1).toString(16) ); // dcb3, between 0xdc00 and 0xdfff +``` + +You will find more ways to deal with surrogate pairs later in the chapter . There are probably special libraries for that too, but nothing famous enough to suggest here. + +### Diacritical marks and normalization + +In many languages there are symbols that are composed of the base character with a mark above/under it. + +For instance, the letter `a` can be the base character for: `àáâäãåā`. Most common "composite" character have their own code in the UTF-16 table. But not all of them, because there are too many possible combinations. + +To support arbitrary compositions, UTF-16 allows us to use several unicode characters: the base character followed by one or many "mark" characters that "decorate" it. + +For instance, if we have `S` followed by the special "dot above" character (code `\u0307`), it is shown as Ṡ. + +```js run +alert( 'S\u0307' ); // Ṡ +``` + +If we need an additional mark above the letter (or below it) -- no problem, just add the necessary mark character. + +For instance, if we append a character "dot below" (code `\u0323`), then we'll have "S with dots above and below": `Ṩ`. + +For example: + +```js run +alert( 'S\u0307\u0323' ); // Ṩ +``` + +This provides great flexibility, but also an interesting problem: two characters may visually look the same, but be represented with different unicode compositions. + +For instance: + +```js run +let s1 = 'S\u0307\u0323'; // Ṩ, S + dot above + dot below +let s2 = 'S\u0323\u0307'; // Ṩ, S + dot below + dot above + +alert( `s1: ${s1}, s2: ${s2}` ); + +alert( s1 == s2 ); // false though the characters look identical (?!) +``` + +To solve this, there exists a "unicode normalization" algorithm that brings each string to the single "normal" form. + +It is implemented by [str.normalize()](mdn:js/String/normalize). + +```js run +alert( "S\u0307\u0323".normalize() == "S\u0323\u0307".normalize() ); // true +``` + +It's funny that in our situation `normalize()` actually brings together a sequence of 3 characters to one: `\u1e68` (S with two dots). + +```js run +alert( "S\u0307\u0323".normalize().length ); // 1 + +alert( "S\u0307\u0323".normalize() == "\u1e68" ); // true +``` + +In reality, this is not always the case. The reason being that the symbol `Ṩ` is "common enough", so UTF-16 creators included it in the main table and gave it the code. + +If you want to learn more about normalization rules and variants -- they are described in the appendix of the Unicode standard: [Unicode Normalization Forms](http://www.unicode.org/reports/tr15/), but for most practical purposes the information from this section is enough. + ## Summary - There are 3 types of quotes. Backticks allow a string to span multiple lines and embed expressions `${…}`. -- We can use special characters, such as a line break `\n`. +- Strings in JavaScript are encoded using UTF-16. +- We can use special characters like `\n` and insert letters by their unicode using `\u...`. - To get a character, use: `[]`. - To get a substring, use: `slice` or `substring`. - To lowercase/uppercase a string, use: `toLowerCase/toUpperCase`. @@ -518,5 +677,3 @@ There are several other helpful methods in strings: - ...and more to be found in the [manual](mdn:js/String). Strings also have methods for doing search/replace with regular expressions. But that's big topic, so it's explained in a separate tutorial section . - -Also, as of now it's important to know that strings are based on Unicode encoding, and hence there're issues with comparisons. There's more about Unicode in the chapter . \ No newline at end of file diff --git a/1-js/05-data-types/04-array/10-maximal-subarray/solution.md b/1-js/05-data-types/04-array/10-maximal-subarray/solution.md index 7e1ca3bd..daadf494 100644 --- a/1-js/05-data-types/04-array/10-maximal-subarray/solution.md +++ b/1-js/05-data-types/04-array/10-maximal-subarray/solution.md @@ -57,9 +57,9 @@ alert( getMaxSubSum([1, 2, 3]) ); // 6 alert( getMaxSubSum([100, -9, 2, -3, 5]) ); // 100 ``` -The solution has a time complexity of [O(n2)](https://en.wikipedia.org/wiki/Big_O_notation). In other words, if we increase the array size 2 times, the algorithm will work 4 times longer. +The solution has a time complexety of [O(n2)](https://en.wikipedia.org/wiki/Big_O_notation). In other words, if we increase the array size 2 times, the algorithm will work 4 times longer. -For big arrays (1000, 10000 or more items) such algorithms can lead to serious sluggishness. +For big arrays (1000, 10000 or more items) such algorithms can lead to a serious sluggishness. # Fast solution @@ -91,4 +91,4 @@ alert( getMaxSubSum([-1, -2, -3]) ); // 0 The algorithm requires exactly 1 array pass, so the time complexity is O(n). -You can find more detailed information about the algorithm here: [Maximum subarray problem](http://en.wikipedia.org/wiki/Maximum_subarray_problem). If it's still not obvious why that works, then please trace the algorithm on the examples above, see how it works, that's better than any words. +You can find more detail information about the algorithm here: [Maximum subarray problem](http://en.wikipedia.org/wiki/Maximum_subarray_problem). If it's still not obvious why that works, then please trace the algorithm on the examples above, see how it works, that's better than any words. diff --git a/1-js/05-data-types/04-array/2-create-array/task.md b/1-js/05-data-types/04-array/2-create-array/task.md index d4551c79..16d14071 100644 --- a/1-js/05-data-types/04-array/2-create-array/task.md +++ b/1-js/05-data-types/04-array/2-create-array/task.md @@ -8,7 +8,7 @@ Let's try 5 array operations. 1. Create an array `styles` with items "Jazz" and "Blues". 2. Append "Rock-n-Roll" to the end. -3. Replace the value in the middle with "Classics". Your code for finding the middle value should work for any arrays with odd length. +3. Replace the value in the middle by "Classics". Your code for finding the middle value should work for any arrays with odd length. 4. Strip off the first value of the array and show it. 5. Prepend `Rap` and `Reggae` to the array. diff --git a/1-js/05-data-types/04-array/3-call-array-this/task.md b/1-js/05-data-types/04-array/3-call-array-this/task.md index f1e13499..340c5fee 100644 --- a/1-js/05-data-types/04-array/3-call-array-this/task.md +++ b/1-js/05-data-types/04-array/3-call-array-this/task.md @@ -11,7 +11,7 @@ let arr = ["a", "b"]; arr.push(function() { alert( this ); -}); +}) arr[2](); // ? ``` diff --git a/1-js/05-data-types/04-array/array-pop.svg b/1-js/05-data-types/04-array/array-pop.svg index 82b112b4..35191605 100644 --- a/1-js/05-data-types/04-array/array-pop.svg +++ b/1-js/05-data-types/04-array/array-pop.svg @@ -1 +1 @@ -0123"Apple""Orange""Pear""Lemon"length = 4clear012"Apple""Orange""Pear"length = 3 \ No newline at end of file +0123"Apple""Orange""Pear""Lemon"length = 4clear012"Apple""Orange""Pear"length = 3 \ No newline at end of file diff --git a/1-js/05-data-types/04-array/array-shift.svg b/1-js/05-data-types/04-array/array-shift.svg index 9485a3c9..09236b9d 100644 --- a/1-js/05-data-types/04-array/array-shift.svg +++ b/1-js/05-data-types/04-array/array-shift.svg @@ -1 +1 @@ -123"Orange""Pear""Lemon"length = 423"Orange""Pear""Lemon"length = 3clearmove elements to the left0"Apple"012"Orange""Pear""Lemon"11 \ No newline at end of file +123"Orange""Pear""Lemon"length = 423"Orange""Pear""Lemon"length = 3clearmove elements to the left0"Apple"012"Orange""Pear""Lemon"11 \ No newline at end of file diff --git a/1-js/05-data-types/04-array/array-speed.svg b/1-js/05-data-types/04-array/array-speed.svg index 41f7d998..5660cd5e 100644 --- a/1-js/05-data-types/04-array/array-speed.svg +++ b/1-js/05-data-types/04-array/array-speed.svg @@ -1 +1 @@ -0123popunshiftpushshift \ No newline at end of file +0123popunshiftpushshift \ No newline at end of file diff --git a/1-js/05-data-types/04-array/article.md b/1-js/05-data-types/04-array/article.md index 4bcab0bc..eaa3ea00 100644 --- a/1-js/05-data-types/04-array/article.md +++ b/1-js/05-data-types/04-array/article.md @@ -92,38 +92,6 @@ let fruits = [ The "trailing comma" style makes it easier to insert/remove items, because all lines become alike. ```` -## Get last elements with "at" - -[recent browser="new"] - -Let's say we want the last element of the array. - -Some programming languages allow to use negative indexes for the same purpose, like `fruits[-1]`. - -Although, in JavaScript it won't work. The result will be `undefined`, because the index in square brackets is treated literally. - -We can explicitly calculate the last element index and then access it: `fruits[fruits.length - 1]`. - -```js run -let fruits = ["Apple", "Orange", "Plum"]; - -alert( fruits[fruits.length-1] ); // Plum -``` - -A bit cumbersome, isn't it? We need to write the variable name twice. - -Luckily, there's a shorter syntax: `fruits.at(-1)`: - -```js run -let fruits = ["Apple", "Orange", "Plum"]; - -// same as fruits[fruits.length-1] -alert( fruits.at(-1) ); // Plum -``` - -In other words, `arr.at(i)`: -- is exactly the same as `arr[i]`, if `i >= 0`. -- for negative values of `i`, it steps back from the end of the array. ## Methods pop/push, shift/unshift @@ -153,9 +121,9 @@ A stack is usually illustrated as a pack of cards: new cards are added to the to For stacks, the latest pushed item is received first, that's also called LIFO (Last-In-First-Out) principle. For queues, we have FIFO (First-In-First-Out). -Arrays in JavaScript can work both as a queue and as a stack. They allow you to add/remove elements, both to/from the beginning or the end. +Arrays in JavaScript can work both as a queue and as a stack. They allow you to add/remove elements both to/from the beginning or the end. -In computer science, the data structure that allows this, is called [deque](https://en.wikipedia.org/wiki/Double-ended_queue). +In computer science the data structure that allows this, is called [deque](https://en.wikipedia.org/wiki/Double-ended_queue). **Methods that work with the end of the array:** @@ -170,8 +138,6 @@ In computer science, the data structure that allows this, is called [deque](http alert( fruits ); // Apple, Orange ``` - Both `fruits.pop()` and `fruits.at(-1)` return the last element of the array, but `fruits.pop()` also modifies the array by removing it. - `push` : Append the element to the end of the array: @@ -243,7 +209,7 @@ arr.push("Pear"); // modify the array by reference alert( fruits ); // Banana, Pear - 2 items now ``` -...But what makes arrays really special is their internal representation. The engine tries to store its elements in the contiguous memory area, one after another, just as depicted on the illustrations in this chapter, and there are other optimizations as well, to make arrays work really fast. +...But what makes arrays really special is their internal representation. The engine tries to store its elements in the contiguous memory area, one after another, just as depicted on the illustrations in this chapter, and there are other optimizations as well, to make arrays work really fast. But they all break if we quit working with an array as with an "ordered collection" and start working with it as if it were a regular object. @@ -281,7 +247,7 @@ Why is it faster to work with the end of an array than with its beginning? Let's fruits.shift(); // take 1 element from the start ``` -It's not enough to take and remove the element with the index `0`. Other elements need to be renumbered as well. +It's not enough to take and remove the element with the number `0`. Other elements need to be renumbered as well. The `shift` operation must do 3 things: @@ -399,11 +365,11 @@ There is one more syntax to create an array: let arr = *!*new Array*/!*("Apple", "Pear", "etc"); ``` -It's rarely used, because square brackets `[]` are shorter. Also, there's a tricky feature with it. +It's rarely used, because square brackets `[]` are shorter. Also there's a tricky feature with it. If `new Array` is called with a single argument which is a number, then it creates an array *without items, but with the given length*. -Let's see how one can shoot themselves in the foot: +Let's see how one can shoot themself in the foot: ```js run let arr = new Array(2); // will it create an array of [2] ? @@ -413,7 +379,9 @@ alert( arr[0] ); // undefined! no elements. alert( arr.length ); // length 2 ``` -To avoid such surprises, we usually use square brackets, unless we really know what we're doing. +In the code above, `new Array(number)` has all elements `undefined`. + +To evade such surprises, we usually use square brackets, unless we really know what we're doing. ## Multidimensional arrays @@ -470,10 +438,10 @@ This operator has no special treatment for arrays, it works with them as with an Let's recall the rules: - Two objects are equal `==` only if they're references to the same object. -- If one of the arguments of `==` is an object, and the other one is a primitive, then the object gets converted to primitive, as explained in the chapter . +- If one of arguments of `==` is an object, and the other one is a primitive, then the object gets converted to primitive, as explained in the chapter . - ...With an exception of `null` and `undefined` that equal `==` each other and nothing else. -The strict comparison `===` is even simpler, as it doesn't convert types. +The strict comparison `===` is even simpler, as it doesn't convert types. So, if we compare arrays with `==`, they are never the same, unless we compare two variables that reference exactly the same array. @@ -493,7 +461,7 @@ alert( 0 == [] ); // true alert('0' == [] ); // false ``` -Here, in both cases, we compare a primitive with an array object. So the array `[]` gets converted to primitive for the purpose of comparison and becomes an empty string `''`. +Here, in both cases, we compare a primitive with an array object. So the array `[]` gets converted to primitive for the purpose of comparison and becomes an empty string `''`. Then the comparison process goes on with the primitives, as described in the chapter : @@ -512,26 +480,21 @@ That's simple: don't use the `==` operator. Instead, compare them item-by-item i Array is a special kind of object, suited to storing and managing ordered data items. -The declaration: +- The declaration: -```js -// square brackets (usual) -let arr = [item1, item2...]; + ```js + // square brackets (usual) + let arr = [item1, item2...]; -// new Array (exceptionally rare) -let arr = new Array(item1, item2...); -``` + // new Array (exceptionally rare) + let arr = new Array(item1, item2...); + ``` -The call to `new Array(number)` creates an array with the given length, but without elements. + The call to `new Array(number)` creates an array with the given length, but without elements. - The `length` property is the array length or, to be precise, its last numeric index plus one. It is auto-adjusted by array methods. - If we shorten `length` manually, the array is truncated. -Getting the elements: - -- we can get element by its index, like `arr[0]` -- also we can use `at(i)` method that allows negative indexes. For negative values of `i`, it steps back from the end of the array. If `i >= 0`, it works same as `arr[i]`. - We can use an array as a deque with the following operations: - `push(...items)` adds `items` to the end. diff --git a/1-js/05-data-types/04-array/queue.svg b/1-js/05-data-types/04-array/queue.svg index e89a3dcd..0ed2f1cd 100644 --- a/1-js/05-data-types/04-array/queue.svg +++ b/1-js/05-data-types/04-array/queue.svg @@ -1 +1 @@ -pushshift \ No newline at end of file +pushshift \ No newline at end of file diff --git a/1-js/05-data-types/04-array/stack.svg b/1-js/05-data-types/04-array/stack.svg index ae0c474d..dcc600e7 100644 --- a/1-js/05-data-types/04-array/stack.svg +++ b/1-js/05-data-types/04-array/stack.svg @@ -1 +1 @@ -pushpop \ No newline at end of file +pushpop \ No newline at end of file diff --git a/1-js/05-data-types/05-array-methods/12-reduce-object/task.md b/1-js/05-data-types/05-array-methods/12-reduce-object/task.md index 7f008235..d3c8f8eb 100644 --- a/1-js/05-data-types/05-array-methods/12-reduce-object/task.md +++ b/1-js/05-data-types/05-array-methods/12-reduce-object/task.md @@ -4,7 +4,7 @@ importance: 4 # Create keyed object from array -Let's say we received an array of users in the form `{id:..., name:..., age:... }`. +Let's say we received an array of users in the form `{id:..., name:..., age... }`. Create a function `groupById(arr)` that creates an object from it, with `id` as the key, and array items as values. diff --git a/1-js/05-data-types/05-array-methods/2-filter-range/task.md b/1-js/05-data-types/05-array-methods/2-filter-range/task.md index 46e47c93..18b2c1d9 100644 --- a/1-js/05-data-types/05-array-methods/2-filter-range/task.md +++ b/1-js/05-data-types/05-array-methods/2-filter-range/task.md @@ -4,7 +4,7 @@ importance: 4 # Filter range -Write a function `filterRange(arr, a, b)` that gets an array `arr`, looks for elements with values higher or equal to `a` and lower or equal to `b` and return a result as an array. +Write a function `filterRange(arr, a, b)` that gets an array `arr`, looks for elements between `a` and `b` in it and returns an array of them. The function should not modify the array. It should return the new array. diff --git a/1-js/05-data-types/05-array-methods/3-filter-range-in-place/_js.view/test.js b/1-js/05-data-types/05-array-methods/3-filter-range-in-place/_js.view/test.js index 241b74c6..db32d9a1 100644 --- a/1-js/05-data-types/05-array-methods/3-filter-range-in-place/_js.view/test.js +++ b/1-js/05-data-types/05-array-methods/3-filter-range-in-place/_js.view/test.js @@ -4,13 +4,13 @@ describe("filterRangeInPlace", function() { let arr = [5, 3, 8, 1]; - filterRangeInPlace(arr, 2, 5); + filterRangeInPlace(arr, 1, 4); - assert.deepEqual(arr, [5, 3]); + assert.deepEqual(arr, [3, 1]); }); it("doesn't return anything", function() { assert.isUndefined(filterRangeInPlace([1,2,3], 1, 4)); }); -}); +}); \ No newline at end of file diff --git a/1-js/05-data-types/05-array-methods/7-map-objects/solution.md b/1-js/05-data-types/05-array-methods/7-map-objects/solution.md index 2d8d4fb0..5d8bf4a1 100644 --- a/1-js/05-data-types/05-array-methods/7-map-objects/solution.md +++ b/1-js/05-data-types/05-array-methods/7-map-objects/solution.md @@ -25,7 +25,7 @@ alert( usersMapped[0].id ); // 1 alert( usersMapped[0].fullName ); // John Smith ``` -Please note that in the arrow functions we need to use additional brackets. +Please note that in for the arrow functions we need to use additional brackets. We can't write like this: ```js diff --git a/1-js/05-data-types/05-array-methods/8-sort-objects/solution.md b/1-js/05-data-types/05-array-methods/8-sort-objects/solution.md index cfaf9761..9f1ade70 100644 --- a/1-js/05-data-types/05-array-methods/8-sort-objects/solution.md +++ b/1-js/05-data-types/05-array-methods/8-sort-objects/solution.md @@ -1,6 +1,6 @@ ```js run no-beautify function sortByAge(arr) { - arr.sort((a, b) => a.age - b.age); + arr.sort((a, b) => a.age > b.age ? 1 : -1); } let john = { name: "John", age: 25 }; diff --git a/1-js/05-data-types/05-array-methods/article.md b/1-js/05-data-types/05-array-methods/article.md index feb626f9..3afba386 100644 --- a/1-js/05-data-types/05-array-methods/article.md +++ b/1-js/05-data-types/05-array-methods/article.md @@ -44,7 +44,7 @@ The syntax is: arr.splice(start[, deleteCount, elem1, ..., elemN]) ``` -It modifies `arr` starting from the index `start`: removes `deleteCount` elements and then inserts `elem1, ..., elemN` at their place. Returns the array of removed elements. +It modified `arr` starting from the index `start`: removes `deleteCount` elements and then inserts `elem1, ..., elemN` at their place. Returns the array of removed elements. This method is easy to grasp by examples. @@ -234,13 +234,12 @@ Now let's cover methods that search in an array. ### indexOf/lastIndexOf and includes -The methods [arr.indexOf](mdn:js/Array/indexOf) and [arr.includes](mdn:js/Array/includes) have the similar syntax and do essentially the same as their string counterparts, but operate on items instead of characters: +The methods [arr.indexOf](mdn:js/Array/indexOf), [arr.lastIndexOf](mdn:js/Array/lastIndexOf) and [arr.includes](mdn:js/Array/includes) have the same syntax and do essentially the same as their string counterparts, but operate on items instead of characters: - `arr.indexOf(item, from)` -- looks for `item` starting from index `from`, and returns the index where it was found, otherwise `-1`. +- `arr.lastIndexOf(item, from)` -- same, but looks for from right to left. - `arr.includes(item, from)` -- looks for `item` starting from index `from`, returns `true` if found. -Usually these methods are used with only one argument: the `item` to search. By default, the search is from the beginning. - For instance: ```js run @@ -253,31 +252,19 @@ alert( arr.indexOf(null) ); // -1 alert( arr.includes(1) ); // true ``` -Please note that `indexOf` uses the strict equality `===` for comparison. So, if we look for `false`, it finds exactly `false` and not the zero. +Note that the methods use `===` comparison. So, if we look for `false`, it finds exactly `false` and not the zero. -If we want to check if `item` exists in the array, and don't need the exact index, then `arr.includes` is preferred. +If we want to check for inclusion, and don't want to know the exact index, then `arr.includes` is preferred. -The method [arr.lastIndexOf](mdn:js/Array/lastIndexOf) is the same as `indexOf`, but looks for from right to left. - -```js run -let fruits = ['Apple', 'Orange', 'Apple'] - -alert( fruits.indexOf('Apple') ); // 0 (first Apple) -alert( fruits.lastIndexOf('Apple') ); // 2 (last Apple) -``` - -````smart header="The `includes` method handles `NaN` correctly" -A minor, but noteworthy feature of `includes` is that it correctly handles `NaN`, unlike `indexOf`: +Also, a very minor difference of `includes` is that it correctly handles `NaN`, unlike `indexOf/lastIndexOf`: ```js run const arr = [NaN]; -alert( arr.indexOf(NaN) ); // -1 (wrong, should be 0) +alert( arr.indexOf(NaN) ); // -1 (should be 0, but === equality doesn't work for NaN) alert( arr.includes(NaN) );// true (correct) ``` -That's because `includes` was added to JavaScript much later and uses the more up to date comparison algorithm internally. -```` -### find and findIndex/findLastIndex +### find and findIndex Imagine we have an array of objects. How do we find an object with the specific condition? @@ -317,28 +304,7 @@ In real life arrays of objects is a common thing, so the `find` method is very u Note that in the example we provide to `find` the function `item => item.id == 1` with one argument. That's typical, other arguments of this function are rarely used. -The [arr.findIndex](mdn:js/Array/findIndex) method has the same syntax, but returns the index where the element was found instead of the element itself. The value of `-1` is returned if nothing is found. - -The [arr.findLastIndex](mdn:js/Array/findLastIndex) method is like `findIndex`, but searches from right to left, similar to `lastIndexOf`. - -Here's an example: - -```js run -let users = [ - {id: 1, name: "John"}, - {id: 2, name: "Pete"}, - {id: 3, name: "Mary"}, - {id: 4, name: "John"} -]; - -// Find the index of the first John -alert(users.findIndex(user => user.name == 'John')); // 0 - -// Find the index of the last John -alert(users.findLastIndex(user => user.name == 'John')); // 3 -``` - - +The [arr.findIndex](mdn:js/Array/findIndex) method is essentially the same, but it returns the index where the element was found instead of the element itself and `-1` is returned when nothing is found. ### filter @@ -423,7 +389,6 @@ Literally, all elements are converted to strings for comparisons. For strings, l To use our own sorting order, we need to supply a function as the argument of `arr.sort()`. The function should compare two arbitrary values and return: - ```js function compare(a, b) { if (a > b) return 1; // if the first value is greater than the second @@ -454,14 +419,13 @@ Now it works as intended. Let's step aside and think what's happening. The `arr` can be array of anything, right? It may contain numbers or strings or objects or whatever. We have a set of *some items*. To sort it, we need an *ordering function* that knows how to compare its elements. The default is a string order. -The `arr.sort(fn)` method implements a generic sorting algorithm. We don't need to care how it internally works (an optimized [quicksort](https://en.wikipedia.org/wiki/Quicksort) or [Timsort](https://en.wikipedia.org/wiki/Timsort) most of the time). It will walk the array, compare its elements using the provided function and reorder them, all we need is to provide the `fn` which does the comparison. +The `arr.sort(fn)` method implements a generic sorting algorithm. We don't need to care how it internally works (an optimized [quicksort](https://en.wikipedia.org/wiki/Quicksort) most of the time). It will walk the array, compare its elements using the provided function and reorder them, all we need is to provide the `fn` which does the comparison. By the way, if we ever want to know which elements are compared -- nothing prevents from alerting them: ```js run [1, -2, 15, 2, 0, 8].sort(function(a, b) { alert( a + " <> " + b ); - return a - b; }); ``` @@ -668,6 +632,7 @@ So it's advised to always specify the initial value. The method [arr.reduceRight](mdn:js/Array/reduceRight) does the same, but goes from right to left. + ## Array.isArray Arrays do not form a separate language type. They are based on objects. @@ -676,7 +641,7 @@ So `typeof` does not help to distinguish a plain object from an array: ```js run alert(typeof {}); // object -alert(typeof []); // object (same) +alert(typeof []); // same ``` ...But arrays are used so often that there's a special method for that: [Array.isArray(value)](mdn:js/Array/isArray). It returns `true` if the `value` is an array, and `false` otherwise. @@ -735,7 +700,7 @@ alert(soldiers[1].age); // 23 If in the example above we used `users.filter(army.canJoin)`, then `army.canJoin` would be called as a standalone function, with `this=undefined`, thus leading to an instant error. -A call to `users.filter(army.canJoin, army)` can be replaced with `users.filter(user => army.canJoin(user))`, that does the same. The latter is used more often, as it's a bit easier to understand for most people. +A call to `users.filter(army.canJoin, army)` can be replaced with `users.filter(user => army.canJoin(user))`, that does the same. The former is used more often, as it's a bit easier to understand for most people. ## Summary @@ -746,7 +711,7 @@ A cheat sheet of array methods: - `pop()` -- extracts an item from the end, - `shift()` -- extracts an item from the beginning, - `unshift(...items)` -- adds items to the beginning. - - `splice(pos, deleteCount, ...items)` -- at index `pos` deletes `deleteCount` elements and inserts `items`. + - `splice(pos, deleteCount, ...items)` -- at index `pos` delete `deleteCount` elements and insert `items`. - `slice(start, end)` -- creates a new array, copies elements from index `start` till `end` (not inclusive) into it. - `concat(...items)` -- returns a new array: copies all members of the current one and adds `items` to it. If any of `items` is an array, then its elements are taken. @@ -764,23 +729,20 @@ A cheat sheet of array methods: - `sort(func)` -- sorts the array in-place, then returns it. - `reverse()` -- reverses the array in-place, then returns it. - `split/join` -- convert a string to array and back. - - `reduce/reduceRight(func, initial)` -- calculate a single value over the array by calling `func` for each element and passing an intermediate result between the calls. + - `reduce(func, initial)` -- calculate a single value over the array by calling `func` for each element and passing an intermediate result between the calls. - Additionally: - - `Array.isArray(value)` checks `value` for being an array, if so returns `true`, otherwise `false`. + - `Array.isArray(arr)` checks `arr` for being an array. Please note that methods `sort`, `reverse` and `splice` modify the array itself. These methods are the most used ones, they cover 99% of use cases. But there are few others: -- [arr.some(fn)](mdn:js/Array/some)/[arr.every(fn)](mdn:js/Array/every) check the array. +- [arr.some(fn)](mdn:js/Array/some)/[arr.every(fn)](mdn:js/Array/every) checks the array. The function `fn` is called on each element of the array similar to `map`. If any/all results are `true`, returns `true`, otherwise `false`. - These methods behave sort of like `||` and `&&` operators: if `fn` returns a truthy value, `arr.some()` immediately returns `true` and stops iterating over the rest of items; if `fn` returns a falsy value, `arr.every()` immediately returns `false` and stops iterating over the rest of items as well. - We can use `every` to compare arrays: - ```js run function arraysEqual(arr1, arr2) { return arr1.length === arr2.length && arr1.every((value, index) => value === arr2[index]); @@ -793,8 +755,6 @@ These methods are the most used ones, they cover 99% of use cases. But there are - [arr.copyWithin(target, start, end)](mdn:js/Array/copyWithin) -- copies its elements from position `start` till position `end` into *itself*, at position `target` (overwrites existing). -- [arr.flat(depth)](mdn:js/Array/flat)/[arr.flatMap(fn)](mdn:js/Array/flatMap) create a new flat array from a multidimensional array. - For the full list, see the [manual](mdn:js/Array). From the first sight it may seem that there are so many methods, quite difficult to remember. But actually that's much easier. diff --git a/1-js/05-data-types/05-array-methods/reduce.svg b/1-js/05-data-types/05-array-methods/reduce.svg index 180941dc..fcac711c 100644 --- a/1-js/05-data-types/05-array-methods/reduce.svg +++ b/1-js/05-data-types/05-array-methods/reduce.svg @@ -1 +1 @@ -1sum 0 current 12sum 0+1 current 23sum 0+1+2 current 34sum 0+1+2+3 current 45sum 0+1+2+3+4 current 50+1+2+3+4+5 = 15 \ No newline at end of file +1sum 0 current 12sum 0+1 current 23sum 0+1+2 current 34sum 0+1+2+3 current 45sum 0+1+2+3+4 current 50+1+2+3+4+5 = 15 \ No newline at end of file diff --git a/1-js/05-data-types/06-iterable/article.md b/1-js/05-data-types/06-iterable/article.md index 76f74036..5e464ac2 100644 --- a/1-js/05-data-types/06-iterable/article.md +++ b/1-js/05-data-types/06-iterable/article.md @@ -1,7 +1,7 @@ # Iterables -*Iterable* objects are a generalization of arrays. That's a concept that allows us to make any object useable in a `for..of` loop. +*Iterable* objects is a generalization of arrays. That's a concept that allows us to make any object useable in a `for..of` loop. Of course, Arrays are iterable. But there are many other built-in objects, that are iterable as well. For instance, strings are also iterable. @@ -26,12 +26,12 @@ let range = { // for(let num of range) ... num=1,2,3,4,5 ``` -To make the `range` object iterable (and thus let `for..of` work) we need to add a method to the object named `Symbol.iterator` (a special built-in symbol just for that). +To make the `range` iterable (and thus let `for..of` work) we need to add a method to the object named `Symbol.iterator` (a special built-in symbol just for that). 1. When `for..of` starts, it calls that method once (or errors if not found). The method must return an *iterator* -- an object with the method `next`. 2. Onward, `for..of` works *only with that returned object*. 3. When `for..of` wants the next value, it calls `next()` on that object. -4. The result of `next()` must have the form `{done: Boolean, value: any}`, where `done=true` means that the loop is finished, otherwise `value` is the next value. +4. The result of `next()` must have the form `{done: Boolean, value: any}`, where `done=true` means that the iteration is finished, otherwise `value` is the next value. Here's the full implementation for `range` with remarks: @@ -45,10 +45,10 @@ let range = { range[Symbol.iterator] = function() { // ...it returns the iterator object: - // 2. Onward, for..of works only with the iterator object below, asking it for next values + // 2. Onward, for..of works only with this iterator, asking it for next values return { current: this.from, - last: this.to, + last: this.to, // 3. next() is called on each iteration by the for..of loop next() { @@ -140,7 +140,7 @@ for (let char of str) { ## Calling an iterator explicitly -For deeper understanding, let's see how to use an iterator explicitly. +For deeper understanding let's see how to use an iterator explicitly. We'll iterate over a string in exactly the same way as `for..of`, but with direct calls. This code creates a string iterator and gets values from it "manually": @@ -165,12 +165,12 @@ That is rarely needed, but gives us more control over the process than `for..of` ## Iterables and array-likes [#array-like] -Two official terms look similar, but are very different. Please make sure you understand them well to avoid the confusion. +There are two official terms that look similar, but are very different. Please make sure you understand them well to avoid the confusion. - *Iterables* are objects that implement the `Symbol.iterator` method, as described above. - *Array-likes* are objects that have indexes and `length`, so they look like arrays. -When we use JavaScript for practical tasks in a browser or any other environment, we may meet objects that are iterables or array-likes, or both. +When we use JavaScript for practical tasks in browser or other environments, we may meet objects that are iterables or array-likes, or both. For instance, strings are both iterable (`for..of` works on them) and array-like (they have numeric indexes and `length`). @@ -218,7 +218,7 @@ alert(arr.pop()); // World (method works) The same happens for an iterable: -```js run +```js // assuming that range is taken from the example above let arr = Array.from(range); alert(arr); // 1,2,3,4,5 (array toString conversion works) @@ -233,7 +233,7 @@ The optional second argument `mapFn` can be a function that will be applied to e For instance: -```js run +```js // assuming that range is taken from the example above // square each number @@ -270,7 +270,7 @@ for (let char of str) { alert(chars); ``` -...But it is shorter. +...But it is shorter. We can even build surrogate-aware `slice` on it: @@ -293,7 +293,7 @@ alert( str.slice(1, 3) ); // garbage (two pieces from different surrogate pairs) Objects that can be used in `for..of` are called *iterable*. - Technically, iterables must implement the method named `Symbol.iterator`. - - The result of `obj[Symbol.iterator]()` is called an *iterator*. It handles further iteration process. + - The result of `obj[Symbol.iterator]()` is called an *iterator*. It handles the further iteration process. - An iterator must have the method named `next()` that returns an object `{done: Boolean, value: any}`, here `done:true` denotes the end of the iteration process, otherwise the `value` is the next value. - The `Symbol.iterator` method is called automatically by `for..of`, but we also can do it directly. - Built-in iterables like strings or arrays, also implement `Symbol.iterator`. diff --git a/1-js/05-data-types/07-map-set/03-iterable-keys/task.md b/1-js/05-data-types/07-map-set/03-iterable-keys/task.md index 81507647..25c74bfc 100644 --- a/1-js/05-data-types/07-map-set/03-iterable-keys/task.md +++ b/1-js/05-data-types/07-map-set/03-iterable-keys/task.md @@ -4,7 +4,7 @@ importance: 5 # Iterable keys -We'd like to get an array of `map.keys()` in a variable and then apply array-specific methods to it, e.g. `.push`. +We'd like to get an array of `map.keys()` in a variable and then do apply array-specific methods to it, e.g. `.push`. But that doesn't work: diff --git a/1-js/05-data-types/07-map-set/article.md b/1-js/05-data-types/07-map-set/article.md index 35407088..e08c8408 100644 --- a/1-js/05-data-types/07-map-set/article.md +++ b/1-js/05-data-types/07-map-set/article.md @@ -1,10 +1,10 @@ # Map and Set -Till now, we've learned about the following complex data structures: +Now we've learned about the following complex data structures: -- Objects are used for storing keyed collections. -- Arrays are used for storing ordered collections. +- Objects for storing keyed collections. +- Arrays for storing ordered collections. But that's not enough for real life. That's why `Map` and `Set` also exist. @@ -15,12 +15,12 @@ But that's not enough for real life. That's why `Map` and `Set` also exist. Methods and properties are: - `new Map()` -- creates the map. -- [`map.set(key, value)`](mdn:js/Map/set) -- stores the value by the key. -- [`map.get(key)`](mdn:js/Map/get) -- returns the value by the key, `undefined` if `key` doesn't exist in map. -- [`map.has(key)`](mdn:js/Map/has) -- returns `true` if the `key` exists, `false` otherwise. -- [`map.delete(key)`](mdn:js/Map/delete) -- removes the value by the key. -- [`map.clear()`](mdn:js/Map/clear) -- removes everything from the map. -- [`map.size`](mdn:js/Map/size) -- returns the current element count. +- `map.set(key, value)` -- stores the value by the key. +- `map.get(key)` -- returns the value by the key, `undefined` if `key` doesn't exist in map. +- `map.has(key)` -- returns `true` if the `key` exists, `false` otherwise. +- `map.delete(key)` -- removes the value by the key. +- `map.clear()` -- removes everything from the map. +- `map.size` -- returns the current element count. For instance: @@ -42,7 +42,7 @@ alert( map.size ); // 3 As we can see, unlike objects, keys are not converted to strings. Any type of key is possible. ```smart header="`map[key]` isn't the right way to use a `Map`" -Although `map[key]` also works, e.g. we can set `map[key] = 2`, this is treating `map` as a plain JavaScript object, so it implies all corresponding limitations (only string/symbol keys and so on). +Although `map[key]` also works, e.g. we can set `map[key] = 2`, this is treating `map` as a plain JavaScript object, so it implies all corresponding limitations (no object keys and so on). So we should use `map` methods: `set`, `get` and so on. ``` @@ -63,26 +63,24 @@ visitsCountMap.set(john, 123); alert( visitsCountMap.get(john) ); // 123 ``` -Using objects as keys is one of the most notable and important `Map` features. The same does not count for `Object`. String as a key in `Object` is fine, but we can't use another `Object` as a key in `Object`. +Using objects as keys is one of most notable and important `Map` features. For string keys, `Object` can be fine, but not for object keys. Let's try: ```js run let john = { name: "John" }; -let ben = { name: "Ben" }; let visitsCountObj = {}; // try to use an object -visitsCountObj[ben] = 234; // try to use ben object as the key -visitsCountObj[john] = 123; // try to use john object as the key, ben object will get replaced +visitsCountObj[john] = 123; // try to use john object as the key *!* // That's what got written! -alert( visitsCountObj["[object Object]"] ); // 123 +alert( visitsCountObj["[object Object]"] ); // 123 */!* ``` -As `visitsCountObj` is an object, it converts all `Object` keys, such as `john` and `ben` above, to same string `"[object Object]"`. Definitely not what we want. +As `visitsCountObj` is an object, it converts all keys, such as `john` to strings, so we've got the string key `"[object Object]"`. Definitely not what we want. ```smart header="How `Map` compares keys" To test keys for equivalence, `Map` uses the algorithm [SameValueZero](https://tc39.github.io/ecma262/#sec-samevaluezero). It is roughly the same as strict equality `===`, but the difference is that `NaN` is considered equal to `NaN`. So `NaN` can be used as the key as well. @@ -105,9 +103,9 @@ map.set('1', 'str1') For looping over a `map`, there are 3 methods: -- [`map.keys()`](mdn:js/Map/keys) -- returns an iterable for keys, -- [`map.values()`](mdn:js/Map/values) -- returns an iterable for values, -- [`map.entries()`](mdn:js/Map/entries) -- returns an iterable for entries `[key, value]`, it's used by default in `for..of`. +- `map.keys()` -- returns an iterable for keys, +- `map.values()` -- returns an iterable for values, +- `map.entries()` -- returns an iterable for entries `[key, value]`, it's used by default in `for..of`. For instance: @@ -238,11 +236,11 @@ A `Set` is a special type collection - "set of values" (without keys), where eac Its main methods are: - `new Set(iterable)` -- creates the set, and if an `iterable` object is provided (usually an array), copies values from it into the set. -- [`set.add(value)`](mdn:js/Set/add) -- adds a value, returns the set itself. -- [`set.delete(value)`](mdn:js/Set/delete) -- removes the value, returns `true` if `value` existed at the moment of the call, otherwise `false`. -- [`set.has(value)`](mdn:js/Set/has) -- returns `true` if the value exists in the set, otherwise `false`. -- [`set.clear()`](mdn:js/Set/clear) -- removes everything from the set. -- [`set.size`](mdn:js/Set/size) -- is the elements count. +- `set.add(value)` -- adds a value, returns the set itself. +- `set.delete(value)` -- removes the value, returns `true` if `value` existed at the moment of the call, otherwise `false`. +- `set.has(value)` -- returns `true` if the value exists in the set, otherwise `false`. +- `set.clear()` -- removes everything from the set. +- `set.size` -- is the elements count. The main feature is that repeated calls of `set.add(value)` with the same value don't do anything. That's the reason why each value appears in a `Set` only once. @@ -291,13 +289,13 @@ set.forEach((value, valueAgain, set) => { Note the funny thing. The callback function passed in `forEach` has 3 arguments: a `value`, then *the same value* `valueAgain`, and then the target object. Indeed, the same value appears in the arguments twice. -That's for compatibility with `Map` where the callback passed `forEach` has three arguments. Looks a bit strange, for sure. But this may help to replace `Map` with `Set` in certain cases with ease, and vice versa. +That's for compatibility with `Map` where the callback passed `forEach` has three arguments. Looks a bit strange, for sure. But may help to replace `Map` with `Set` in certain cases with ease, and vice versa. The same methods `Map` has for iterators are also supported: -- [`set.keys()`](mdn:js/Set/keys) -- returns an iterable object for values, -- [`set.values()`](mdn:js/Set/values) -- same as `set.keys()`, for compatibility with `Map`, -- [`set.entries()`](mdn:js/Set/entries) -- returns an iterable object for entries `[value, value]`, exists for compatibility with `Map`. +- `set.keys()` -- returns an iterable object for values, +- `set.values()` -- same as `set.keys()`, for compatibility with `Map`, +- `set.entries()` -- returns an iterable object for entries `[value, value]`, exists for compatibility with `Map`. ## Summary @@ -306,12 +304,12 @@ The same methods `Map` has for iterators are also supported: Methods and properties: - `new Map([iterable])` -- creates the map, with optional `iterable` (e.g. array) of `[key,value]` pairs for initialization. -- [`map.set(key, value)`](mdn:js/Map/set) -- stores the value by the key, returns the map itself. -- [`map.get(key)`](mdn:js/Map/get) -- returns the value by the key, `undefined` if `key` doesn't exist in map. -- [`map.has(key)`](mdn:js/Map/has) -- returns `true` if the `key` exists, `false` otherwise. -- [`map.delete(key)`](mdn:js/Map/delete) -- removes the value by the key, returns `true` if `key` existed at the moment of the call, otherwise `false`. -- [`map.clear()`](mdn:js/Map/clear) -- removes everything from the map. -- [`map.size`](mdn:js/Map/size) -- returns the current element count. +- `map.set(key, value)` -- stores the value by the key. +- `map.get(key)` -- returns the value by the key, `undefined` if `key` doesn't exist in map. +- `map.has(key)` -- returns `true` if the `key` exists, `false` otherwise. +- `map.delete(key)` -- removes the value by the key. +- `map.clear()` -- removes everything from the map. +- `map.size` -- returns the current element count. The differences from a regular `Object`: @@ -323,10 +321,10 @@ The differences from a regular `Object`: Methods and properties: - `new Set([iterable])` -- creates the set, with optional `iterable` (e.g. array) of values for initialization. -- [`set.add(value)`](mdn:js/Set/add) -- adds a value (does nothing if `value` exists), returns the set itself. -- [`set.delete(value)`](mdn:js/Set/delete) -- removes the value, returns `true` if `value` existed at the moment of the call, otherwise `false`. -- [`set.has(value)`](mdn:js/Set/has) -- returns `true` if the value exists in the set, otherwise `false`. -- [`set.clear()`](mdn:js/Set/clear) -- removes everything from the set. -- [`set.size`](mdn:js/Set/size) -- is the elements count. +- `set.add(value)` -- adds a value (does nothing if `value` exists), returns the set itself. +- `set.delete(value)` -- removes the value, returns `true` if `value` existed at the moment of the call, otherwise `false`. +- `set.has(value)` -- returns `true` if the value exists in the set, otherwise `false`. +- `set.clear()` -- removes everything from the set. +- `set.size` -- is the elements count. Iteration over `Map` and `Set` is always in the insertion order, so we can't say that these collections are unordered, but we can't reorder elements or directly get an element by its number. diff --git a/1-js/05-data-types/08-weakmap-weakset/01-recipients-read/solution.md b/1-js/05-data-types/08-weakmap-weakset/01-recipients-read/solution.md index e2147ccf..6a4c20ba 100644 --- a/1-js/05-data-types/08-weakmap-weakset/01-recipients-read/solution.md +++ b/1-js/05-data-types/08-weakmap-weakset/01-recipients-read/solution.md @@ -25,7 +25,7 @@ messages.shift(); // now readMessages has 1 element (technically memory may be cleaned later) ``` -The `WeakSet` allows to store a set of messages and easily check for the existence of a message in it. +The `WeakSet` allows to store a set of messages and easily check for the existance of a message in it. It cleans up itself automatically. The tradeoff is that we can't iterate over it, can't get "all read messages" from it directly. But we can do it by iterating over all messages and filtering those that are in the set. diff --git a/1-js/05-data-types/08-weakmap-weakset/article.md b/1-js/05-data-types/08-weakmap-weakset/article.md index 8d5a8698..bcc5e5e6 100644 --- a/1-js/05-data-types/08-weakmap-weakset/article.md +++ b/1-js/05-data-types/08-weakmap-weakset/article.md @@ -1,6 +1,6 @@ # WeakMap and WeakSet -As we know from the chapter , JavaScript engine keeps a value in memory while it is "reachable" and can potentially be used. +As we know from the chapter , JavaScript engine stores a value in memory while it is reachable (and can potentially be used). For instance: ```js @@ -30,8 +30,7 @@ let array = [ john ]; john = null; // overwrite the reference *!* -// the object previously referenced by john is stored inside the array -// therefore it won't be garbage-collected +// john is stored inside the array, so it won't be garbage-collected // we can get it as array[0] */!* ``` @@ -60,7 +59,7 @@ Let's see what it means on examples. ## WeakMap -The first difference between `Map` and `WeakMap` is that keys must be objects, not primitive values: +The first difference from `Map` is that `WeakMap` keys must be objects, not primitive values: ```js run let weakMap = new WeakMap(); @@ -101,9 +100,9 @@ Compare it with the regular `Map` example above. Now if `john` only exists as th Why such a limitation? That's for technical reasons. If an object has lost all other references (like `john` in the code above), then it is to be garbage-collected automatically. But technically it's not exactly specified *when the cleanup happens*. -The JavaScript engine decides that. It may choose to perform the memory cleanup immediately or to wait and do the cleaning later when more deletions happen. So, technically, the current element count of a `WeakMap` is not known. The engine may have cleaned it up or not, or did it partially. For that reason, methods that access all keys/values are not supported. +The JavaScript engine decides that. It may choose to perform the memory cleanup immediately or to wait and do the cleaning later when more deletions happen. So, technically the current element count of a `WeakMap` is not known. The engine may have cleaned it up or not, or did it partially. For that reason, methods that access all keys/values are not supported. -Now, where do we need such a data structure? +Now where do we need such data structure? ## Use case: additional data @@ -147,7 +146,7 @@ countUser(john); // count his visits john = null; ``` -Now, `john` object should be garbage collected, but remains in memory, as it's a key in `visitsCountMap`. +Now `john` object should be garbage collected, but remains in memory, as it's a key in `visitsCountMap`. We need to clean `visitsCountMap` when we remove users, otherwise it will grow in memory indefinitely. Such cleaning can become a tedious task in complex architectures. @@ -164,13 +163,13 @@ function countUser(user) { } ``` -Now we don't have to clean `visitsCountMap`. After `john` object becomes unreachable, by all means except as a key of `WeakMap`, it gets removed from memory, along with the information by that key from `WeakMap`. +Now we don't have to clean `visitsCountMap`. After `john` object becomes unreachable by all means except as a key of `WeakMap`, it gets removed from memory, along with the information by that key from `WeakMap`. ## Use case: caching -Another common example is caching. We can store ("cache") results from a function, so that future calls on the same object can reuse it. +Another common example is caching: when a function result should be remembered ("cached"), so that future calls on the same object reuse it. -To achieve that, we can use `Map` (not optimal scenario): +We can use `Map` to store results, like this: ```js run // 📁 cache.js @@ -207,7 +206,7 @@ alert(cache.size); // 1 (Ouch! The object is still in cache, taking memory!) For multiple calls of `process(obj)` with the same object, it only calculates the result the first time, and then just takes it from `cache`. The downside is that we need to clean `cache` when the object is not needed any more. -If we replace `Map` with `WeakMap`, then this problem disappears. The cached result will be removed from memory automatically after the object gets garbage collected. +If we replace `Map` with `WeakMap`, then this problem disappears: the cached result will be removed from memory automatically after the object gets garbage collected. ```js run // 📁 cache.js @@ -248,7 +247,7 @@ obj = null; - An object exists in the set while it is reachable from somewhere else. - Like `Set`, it supports `add`, `has` and `delete`, but not `size`, `keys()` and no iterations. -Being "weak", it also serves as additional storage. But not for arbitrary data, rather for "yes/no" facts. A membership in `WeakSet` may mean something about the object. +Being "weak", it also serves as an additional storage. But not for an arbitrary data, but rather for "yes/no" facts. A membership in `WeakSet` may mean something about the object. For instance, we can add users to `WeakSet` to keep track of those who visited our site: @@ -276,7 +275,7 @@ john = null; // visitedSet will be cleaned automatically ``` -The most notable limitation of `WeakMap` and `WeakSet` is the absence of iterations, and the inability to get all current content. That may appear inconvenient, but does not prevent `WeakMap/WeakSet` from doing their main job -- be an "additional" storage of data for objects which are stored/managed at another place. +The most notable limitation of `WeakMap` and `WeakSet` is the absence of iterations, and inability to get all current content. That may appear inconvenient, but does not prevent `WeakMap/WeakSet` from doing their main job -- be an "additional" storage of data for objects which are stored/managed at another place. ## Summary @@ -284,8 +283,6 @@ The most notable limitation of `WeakMap` and `WeakSet` is the absence of iterati `WeakSet` is `Set`-like collection that stores only objects and removes them once they become inaccessible by other means. -Their main advantages are that they have weak reference to objects, so they can easily be removed by garbage collector. +Both of them do not support methods and properties that refer to all keys or their count. Only individual operations are allowed. -That comes at the cost of not having support for `clear`, `size`, `keys`, `values`... - -`WeakMap` and `WeakSet` are used as "secondary" data structures in addition to the "primary" object storage. Once the object is removed from the primary storage, if it is only found as the key of `WeakMap` or in a `WeakSet`, it will be cleaned up automatically. +`WeakMap` and `WeakSet` are used as "secondary" data structures in addition to the "main" object storage. Once the object is removed from the main storage, if it is only found as the key of `WeakMap` or in a `WeakSet`, it will be cleaned up automatically. diff --git a/1-js/05-data-types/09-keys-values-entries/article.md b/1-js/05-data-types/09-keys-values-entries/article.md index bef678f5..b633dc27 100644 --- a/1-js/05-data-types/09-keys-values-entries/article.md +++ b/1-js/05-data-types/09-keys-values-entries/article.md @@ -77,7 +77,7 @@ Objects lack many methods that exist for arrays, e.g. `map`, `filter` and others If we'd like to apply them, then we can use `Object.entries` followed by `Object.fromEntries`: 1. Use `Object.entries(obj)` to get an array of key/value pairs from `obj`. -2. Use array methods on that array, e.g. `map`, to transform these key/value pairs. +2. Use array methods on that array, e.g. `map`. 3. Use `Object.fromEntries(array)` on the resulting array to turn it back into an object. For example, we have an object with prices, and would like to double them: @@ -91,13 +91,12 @@ let prices = { *!* let doublePrices = Object.fromEntries( - // convert prices to array, map each key/value pair into another pair - // and then fromEntries gives back the object - Object.entries(prices).map(entry => [entry[0], entry[1] * 2]) + // convert to array, map, and then fromEntries gives back the object + Object.entries(prices).map(([key, value]) => [key, value * 2]) ); */!* alert(doublePrices.meat); // 8 -``` +``` -It may look difficult at first sight, but becomes easy to understand after you use it once or twice. We can make powerful chains of transforms this way. +It may look difficult from the first sight, but becomes easy to understand after you use it once or twice. We can make powerful chains of transforms this way. diff --git a/1-js/05-data-types/10-destructuring-assignment/6-max-salary/_js.view/solution.js b/1-js/05-data-types/10-destructuring-assignment/6-max-salary/_js.view/solution.js index 6538af42..f4bd5c76 100644 --- a/1-js/05-data-types/10-destructuring-assignment/6-max-salary/_js.view/solution.js +++ b/1-js/05-data-types/10-destructuring-assignment/6-max-salary/_js.view/solution.js @@ -1,14 +1,16 @@ function topSalary(salaries) { - let maxSalary = 0; + let max = 0; let maxName = null; for(const [name, salary] of Object.entries(salaries)) { - if (maxSalary < salary) { - maxSalary = salary; + if (max < salary) { + max = salary; maxName = name; } } return maxName; -} \ No newline at end of file +} + + diff --git a/1-js/05-data-types/10-destructuring-assignment/article.md b/1-js/05-data-types/10-destructuring-assignment/article.md index 41e36db2..46aa760a 100644 --- a/1-js/05-data-types/10-destructuring-assignment/article.md +++ b/1-js/05-data-types/10-destructuring-assignment/article.md @@ -2,22 +2,19 @@ The two most used data structures in JavaScript are `Object` and `Array`. -- Objects allow us to create a single entity that stores data items by key. -- Arrays allow us to gather data items into an ordered list. +Objects allow us to create a single entity that stores data items by key, and arrays allow us to gather data items into an ordered collection. -Although, when we pass those to a function, it may need not be an object/array as a whole. It may need individual pieces. +But when we pass those to a function, it may need not an object/array as a whole, but rather individual pieces. -*Destructuring assignment* is a special syntax that allows us to "unpack" arrays or objects into a bunch of variables, as sometimes that's more convenient. - -Destructuring also works great with complex functions that have a lot of parameters, default values, and so on. Soon we'll see that. +*Destructuring assignment* is a special syntax that allows us to "unpack" arrays or objects into a bunch of variables, as sometimes that's more convenient. Destructuring also works great with complex functions that have a lot of parameters, default values, and so on. ## Array destructuring -Here's an example of how an array is destructured into variables: +An example of how the array is destructured into variables: ```js // we have an array with the name and surname -let arr = ["John", "Smith"] +let arr = ["Ilya", "Kantor"] *!* // destructuring assignment @@ -26,22 +23,18 @@ let arr = ["John", "Smith"] let [firstName, surname] = arr; */!* -alert(firstName); // John -alert(surname); // Smith +alert(firstName); // Ilya +alert(surname); // Kantor ``` Now we can work with variables instead of array members. It looks great when combined with `split` or other array-returning methods: -```js run -let [firstName, surname] = "John Smith".split(' '); -alert(firstName); // John -alert(surname); // Smith +```js +let [firstName, surname] = "Ilya Kantor".split(' '); ``` -As you can see, the syntax is simple. There are several peculiar details though. Let's see more examples, to better understand it. - ````smart header="\"Destructuring\" does not mean \"destructive\"." It's called "destructuring assignment," because it "destructurizes" by copying items into variables. But the array itself is not modified. @@ -76,25 +69,26 @@ In the code above, the second element of the array is skipped, the third one is let [a, b, c] = "abc"; // ["a", "b", "c"] let [one, two, three] = new Set([1, 2, 3]); ``` -That works, because internally a destructuring assignment works by iterating over the right value. It's a kind of syntax sugar for calling `for..of` over the value to the right of `=` and assigning the values. + ```` ````smart header="Assign to anything at the left-side" -We can use any "assignables" on the left side. + +We can use any "assignables" at the left side. For instance, an object property: ```js run let user = {}; -[user.name, user.surname] = "John Smith".split(' '); +[user.name, user.surname] = "Ilya Kantor".split(' '); -alert(user.name); // John -alert(user.surname); // Smith +alert(user.name); // Ilya ``` ```` ````smart header="Looping with .entries()" + In the previous chapter we saw the [Object.entries(obj)](mdn:js/Object/entries) method. We can use it with destructuring to loop over keys-and-values of an object: @@ -113,7 +107,7 @@ for (let [key, value] of Object.entries(user)) { } ``` -The similar code for a `Map` is simpler, as it's iterable: +...And the same for a map: ```js run let user = new Map(); @@ -121,7 +115,6 @@ user.set("name", "John"); user.set("age", "30"); *!* -// Map iterates as [key, value] pairs, very convenient for destructuring for (let [key, value] of user) { */!* alert(`${key}:${value}`); // name:John, then age:30 @@ -129,17 +122,15 @@ for (let [key, value] of user) { ``` ```` -````smart header="Swap variables trick" -There's a well-known trick for swapping values of two variables using a destructuring assignment: +```smart header="Swap variables trick" +A well-known trick for swapping values of two variables: ```js run let guest = "Jane"; let admin = "Pete"; -// Let's swap the values: make guest=Pete, admin=Jane -*!* +// Swap values: make guest=Pete, admin=Jane [guest, admin] = [admin, guest]; -*/!* alert(`${guest} ${admin}`); // Pete Jane (successfully swapped!) ``` @@ -147,47 +138,31 @@ alert(`${guest} ${admin}`); // Pete Jane (successfully swapped!) Here we create a temporary array of two variables and immediately destructure it in swapped order. We can swap more than two variables this way. -```` + ### The rest '...' -Usually, if the array is longer than the list at the left, the "extra" items are omitted. - -For example, here only two items are taken, and the rest is just ignored: - -```js run -let [name1, name2] = ["Julius", "Caesar", "Consul", "of the Roman Republic"]; - -alert(name1); // Julius -alert(name2); // Caesar -// Further items aren't assigned anywhere -``` - -If we'd like also to gather all that follows -- we can add one more parameter that gets "the rest" using three dots `"..."`: +If we want not just to get first values, but also to gather all that follows -- we can add one more parameter that gets "the rest" using three dots `"..."`: ```js run let [name1, name2, *!*...rest*/!*] = ["Julius", "Caesar", *!*"Consul", "of the Roman Republic"*/!*]; +alert(name1); // Julius +alert(name2); // Caesar + *!* -// rest is array of items, starting from the 3rd one +// Note that type of `rest` is Array. alert(rest[0]); // Consul alert(rest[1]); // of the Roman Republic alert(rest.length); // 2 */!* ``` -The value of `rest` is the array of the remaining array elements. - -We can use any other variable name in place of `rest`, just make sure it has three dots before it and goes last in the destructuring assignment. - -```js run -let [name1, name2, *!*...titles*/!*] = ["Julius", "Caesar", "Consul", "of the Roman Republic"]; -// now titles = ["Consul", "of the Roman Republic"] -``` +The value of `rest` is the array of the remaining array elements. We can use any other variable name in place of `rest`, just make sure it has three dots before it and goes last in the destructuring assignment. ### Default values -If the array is shorter than the list of variables at the left, there'll be no errors. Absent values are considered undefined: +If there are fewer values in the array than variables in the assignment, there will be no error. Absent values are considered undefined: ```js run *!* @@ -212,7 +187,7 @@ alert(surname); // Anonymous (default used) Default values can be more complex expressions or even function calls. They are evaluated only if the value is not provided. -For instance, here we use the `prompt` function for two defaults: +For instance, here we use the `prompt` function for two defaults. But it will run only for the missing one: ```js run // runs only prompt for surname @@ -222,7 +197,7 @@ alert(name); // Julius (from array) alert(surname); // whatever prompt gets ``` -Please note: the `prompt` will run only for the missing value (`surname`). + ## Object destructuring @@ -234,7 +209,7 @@ The basic syntax is: let {var1, var2} = {var1:…, var2:…} ``` -We should have an existing object on the right side, that we want to split into variables. The left side contains an object-like "pattern" for corresponding properties. In the simplest case, that's a list of variable names in `{...}`. +We have an existing object at the right side, that we want to split into variables. The left side contains a "pattern" for corresponding properties. In the simple case, that's a list of variable names in `{...}`. For instance: @@ -254,9 +229,7 @@ alert(width); // 100 alert(height); // 200 ``` -Properties `options.title`, `options.width` and `options.height` are assigned to the corresponding variables. - -The order does not matter. This works too: +Properties `options.title`, `options.width` and `options.height` are assigned to the corresponding variables. The order does not matter. This works too: ```js // changed the order in let {...} @@ -265,7 +238,7 @@ let {height, width, title} = { title: "Menu", height: 200, width: 100 } The pattern on the left side may be more complex and specify the mapping between properties and variables. -If we want to assign a property to a variable with another name, for instance, make `options.width` go into the variable named `w`, then we can set the variable name using a colon: +If we want to assign a property to a variable with another name, for instance, `options.width` to go into the variable named `w`, then we can set it using a colon: ```js run let options = { @@ -420,7 +393,7 @@ alert( title ); // Menu If an object or an array contain other nested objects and arrays, we can use more complex left-side patterns to extract deeper portions. -In the code below `options` has another object in the property `size` and an array in the property `items`. The pattern on the left side of the assignment has the same structure to extract values from them: +In the code below `options` has another object in the property `size` and an array in the property `items`. The pattern at the left side of the assignment has the same structure to extract values from them: ```js run let options = { @@ -429,7 +402,7 @@ let options = { height: 200 }, items: ["Cake", "Donut"], - extra: true + extra: true }; // destructuring assignment split in multiple lines for clarity diff --git a/1-js/05-data-types/10-destructuring-assignment/destructuring-complex.svg b/1-js/05-data-types/10-destructuring-assignment/destructuring-complex.svg index 8a1ff1a9..cb496bf6 100644 --- a/1-js/05-data-types/10-destructuring-assignment/destructuring-complex.svg +++ b/1-js/05-data-types/10-destructuring-assignment/destructuring-complex.svg @@ -1 +1 @@ - \ No newline at end of file + \ No newline at end of file diff --git a/1-js/05-data-types/11-date/1-new-date/solution.md b/1-js/05-data-types/11-date/1-new-date/solution.md index 18286c33..bed44945 100644 --- a/1-js/05-data-types/11-date/1-new-date/solution.md +++ b/1-js/05-data-types/11-date/1-new-date/solution.md @@ -13,6 +13,6 @@ We could also create a date from a string, like this: ```js run //new Date(datastring) -let d2 = new Date("2012-02-20T03:12"); +let d2 = new Date("February 20, 2012 03:12:00"); alert( d2 ); ``` diff --git a/1-js/05-data-types/11-date/article.md b/1-js/05-data-types/11-date/article.md index 2266c077..2193b7cc 100644 --- a/1-js/05-data-types/11-date/article.md +++ b/1-js/05-data-types/11-date/article.md @@ -57,7 +57,7 @@ To create a new `Date` object call `new Date()` with one of the following argume `new Date(year, month, date, hours, minutes, seconds, ms)` : Create the date with the given components in the local time zone. Only the first two arguments are obligatory. - - The `year` should have 4 digits. For compatibility, 2 digits are also accepted and considered `19xx`, e.g. `98` is the same as `1998` here, but always using 4 digits is strongly encouraged. + - The `year` must have 4 digits: `2013` is okay, `98` is not. - The `month` count starts with `0` (Jan), up to `11` (Dec). - The `date` parameter is actually the day of month, if absent then `1` is assumed. - If `hours/minutes/seconds/ms` is absent, they are assumed to be equal `0`. @@ -348,7 +348,7 @@ let time1 = 0; let time2 = 0; *!* -// run bench(diffSubtract) and bench(diffGetTime) each 10 times alternating +// run bench(upperSlice) and bench(upperLoop) each 10 times alternating for (let i = 0; i < 10; i++) { time1 += bench(diffSubtract); time2 += bench(diffGetTime); @@ -388,7 +388,7 @@ The string format should be: `YYYY-MM-DDTHH:mm:ss.sssZ`, where: - `YYYY-MM-DD` -- is the date: year-month-day. - The character `"T"` is used as the delimiter. - `HH:mm:ss.sss` -- is the time: hours, minutes, seconds and milliseconds. -- The optional `'Z'` part denotes the time zone in the format `+-hh:mm`. A single letter `Z` would mean UTC+0. +- The optional `'Z'` part denotes the time zone in the format `+-hh:mm`. A single letter `Z` that would mean UTC+0. Shorter variants are also possible, like `YYYY-MM-DD` or `YYYY-MM` or even `YYYY`. @@ -407,7 +407,7 @@ We can instantly create a `new Date` object from the timestamp: ```js run let date = new Date( Date.parse('2012-01-26T13:51:50.417-07:00') ); -alert(date); +alert(date); ``` ## Summary @@ -427,7 +427,7 @@ Sometimes we need more precise time measurements. JavaScript itself does not hav alert(`Loading started ${performance.now()}ms ago`); // Something like: "Loading started 34731.26000000001ms ago" // .26 is microseconds (260 microseconds) -// more than 3 digits after the decimal point are precision errors, only the first 3 are correct +// more than 3 digits after the decimal point are precision errors, but only the first 3 are correct ``` Node.js has `microtime` module and other ways. Technically, almost any device and environment allows to get more precision, it's just not in `Date`. diff --git a/1-js/05-data-types/12-json/article.md b/1-js/05-data-types/12-json/article.md index 50374535..a5f2974a 100644 --- a/1-js/05-data-types/12-json/article.md +++ b/1-js/05-data-types/12-json/article.md @@ -27,7 +27,7 @@ Luckily, there's no need to write the code to handle all this. The task has been ## JSON.stringify -The [JSON](https://en.wikipedia.org/wiki/JSON) (JavaScript Object Notation) is a general format to represent values and objects. It is described as in [RFC 4627](https://tools.ietf.org/html/rfc4627) standard. Initially it was made for JavaScript, but many other languages have libraries to handle it as well. So it's easy to use JSON for data exchange when the client uses JavaScript and the server is written on Ruby/PHP/Java/Whatever. +The [JSON](http://en.wikipedia.org/wiki/JSON) (JavaScript Object Notation) is a general format to represent values and objects. It is described as in [RFC 4627](http://tools.ietf.org/html/rfc4627) standard. Initially it was made for JavaScript, but many other languages have libraries to handle it as well. So it's easy to use JSON for data exchange when the client uses JavaScript and the server is written on Ruby/PHP/Java/Whatever. JavaScript provides methods: @@ -41,7 +41,7 @@ let student = { age: 30, isAdmin: false, courses: ['html', 'css', 'js'], - spouse: null + wife: null }; *!* @@ -58,7 +58,7 @@ alert(json); "age": 30, "isAdmin": false, "courses": ["html", "css", "js"], - "spouse": null + "wife": null } */ */!* @@ -105,7 +105,7 @@ JSON is data-only language-independent specification, so some JavaScript-specifi Namely: - Function properties (methods). -- Symbolic keys and values. +- Symbolic properties. - Properties that store `undefined`. ```js run @@ -276,7 +276,6 @@ name: John name: Alice place: [object Object] number: 23 -occupiedBy: [object Object] */ ``` @@ -329,8 +328,6 @@ alert(JSON.stringify(user, null, 2)); */ ``` -The third argument can also be a string. In this case, the string is used for indentation instead of a number of spaces. - The `space` parameter is used solely for logging and nice-output purposes. ## Custom "toJSON" diff --git a/1-js/05-data-types/12-json/json-meetup.svg b/1-js/05-data-types/12-json/json-meetup.svg index 3fa32a26..e4467490 100644 --- a/1-js/05-data-types/12-json/json-meetup.svg +++ b/1-js/05-data-types/12-json/json-meetup.svg @@ -1 +1 @@ -number: 23title: "Conference"...placeoccupiedByparticipants \ No newline at end of file +number: 23title: "Conference"...placeoccupiedByparticipants \ No newline at end of file diff --git a/1-js/06-advanced-functions/01-recursion/01-sum-to/solution.md b/1-js/06-advanced-functions/01-recursion/01-sum-to/solution.md index 11667f94..3a281ef3 100644 --- a/1-js/06-advanced-functions/01-recursion/01-sum-to/solution.md +++ b/1-js/06-advanced-functions/01-recursion/01-sum-to/solution.md @@ -37,4 +37,4 @@ P.S. Naturally, the formula is the fastest solution. It uses only 3 operations f The loop variant is the second in terms of speed. In both the recursive and the loop variant we sum the same numbers. But the recursion involves nested calls and execution stack management. That also takes resources, so it's slower. -P.P.S. Some engines support the "tail call" optimization: if a recursive call is the very last one in the function, with no other calculations performed, then the outer function will not need to resume the execution, so the engine doesn't need to remember its execution context. That removes the burden on memory. But if the JavaScript engine does not support tail call optimization (most of them don't), there will be an error: maximum stack size exceeded, because there's usually a limitation on the total stack size. +P.P.S. Some engines support the "tail call" optimization: if a recursive call is the very last one in the function (like in `sumTo` above), then the outer function will not need to resume the execution, so the engine doesn't need to remember its execution context. That removes the burden on memory, so counting `sumTo(100000)` becomes possible. But if the JavaScript engine does not support tail call optimization (most of them don't), there will be an error: maximum stack size exceeded, because there's usually a limitation on the total stack size. diff --git a/1-js/06-advanced-functions/01-recursion/02-factorial/solution.md b/1-js/06-advanced-functions/01-recursion/02-factorial/solution.md index 09e511db..59040a2b 100644 --- a/1-js/06-advanced-functions/01-recursion/02-factorial/solution.md +++ b/1-js/06-advanced-functions/01-recursion/02-factorial/solution.md @@ -1,4 +1,4 @@ -By definition, a factorial `n!` can be written as `n * (n-1)!`. +By definition, a factorial is `n!` can be written as `n * (n-1)!`. In other words, the result of `factorial(n)` can be calculated as `n` multiplied by the result of `factorial(n-1)`. And the call for `n-1` can recursively descend lower, and lower, till `1`. diff --git a/1-js/06-advanced-functions/01-recursion/03-fibonacci-numbers/fibonacci-recursion-tree.svg b/1-js/06-advanced-functions/01-recursion/03-fibonacci-numbers/fibonacci-recursion-tree.svg index 1313837f..59e6a52c 100644 --- a/1-js/06-advanced-functions/01-recursion/03-fibonacci-numbers/fibonacci-recursion-tree.svg +++ b/1-js/06-advanced-functions/01-recursion/03-fibonacci-numbers/fibonacci-recursion-tree.svg @@ -1 +1 @@ -fib ( 5 )fib(4)fib(3)fib(3)fib(2)fib(0)fib(1)fib(1)fib(2)fib(0)fib(1)fib(1)fib(2)fib(0)fib(1) \ No newline at end of file +fib ( 5 )fib(4)fib(3)fib(3)fib(2)fib(0)fib(1)fib(1)fib(2)fib(0)fib(1)fib(1)fib(2)fib(0)fib(1) \ No newline at end of file diff --git a/1-js/06-advanced-functions/01-recursion/05-output-single-linked-list-reverse/solution.md b/1-js/06-advanced-functions/01-recursion/05-output-single-linked-list-reverse/solution.md index 0eb76ea1..4357ff20 100644 --- a/1-js/06-advanced-functions/01-recursion/05-output-single-linked-list-reverse/solution.md +++ b/1-js/06-advanced-functions/01-recursion/05-output-single-linked-list-reverse/solution.md @@ -33,7 +33,7 @@ printReverseList(list); # Using a loop -The loop variant is also a little bit more complicated than the direct output. +The loop variant is also a little bit more complicated then the direct output. There is no way to get the last value in our `list`. We also can't "go back". diff --git a/1-js/06-advanced-functions/01-recursion/article.md b/1-js/06-advanced-functions/01-recursion/article.md index 5ae89447..320de62f 100644 --- a/1-js/06-advanced-functions/01-recursion/article.md +++ b/1-js/06-advanced-functions/01-recursion/article.md @@ -61,7 +61,7 @@ When `pow(x, n)` is called, the execution splits into two branches: if n==1 = x / pow(x, n) = - \ + \ else = x * pow(x, n - 1) ``` @@ -132,7 +132,7 @@ We can sketch it as: -That's when the function starts to execute. The condition `n == 1` is falsy, so the flow continues into the second branch of `if`: +That's when the function starts to execute. The condition `n == 1` is false, so the flow continues into the second branch of `if`: ```js run function pow(x, n) { @@ -188,7 +188,7 @@ The new current execution context is on top (and bold), and previous remembered When we finish the subcall -- it is easy to resume the previous context, because it keeps both variables and the exact place of the code where it stopped. ```smart -Here in the picture we use the word "line", as in our example there's only one subcall in line, but generally a single line of code may contain multiple subcalls, like `pow(…) + pow(…) + somethingElse(…)`. +Here in the picture we use the word "line", as our example there's only one subcall in line, but generally a single line of code may contain multiple subcalls, like `pow(…) + pow(…) + somethingElse(…)`. So it would be more precise to say that the execution resumes "immediately after the subcall". ``` @@ -285,7 +285,7 @@ The iterative `pow` uses a single context changing `i` and `result` in the proce **Any recursion can be rewritten as a loop. The loop variant usually can be made more effective.** -...But sometimes the rewrite is non-trivial, especially when a function uses different recursive subcalls depending on conditions and merges their results or when the branching is more intricate. And the optimization may be unneeded and totally not worth the efforts. +...But sometimes the rewrite is non-trivial, especially when function uses different recursive subcalls depending on conditions and merges their results or when the branching is more intricate. And the optimization may be unneeded and totally not worth the efforts. Recursion can give a shorter code, easier to understand and support. Optimizations are not required in every place, mostly we need a good code, that's why it's used. @@ -535,7 +535,7 @@ Terms: list = { value, next -> list } ``` - Trees like HTML elements tree or the department tree from this chapter are also naturally recursive: they have branches and every branch can have other branches. + Trees like HTML elements tree or the department tree from this chapter are also naturally recursive: they branch and every branch can have other branches. Recursive functions can be used to walk them as we've seen in the `sumSalary` example. diff --git a/1-js/06-advanced-functions/01-recursion/linked-list-0.svg b/1-js/06-advanced-functions/01-recursion/linked-list-0.svg index 5d23c7a4..f18c6ffb 100644 --- a/1-js/06-advanced-functions/01-recursion/linked-list-0.svg +++ b/1-js/06-advanced-functions/01-recursion/linked-list-0.svg @@ -1 +1 @@ -value1nextvalue"new item"nextvalue2nextvalue3nextvalue4nextnulllist \ No newline at end of file +value1nextvalue"new item"nextvalue2nextvalue3nextvalue4nextnulllist \ No newline at end of file diff --git a/1-js/06-advanced-functions/01-recursion/linked-list-remove-1.svg b/1-js/06-advanced-functions/01-recursion/linked-list-remove-1.svg index 2f37449c..edec2391 100644 --- a/1-js/06-advanced-functions/01-recursion/linked-list-remove-1.svg +++ b/1-js/06-advanced-functions/01-recursion/linked-list-remove-1.svg @@ -1 +1 @@ -value"new item"nextvalue1nextvalue2nextvalue3nextvalue4nextnulllist \ No newline at end of file +value"new item"nextvalue1nextvalue2nextvalue3nextvalue4nextnulllist \ No newline at end of file diff --git a/1-js/06-advanced-functions/01-recursion/linked-list-split.svg b/1-js/06-advanced-functions/01-recursion/linked-list-split.svg index 6c307213..cba81e64 100644 --- a/1-js/06-advanced-functions/01-recursion/linked-list-split.svg +++ b/1-js/06-advanced-functions/01-recursion/linked-list-split.svg @@ -1 +1 @@ -value1nextvalue2nextvalue3nextvalue4nextnullnullsecondListlist \ No newline at end of file +value1nextvalue2nextvalue3nextvalue4nextnullnullsecondListlist \ No newline at end of file diff --git a/1-js/06-advanced-functions/01-recursion/linked-list.svg b/1-js/06-advanced-functions/01-recursion/linked-list.svg index c02744f3..63a070fd 100644 --- a/1-js/06-advanced-functions/01-recursion/linked-list.svg +++ b/1-js/06-advanced-functions/01-recursion/linked-list.svg @@ -1 +1 @@ -value1nextvalue2nextvalue3nextvalue4nextnulllist \ No newline at end of file +value1nextvalue2nextvalue3nextvalue4nextnulllist \ No newline at end of file diff --git a/1-js/06-advanced-functions/01-recursion/recursion-pow.svg b/1-js/06-advanced-functions/01-recursion/recursion-pow.svg index 2b970a04..8bd4a43f 100644 --- a/1-js/06-advanced-functions/01-recursion/recursion-pow.svg +++ b/1-js/06-advanced-functions/01-recursion/recursion-pow.svg @@ -1 +1 @@ -pow(x,n)xx * pow(x, n-1)n == 1 ?YesNorecursive call until n==1 \ No newline at end of file +pow(x,n)xx * pow(x, n-1)n == 1 ?YesNorecursive call until n==1 \ No newline at end of file diff --git a/1-js/06-advanced-functions/01-recursion/recursive-salaries.svg b/1-js/06-advanced-functions/01-recursion/recursive-salaries.svg index bd874c5b..f47f0668 100644 --- a/1-js/06-advanced-functions/01-recursion/recursive-salaries.svg +++ b/1-js/06-advanced-functions/01-recursion/recursive-salaries.svg @@ -1 +1 @@ - \ No newline at end of file + \ No newline at end of file diff --git a/1-js/06-advanced-functions/02-rest-parameters-spread/article.md b/1-js/06-advanced-functions/02-rest-parameters-spread/article.md index c63fe70c..1f139d7a 100644 --- a/1-js/06-advanced-functions/02-rest-parameters-spread/article.md +++ b/1-js/06-advanced-functions/02-rest-parameters-spread/article.md @@ -225,7 +225,7 @@ But there's a subtle difference between `Array.from(obj)` and `[...obj]`: So, for the task of turning something into an array, `Array.from` tends to be more universal. -## Copy an array/object +## Get a new copy of an array/object Remember when we talked about `Object.assign()` [in the past](info:object-copy#cloning-and-merging-object-assign)? @@ -233,11 +233,8 @@ It is possible to do the same thing with the spread syntax. ```js run let arr = [1, 2, 3]; - -*!* let arrCopy = [...arr]; // spread the array into a list of parameters // then put the result into a new array -*/!* // do the arrays have the same contents? alert(JSON.stringify(arr) === JSON.stringify(arrCopy)); // true @@ -255,11 +252,8 @@ Note that it is possible to do the same thing to make a copy of an object: ```js run let obj = { a: 1, b: 2, c: 3 }; - -*!* let objCopy = { ...obj }; // spread the object into a list of parameters // then return the result in a new object -*/!* // do the objects have the same contents? alert(JSON.stringify(obj) === JSON.stringify(objCopy)); // true @@ -273,7 +267,7 @@ alert(JSON.stringify(obj)); // {"a":1,"b":2,"c":3,"d":4} alert(JSON.stringify(objCopy)); // {"a":1,"b":2,"c":3} ``` -This way of copying an object is much shorter than `let objCopy = Object.assign({}, obj)` or for an array `let arrCopy = Object.assign([], arr)` so we prefer to use it whenever we can. +This way of copying an object is much shorter than `let objCopy = Object.assign({}, obj);` or for an array `let arrCopy = Object.assign([], arr);` so we prefer to use it whenever we can. ## Summary diff --git a/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-empty.svg b/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-empty.svg index f8c7bd6a..c4e0e5b9 100644 --- a/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-empty.svg +++ b/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-empty.svg @@ -1 +1 @@ -outer<empty>makeArmy() LexicalEnvironmentwhile iteration LexicalEnvironment<empty><empty><empty>i: 10 \ No newline at end of file +outer<empty>makeArmy() LexicalEnvironmentwhile iteration LexicalEnvironment<empty><empty><empty>i: 10 \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-for-fixed.svg b/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-for-fixed.svg index 7611d0ef..3d1f3022 100644 --- a/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-for-fixed.svg +++ b/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-for-fixed.svg @@ -1 +1 @@ -outermakeArmy() LexicalEnvironmentfor iteration LexicalEnvironmenti: 0i: 1i: 2i: 10... \ No newline at end of file +outermakeArmy() LexicalEnvironmentfor iteration LexicalEnvironmenti: 0i: 1i: 2i: 10... \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-while-fixed.svg b/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-while-fixed.svg index d83ecbe7..c5185e4f 100644 --- a/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-while-fixed.svg +++ b/1-js/06-advanced-functions/03-closure/10-make-army/lexenv-makearmy-while-fixed.svg @@ -1 +1 @@ -outerj: 0j: 1j: 2j: 10...makeArmy() LexicalEnvironmentwhile iteration LexicalEnvironment \ No newline at end of file +outerj: 0j: 1j: 2j: 10...makeArmy() LexicalEnvironmentwhile iteration LexicalEnvironment \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/10-make-army/solution.md b/1-js/06-advanced-functions/03-closure/10-make-army/solution.md index 9d99aa71..3dbefb52 100644 --- a/1-js/06-advanced-functions/03-closure/10-make-army/solution.md +++ b/1-js/06-advanced-functions/03-closure/10-make-army/solution.md @@ -88,11 +88,11 @@ Let's examine what exactly happens inside `makeArmy`, and the solution will beco Here `let j = i` declares an "iteration-local" variable `j` and copies `i` into it. Primitives are copied "by value", so we actually get an independent copy of `i`, belonging to the current loop iteration. - The shooters work correctly, because the value of `i` now lives a little bit closer. Not in `makeArmy()` Lexical Environment, but in the Lexical Environment that corresponds to the current loop iteration: + The shooters work correctly, because the value of `i` now lives a little bit closer. Not in `makeArmy()` Lexical Environment, but in the Lexical Environment that corresponds the current loop iteration: ![](lexenv-makearmy-while-fixed.svg) - Such a problem could also be avoided if we used `for` in the beginning, like this: + Such problem could also be avoided if we used `for` in the beginning, like this: ```js run demo function makeArmy() { diff --git a/1-js/06-advanced-functions/03-closure/2-closure-variable-access/lexenv-nested-work.svg b/1-js/06-advanced-functions/03-closure/2-closure-variable-access/lexenv-nested-work.svg index 8dfd8bd6..5cdf7f1a 100644 --- a/1-js/06-advanced-functions/03-closure/2-closure-variable-access/lexenv-nested-work.svg +++ b/1-js/06-advanced-functions/03-closure/2-closure-variable-access/lexenv-nested-work.svg @@ -1 +1 @@ -makeWorker: function name: "John"<empty>outerouterouternullname: "Pete" \ No newline at end of file +makeWorker: function name: "John"<empty>outerouterouternullname: "Pete" \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/5-function-in-if/task.md b/1-js/06-advanced-functions/03-closure/5-function-in-if/task.md index 4e386eec..d02c53b9 100644 --- a/1-js/06-advanced-functions/03-closure/5-function-in-if/task.md +++ b/1-js/06-advanced-functions/03-closure/5-function-in-if/task.md @@ -1,6 +1,4 @@ -importance: 5 ---- # Function in if Look at the code. What will be the result of the call at the last line? diff --git a/1-js/06-advanced-functions/03-closure/7-let-scope/solution.md b/1-js/06-advanced-functions/03-closure/7-let-scope/solution.md index b16b3529..346e4060 100644 --- a/1-js/06-advanced-functions/03-closure/7-let-scope/solution.md +++ b/1-js/06-advanced-functions/03-closure/7-let-scope/solution.md @@ -27,7 +27,7 @@ The code above demonstrates it. function func() { *!* // the local variable x is known to the engine from the beginning of the function, - // but "uninitialized" (unusable) until let ("dead zone") + // but "unitialized" (unusable) until let ("dead zone") // hence the error */!* diff --git a/1-js/06-advanced-functions/03-closure/9-sort-by-field/_js.view/test.js b/1-js/06-advanced-functions/03-closure/9-sort-by-field/_js.view/test.js index 802f28c4..e3c335e0 100644 --- a/1-js/06-advanced-functions/03-closure/9-sort-by-field/_js.view/test.js +++ b/1-js/06-advanced-functions/03-closure/9-sort-by-field/_js.view/test.js @@ -23,7 +23,7 @@ describe("byField", function(){ { name: "John", age: 20, surname: "Johnson"}, ]; let ageSortedAnswer = users.sort(byField("age")); - assert.deepEqual(ageSortedKey, ageSortedAnswer); + assert.deepEqual(ageSortedKey, ageSortedKey); }); it("sorts users by surname", function(){ diff --git a/1-js/06-advanced-functions/03-closure/article.md b/1-js/06-advanced-functions/03-closure/article.md index cb43a796..3a6a7e4e 100644 --- a/1-js/06-advanced-functions/03-closure/article.md +++ b/1-js/06-advanced-functions/03-closure/article.md @@ -7,7 +7,7 @@ We already know that a function can access variables outside of it ("outer" vari But what happens if outer variables change since a function is created? Will the function get newer values or the old ones? -And what if a function is passed along as an argument and called from another place of code, will it get access to outer variables at the new place? +And what if a function is passed along as a parameter and called from another place of code, will it get access to outer variables at the new place? Let's expand our knowledge to understand these scenarios and more complex ones. @@ -146,7 +146,7 @@ Despite being simple, slightly modified variants of that code have practical use How does this work? If we create multiple counters, will they be independent? What's going on with the variables here? -Understanding such things is great for the overall knowledge of JavaScript and beneficial for more complex scenarios. So let's go a bit in-depth. +Undestanding such things is great for the overall knowledge of JavaScript and beneficial for more complex scenarios. So let's go a bit in-depth. ## Lexical Environment diff --git a/1-js/06-advanced-functions/03-closure/closure-function-declaration.svg b/1-js/06-advanced-functions/03-closure/closure-function-declaration.svg index 3ef78787..97f76e56 100644 --- a/1-js/06-advanced-functions/03-closure/closure-function-declaration.svg +++ b/1-js/06-advanced-functions/03-closure/closure-function-declaration.svg @@ -1 +1 @@ -outernullexecution startphrase: <uninitialized> say: function... \ No newline at end of file +outernullexecution startphrase: <uninitialized> say: function... \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/closure-makecounter-environment.svg b/1-js/06-advanced-functions/03-closure/closure-makecounter-environment.svg index f7844171..b9060bc8 100644 --- a/1-js/06-advanced-functions/03-closure/closure-makecounter-environment.svg +++ b/1-js/06-advanced-functions/03-closure/closure-makecounter-environment.svg @@ -1 +1 @@ -null[[Environment]]makeCounter: function counter: undefinedcount: 0outerouter \ No newline at end of file +null[[Environment]]makeCounter: function counter: undefinedcount: 0outerouter \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/closure-makecounter-nested-call-2.svg b/1-js/06-advanced-functions/03-closure/closure-makecounter-nested-call-2.svg index 3950a8fa..3e4206ca 100644 --- a/1-js/06-advanced-functions/03-closure/closure-makecounter-nested-call-2.svg +++ b/1-js/06-advanced-functions/03-closure/closure-makecounter-nested-call-2.svg @@ -1 +1 @@ -count: 1<empty>nullouterouteroutermakeCounter: function counter: functionmodified here \ No newline at end of file +count: 1<empty>nullouterouteroutermakeCounter: function counter: functionmodified here \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/closure-makecounter-nested-call.svg b/1-js/06-advanced-functions/03-closure/closure-makecounter-nested-call.svg index 24315bf2..e1bb8cc8 100644 --- a/1-js/06-advanced-functions/03-closure/closure-makecounter-nested-call.svg +++ b/1-js/06-advanced-functions/03-closure/closure-makecounter-nested-call.svg @@ -1 +1 @@ -count: 0<empty>nullouterouteroutermakeCounter: function counter: function \ No newline at end of file +count: 0<empty>nullouterouteroutermakeCounter: function counter: function \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/closure-makecounter.svg b/1-js/06-advanced-functions/03-closure/closure-makecounter.svg index 2ca06455..2a1c4a72 100644 --- a/1-js/06-advanced-functions/03-closure/closure-makecounter.svg +++ b/1-js/06-advanced-functions/03-closure/closure-makecounter.svg @@ -1 +1 @@ -makeCounter: function counter: undefinedcount: 0nullglobal LexicalEnvironmentLexicalEnvironment of makeCounter() callouterouter \ No newline at end of file +makeCounter: function counter: undefinedcount: 0nullglobal LexicalEnvironmentLexicalEnvironment of makeCounter() callouterouter \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/closure-variable-phrase.svg b/1-js/06-advanced-functions/03-closure/closure-variable-phrase.svg index b9bb12ff..741c0544 100644 --- a/1-js/06-advanced-functions/03-closure/closure-variable-phrase.svg +++ b/1-js/06-advanced-functions/03-closure/closure-variable-phrase.svg @@ -1 +1 @@ -phrase: "Bye"phrase: "Hello"phrase: undefinedphrase: <uninitialized>outernullexecution start \ No newline at end of file +phrase: "Bye"phrase: "Hello"phrase: undefinedphrase: <uninitialized>outernullexecution start \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexenv-if.svg b/1-js/06-advanced-functions/03-closure/lexenv-if.svg index 3d4d6d7c..b644fe15 100644 --- a/1-js/06-advanced-functions/03-closure/lexenv-if.svg +++ b/1-js/06-advanced-functions/03-closure/lexenv-if.svg @@ -1 +1 @@ -phrase: "Hello"outerouternulluser: "John" \ No newline at end of file +phrase: "Hello"outerouternulluser: "John" \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-1.svg b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-1.svg index f15e77a8..a14df709 100644 --- a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-1.svg +++ b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-1.svg @@ -1 +1 @@ -makeCounter: function[[Environment]]outernull \ No newline at end of file +makeCounter: function[[Environment]]outernull \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-2.svg b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-2.svg index f3748853..66e5200f 100644 --- a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-2.svg +++ b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-2.svg @@ -1 +1 @@ -makeCounter: functioncounter: undefinedcount: 0outerouternullglobal LexicalEnvironmentLexicalEnvironment of makeCounter() call \ No newline at end of file +makeCounter: functioncounter: undefinedcount: 0outerouternullglobal LexicalEnvironmentLexicalEnvironment of makeCounter() call \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-3.svg b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-3.svg index 54f1d97b..28c526c4 100644 --- a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-3.svg +++ b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-3.svg @@ -1 +1 @@ -makeCounter: functioncounter: undefinedcount: 0outerouternull[[Environment]] \ No newline at end of file +makeCounter: functioncounter: undefinedcount: 0outerouternull[[Environment]] \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-4.svg b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-4.svg index fb60a785..acc1e8fb 100644 --- a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-4.svg +++ b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-4.svg @@ -1 +1 @@ -makeCounter: functioncounter: functioncount: 0outerouternull[[Environment]] \ No newline at end of file +makeCounter: functioncounter: functioncount: 0outerouternull[[Environment]] \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-5.svg b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-5.svg index 79c440da..cf91c331 100644 --- a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-5.svg +++ b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-5.svg @@ -1 +1 @@ -makeCounter: functioncounter: functioncount: 0<empty>outerouterouternull[[Environment]] \ No newline at end of file +makeCounter: functioncounter: functioncount: 0<empty>outerouterouternull[[Environment]] \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-6.svg b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-6.svg index 06d5b506..def542ce 100644 --- a/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-6.svg +++ b/1-js/06-advanced-functions/03-closure/lexenv-nested-makecounter-6.svg @@ -1 +1 @@ -makeCounter: functioncounter: functioncount: 1outerouternull[[Environment]]modified here \ No newline at end of file +makeCounter: functioncounter: functioncount: 1outerouternull[[Environment]]modified here \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexical-environment-global-2.svg b/1-js/06-advanced-functions/03-closure/lexical-environment-global-2.svg index b6e576f0..2e956cbb 100644 --- a/1-js/06-advanced-functions/03-closure/lexical-environment-global-2.svg +++ b/1-js/06-advanced-functions/03-closure/lexical-environment-global-2.svg @@ -1 +1 @@ -phrase: "Bye"phrase: "Hello"phrase: undefined<empty>outernullexecution start \ No newline at end of file +phrase: "Bye"phrase: "Hello"phrase: undefined<empty>outernullexecution start \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexical-environment-global-3.svg b/1-js/06-advanced-functions/03-closure/lexical-environment-global-3.svg index 1942a7e3..d0f4a8e6 100644 --- a/1-js/06-advanced-functions/03-closure/lexical-environment-global-3.svg +++ b/1-js/06-advanced-functions/03-closure/lexical-environment-global-3.svg @@ -1 +1 @@ -say: function phrase: "Hello"say: functionouternullexecution start \ No newline at end of file +say: function phrase: "Hello"say: functionouternullexecution start \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexical-environment-global.svg b/1-js/06-advanced-functions/03-closure/lexical-environment-global.svg index 7bddc223..9620f048 100644 --- a/1-js/06-advanced-functions/03-closure/lexical-environment-global.svg +++ b/1-js/06-advanced-functions/03-closure/lexical-environment-global.svg @@ -1 +1 @@ -phrase: "Hello"outernullLexical Environment \ No newline at end of file +phrase: "Hello"outernullLexical Environment \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexical-environment-simple-lookup.svg b/1-js/06-advanced-functions/03-closure/lexical-environment-simple-lookup.svg index 79501a5b..ff0486ed 100644 --- a/1-js/06-advanced-functions/03-closure/lexical-environment-simple-lookup.svg +++ b/1-js/06-advanced-functions/03-closure/lexical-environment-simple-lookup.svg @@ -1 +1 @@ -say: function phrase: "Hello"name: "John"outerouternull \ No newline at end of file +say: function phrase: "Hello"name: "John"outerouternull \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexical-environment-simple.svg b/1-js/06-advanced-functions/03-closure/lexical-environment-simple.svg index dea6ac46..abd77fff 100644 --- a/1-js/06-advanced-functions/03-closure/lexical-environment-simple.svg +++ b/1-js/06-advanced-functions/03-closure/lexical-environment-simple.svg @@ -1 +1 @@ -say: function phrase: "Hello"name: "John"outerouternullLexical Environment of the call \ No newline at end of file +say: function phrase: "Hello"name: "John"outerouternullLexical Environment of the call \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/lexical-search-order.svg b/1-js/06-advanced-functions/03-closure/lexical-search-order.svg index d9884ec4..89a9d110 100644 --- a/1-js/06-advanced-functions/03-closure/lexical-search-order.svg +++ b/1-js/06-advanced-functions/03-closure/lexical-search-order.svg @@ -1 +1 @@ -123 \ No newline at end of file +123 \ No newline at end of file diff --git a/1-js/06-advanced-functions/03-closure/variable-scope-lookup.svg b/1-js/06-advanced-functions/03-closure/variable-scope-lookup.svg index f1f1d3b1..67443719 100644 --- a/1-js/06-advanced-functions/03-closure/variable-scope-lookup.svg +++ b/1-js/06-advanced-functions/03-closure/variable-scope-lookup.svg @@ -1 +1 @@ -functionUser(name){this.sayHi=function(){alert(name);};}letuser=newUser("John");user.sayHi(); \ No newline at end of file +functionUser(name){this.sayHi=function(){alert(name);};}letuser=newUser("John");user.sayHi(); \ No newline at end of file diff --git a/1-js/06-advanced-functions/04-var/article.md b/1-js/06-advanced-functions/04-var/article.md index 1579afb6..b2559989 100644 --- a/1-js/06-advanced-functions/04-var/article.md +++ b/1-js/06-advanced-functions/04-var/article.md @@ -4,7 +4,7 @@ ```smart header="This article is for understanding old scripts" The information in this article is useful for understanding old scripts. -That's not how we write new code. +That's not how we write a new code. ``` In the very first chapter about [variables](info:variables), we mentioned three ways of variable declaration: @@ -52,7 +52,7 @@ if (true) { } *!* -alert(test); // ReferenceError: test is not defined +alert(test); // Error: test is not defined */!* ``` @@ -82,7 +82,7 @@ function sayHi() { } sayHi(); -alert(phrase); // ReferenceError: phrase is not defined +alert(phrase); // Error: phrase is not defined (Check the Developer Console) ``` As we can see, `var` pierces through `if`, `for` or other code blocks. That's because a long time ago in JavaScript, blocks had no Lexical Environments, and `var` is a remnant of that. @@ -231,7 +231,7 @@ The Function Expression is wrapped with parenthesis `(function {...})`, because ```js run // Tries to declare and immediately call a function -function() { // <-- SyntaxError: Function statements require a function name +function() { // <-- Error: Function statements require a function name var message = "Hello"; @@ -256,11 +256,11 @@ There exist other ways besides parentheses to tell JavaScript that we mean a Fun ```js run // Ways to create IIFE -*!*(*/!*function() { +(function() { alert("Parentheses around the function"); }*!*)*/!*(); -*!*(*/!*function() { +(function() { alert("Parentheses around the whole thing"); }()*!*)*/!*; @@ -279,7 +279,7 @@ In all the above cases we declare a Function Expression and run it immediately. There are two main differences of `var` compared to `let/const`: -1. `var` variables have no block scope, their visibility is scoped to current function, or global, if declared outside function. +1. `var` variables have no block scope; their visibility is scoped to current function, or global, if declared outside function. 2. `var` declarations are processed at function start (script start for globals). There's one more very minor difference related to the global object, that we'll cover in the next chapter. diff --git a/1-js/06-advanced-functions/05-global-object/article.md b/1-js/06-advanced-functions/05-global-object/article.md index cf4839d9..679db05c 100644 --- a/1-js/06-advanced-functions/05-global-object/article.md +++ b/1-js/06-advanced-functions/05-global-object/article.md @@ -5,7 +5,7 @@ The global object provides variables and functions that are available anywhere. In a browser it is named `window`, for Node.js it is `global`, for other environments it may have another name. -Recently, `globalThis` was added to the language, as a standardized name for a global object, that should be supported across all environments. It's supported in all major browsers. +Recently, `globalThis` was added to the language, as a standardized name for a global object, that should be supported across all environments. It's supported in all major browsers. We'll use `window` here, assuming that our environment is a browser. If your script may run in other environments, it's better to use `globalThis` instead. @@ -25,9 +25,7 @@ var gVar = 5; alert(window.gVar); // 5 (became a property of the global object) ``` -Function declarations have the same effect (statements with `function` keyword in the main code flow, not function expressions). - -Please don't rely on that! This behavior exists for compatibility reasons. Modern scripts use [JavaScript modules](info:modules) where such a thing doesn't happen. +Please don't rely on that! This behavior exists for compatibility reasons. Modern scripts use [JavaScript modules](info:modules) where such thing doesn't happen. If we used `let` instead, such thing wouldn't happen: diff --git a/1-js/06-advanced-functions/06-function-object/5-sum-many-brackets/solution.md b/1-js/06-advanced-functions/06-function-object/5-sum-many-brackets/solution.md index e97039f7..5c932691 100644 --- a/1-js/06-advanced-functions/06-function-object/5-sum-many-brackets/solution.md +++ b/1-js/06-advanced-functions/06-function-object/5-sum-many-brackets/solution.md @@ -5,7 +5,7 @@ Now the code: -```js demo run +```js run function sum(a) { let currentSum = a; @@ -52,4 +52,4 @@ function f(b) { } ``` -This `f` will be used in the next call, again return itself, as many times as needed. Then, when used as a number or a string -- the `toString` returns the `currentSum`. We could also use `Symbol.toPrimitive` or `valueOf` here for the conversion. +This `f` will be used in the next call, again return itself, so many times as needed. Then, when used as a number or a string -- the `toString` returns the `currentSum`. We could also use `Symbol.toPrimitive` or `valueOf` here for the conversion. diff --git a/1-js/06-advanced-functions/06-function-object/article.md b/1-js/06-advanced-functions/06-function-object/article.md index c84f4e52..ed848c0c 100644 --- a/1-js/06-advanced-functions/06-function-object/article.md +++ b/1-js/06-advanced-functions/06-function-object/article.md @@ -326,7 +326,7 @@ welcome(); // Hello, Guest (nested call works) Now it works, because the name `"func"` is function-local. It is not taken from outside (and not visible there). The specification guarantees that it will always reference the current function. -The outer code still has its variable `sayHi` or `welcome`. And `func` is an "internal function name", the way for the function to can call itself reliably. +The outer code still has it's variable `sayHi` or `welcome`. And `func` is an "internal function name", how the function can call itself internally. ```smart header="There's no such thing for Function Declaration" The "internal name" feature described here is only available for Function Expressions, not for Function Declarations. For Function Declarations, there is no syntax for adding an "internal" name. @@ -347,7 +347,7 @@ If the function is declared as a Function Expression (not in the main code flow) Also, functions may carry additional properties. Many well-known JavaScript libraries make great use of this feature. -They create a "main" function and attach many other "helper" functions to it. For instance, the [jQuery](https://jquery.com) library creates a function named `$`. The [lodash](https://lodash.com) library creates a function `_`, and then adds `_.clone`, `_.keyBy` and other properties to it (see the [docs](https://lodash.com/docs) when you want to learn more about them). Actually, they do it to lessen their pollution of the global space, so that a single library gives only one global variable. That reduces the possibility of naming conflicts. +They create a "main" function and attach many other "helper" functions to it. For instance, the [jQuery](https://jquery.com) library creates a function named `$`. The [lodash](https://lodash.com) library creates a function `_`, and then adds `_.clone`, `_.keyBy` and other properties to it (see the [docs](https://lodash.com/docs) when you want learn more about them). Actually, they do it to lessen their pollution of the global space, so that a single library gives only one global variable. That reduces the possibility of naming conflicts. So, a function can do a useful job by itself and also carry a bunch of other functionality in properties. diff --git a/1-js/06-advanced-functions/07-new-function/article.md b/1-js/06-advanced-functions/07-new-function/article.md index ffe264a4..3214ba37 100644 --- a/1-js/06-advanced-functions/07-new-function/article.md +++ b/1-js/06-advanced-functions/07-new-function/article.md @@ -92,7 +92,7 @@ What if it could access the outer variables? The problem is that before JavaScript is published to production, it's compressed using a *minifier* -- a special program that shrinks code by removing extra comments, spaces and -- what's important, renames local variables into shorter ones. -For instance, if a function has `let userName`, minifier replaces it with `let a` (or another letter if this one is occupied), and does it everywhere. That's usually a safe thing to do, because the variable is local, nothing outside the function can access it. And inside the function, minifier replaces every mention of it. Minifiers are smart, they analyze the code structure, so they don't break anything. They're not just a dumb find-and-replace. +For instance, if a function has `let userName`, minifier replaces it `let a` (or another letter if this one is occupied), and does it everywhere. That's usually a safe thing to do, because the variable is local, nothing outside the function can access it. And inside the function, minifier replaces every mention of it. Minifiers are smart, they analyze the code structure, so they don't break anything. They're not just a dumb find-and-replace. So if `new Function` had access to outer variables, it would be unable to find renamed `userName`. diff --git a/1-js/06-advanced-functions/08-settimeout-setinterval/article.md b/1-js/06-advanced-functions/08-settimeout-setinterval/article.md index 5a40238b..95fddea6 100644 --- a/1-js/06-advanced-functions/08-settimeout-setinterval/article.md +++ b/1-js/06-advanced-functions/08-settimeout-setinterval/article.md @@ -102,7 +102,7 @@ As we can see from `alert` output, in a browser the timer identifier is a number Again, there is no universal specification for these methods, so that's fine. -For browsers, timers are described in the [timers section](https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timers) of HTML Living Standard. +For browsers, timers are described in the [timers section](https://www.w3.org/TR/html5/webappapis.html#timers) of HTML5 standard. ## setInterval @@ -129,7 +129,7 @@ setTimeout(() => { clearInterval(timerId); alert('stop'); }, 5000); ```smart header="Time goes on while `alert` is shown" In most browsers, including Chrome and Firefox the internal timer continues "ticking" while showing `alert/confirm/prompt`. -So if you run the code above and don't dismiss the `alert` window for some time, then the next `alert` will be shown immediately as you do it. The actual interval between alerts will be shorter than 2 seconds. +So if you run the code above and don't dismiss the `alert` window for some time, then in the next `alert` will be shown immediately as you do it. The actual interval between alerts will be shorter than 2 seconds. ``` ## Nested setTimeout @@ -232,7 +232,7 @@ setTimeout(function() {...}, 100); For `setInterval` the function stays in memory until `clearInterval` is called. -There's a side effect. A function references the outer lexical environment, so, while it lives, outer variables live too. They may take much more memory than the function itself. So when we don't need the scheduled function anymore, it's better to cancel it, even if it's very small. +There's a side-effect. A function references the outer lexical environment, so, while it lives, outer variables live too. They may take much more memory than the function itself. So when we don't need the scheduled function anymore, it's better to cancel it, even if it's very small. ```` ## Zero delay setTimeout @@ -256,7 +256,7 @@ The first line "puts the call into calendar after 0ms". But the scheduler will o There are also advanced browser-related use cases of zero-delay timeout, that we'll discuss in the chapter . ````smart header="Zero delay is in fact not zero (in a browser)" -In the browser, there's a limitation of how often nested timers can run. The [HTML Living Standard](https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timers) says: "after five nested timers, the interval is forced to be at least 4 milliseconds.". +In the browser, there's a limitation of how often nested timers can run. The [HTML5 standard](https://html.spec.whatwg.org/multipage/timers-and-user-prompts.html#timers) says: "after five nested timers, the interval is forced to be at least 4 milliseconds.". Let's demonstrate what it means with the example below. The `setTimeout` call in it re-schedules itself with zero delay. Each call remembers the real time from the previous one in the `times` array. What do the real delays look like? Let's see: @@ -281,7 +281,7 @@ The similar thing happens if we use `setInterval` instead of `setTimeout`: `setI That limitation comes from ancient times and many scripts rely on it, so it exists for historical reasons. -For server-side JavaScript, that limitation does not exist, and there exist other ways to schedule an immediate asynchronous job, like [setImmediate](https://nodejs.org/api/timers.html#timers_setimmediate_callback_args) for Node.js. So this note is browser-specific. +For server-side JavaScript, that limitation does not exist, and there exist other ways to schedule an immediate asynchronous job, like [setImmediate](https://nodejs.org/api/timers.html) for Node.js. So this note is browser-specific. ```` ## Summary @@ -290,13 +290,13 @@ For server-side JavaScript, that limitation does not exist, and there exist othe - To cancel the execution, we should call `clearTimeout/clearInterval` with the value returned by `setTimeout/setInterval`. - Nested `setTimeout` calls are a more flexible alternative to `setInterval`, allowing us to set the time *between* executions more precisely. - Zero delay scheduling with `setTimeout(func, 0)` (the same as `setTimeout(func)`) is used to schedule the call "as soon as possible, but after the current script is complete". -- The browser limits the minimal delay for five or more nested calls of `setTimeout` or for `setInterval` (after 5th call) to 4ms. That's for historical reasons. +- The browser limits the minimal delay for five or more nested call of `setTimeout` or for `setInterval` (after 5th call) to 4ms. That's for historical reasons. Please note that all scheduling methods do not *guarantee* the exact delay. For example, the in-browser timer may slow down for a lot of reasons: - The CPU is overloaded. - The browser tab is in the background mode. -- The laptop is on battery saving mode. +- The laptop is on battery. All that may increase the minimal timer resolution (the minimal delay) to 300ms or even 1000ms depending on the browser and OS-level performance settings. diff --git a/1-js/06-advanced-functions/08-settimeout-setinterval/setinterval-interval.svg b/1-js/06-advanced-functions/08-settimeout-setinterval/setinterval-interval.svg index bce7d6a8..9a214c54 100644 --- a/1-js/06-advanced-functions/08-settimeout-setinterval/setinterval-interval.svg +++ b/1-js/06-advanced-functions/08-settimeout-setinterval/setinterval-interval.svg @@ -1 +1 @@ -func(1)func(2)func(3)100200300 \ No newline at end of file +func(1)func(2)func(3)100200300 \ No newline at end of file diff --git a/1-js/06-advanced-functions/08-settimeout-setinterval/settimeout-interval.svg b/1-js/06-advanced-functions/08-settimeout-setinterval/settimeout-interval.svg index d6d233b2..a559f616 100644 --- a/1-js/06-advanced-functions/08-settimeout-setinterval/settimeout-interval.svg +++ b/1-js/06-advanced-functions/08-settimeout-setinterval/settimeout-interval.svg @@ -1 +1 @@ -func(1)func(2)func(3)100100 \ No newline at end of file +func(1)func(2)func(3)100100 \ No newline at end of file diff --git a/1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/debounce.svg b/1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/debounce.svg index e624ce02..5896a5fa 100644 --- a/1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/debounce.svg +++ b/1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/debounce.svg @@ -1 +1 @@ -200ms1500ms1000ms0cf(a)f(b)f(c)500mstimecalls: after 1000ms \ No newline at end of file +200ms1500ms1000ms0cf(a)f(b)f(c)500mstimecalls: after 1000ms \ No newline at end of file diff --git a/1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/task.md b/1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/task.md index 5b0fcc5f..347a5e64 100644 --- a/1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/task.md +++ b/1-js/06-advanced-functions/09-call-apply-decorators/03-debounce/task.md @@ -21,9 +21,9 @@ Here's the code for it (uses the debounce decorator from the [Lodash library](ht ```js let f = _.debounce(alert, 1000); -f("a"); +f("a"); setTimeout( () => f("b"), 200); -setTimeout( () => f("c"), 500); +setTimeout( () => f("c"), 500); // debounced function waits 1000ms after the last call and then runs: alert("c") ``` diff --git a/1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/solution.md b/1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/solution.md index 6950664b..cf851f77 100644 --- a/1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/solution.md +++ b/1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/solution.md @@ -12,10 +12,11 @@ function throttle(func, ms) { savedThis = this; return; } - isThrottled = true; func.apply(this, arguments); // (1) + isThrottled = true; + setTimeout(function() { isThrottled = false; // (3) if (savedArgs) { diff --git a/1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/task.md b/1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/task.md index cbd47319..6df7af13 100644 --- a/1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/task.md +++ b/1-js/06-advanced-functions/09-call-apply-decorators/04-throttle/task.md @@ -8,7 +8,7 @@ Create a "throttling" decorator `throttle(f, ms)` -- that returns a wrapper. When it's called multiple times, it passes the call to `f` at maximum once per `ms` milliseconds. -Compared to the debounce decorator, the behavior is completely different: +The difference with debounce is that it's completely different decorator: - `debounce` runs the function once after the "cooldown" period. Good for processing the final result. - `throttle` runs it not more often than given `ms` time. Good for regular updates that shouldn't be very often. diff --git a/1-js/06-advanced-functions/09-call-apply-decorators/article.md b/1-js/06-advanced-functions/09-call-apply-decorators/article.md index c5d78549..d0dda4df 100644 --- a/1-js/06-advanced-functions/09-call-apply-decorators/article.md +++ b/1-js/06-advanced-functions/09-call-apply-decorators/article.md @@ -36,11 +36,11 @@ function cachingDecorator(func) { slow = cachingDecorator(slow); -alert( slow(1) ); // slow(1) is cached and the result returned -alert( "Again: " + slow(1) ); // slow(1) result returned from cache +alert( slow(1) ); // slow(1) is cached +alert( "Again: " + slow(1) ); // the same -alert( slow(2) ); // slow(2) is cached and the result returned -alert( "Again: " + slow(2) ); // slow(2) result returned from cache +alert( slow(2) ); // slow(2) is cached +alert( "Again: " + slow(2) ); // the same as the previous line ``` In the code above `cachingDecorator` is a *decorator*: a special function that takes another function and alters its behavior. @@ -301,18 +301,18 @@ The only syntax difference between `call` and `apply` is that `call` expects a l So these two calls are almost equivalent: ```js -func.call(context, ...args); -func.apply(context, args); +func.call(context, ...args); // pass an array as list with spread syntax +func.apply(context, args); // is same as using call ``` -They perform the same call of `func` with given context and arguments. - -There's only a subtle difference regarding `args`: +There's only a subtle difference: - The spread syntax `...` allows to pass *iterable* `args` as the list to `call`. - The `apply` accepts only *array-like* `args`. -...And for objects that are both iterable and array-like, such as a real array, we can use any of them, but `apply` will probably be faster, because most JavaScript engines internally optimize it better. +So, where we expect an iterable, `call` works, and where we expect an array-like, `apply` works. + +And for objects that are both iterable and array-like, like a real array, we can use any of them, but `apply` will probably be faster, because most JavaScript engines internally optimize it better. Passing all arguments along with the context to another function is called *call forwarding*. diff --git a/1-js/06-advanced-functions/09-call-apply-decorators/decorator-makecaching-wrapper.svg b/1-js/06-advanced-functions/09-call-apply-decorators/decorator-makecaching-wrapper.svg index 9b63cb98..258fcfdf 100644 --- a/1-js/06-advanced-functions/09-call-apply-decorators/decorator-makecaching-wrapper.svg +++ b/1-js/06-advanced-functions/09-call-apply-decorators/decorator-makecaching-wrapper.svg @@ -1 +1 @@ -wrapperaround the function \ No newline at end of file +wrapperaround the function \ No newline at end of file diff --git a/1-js/06-advanced-functions/10-bind/article.md b/1-js/06-advanced-functions/10-bind/article.md index 9d705cdc..8de8e6fd 100644 --- a/1-js/06-advanced-functions/10-bind/article.md +++ b/1-js/06-advanced-functions/10-bind/article.md @@ -187,8 +187,8 @@ let user = { let say = user.say.bind(user); -say("Hello"); // Hello, John! ("Hello" argument is passed to say) -say("Bye"); // Bye, John! ("Bye" is passed to say) +say("Hello"); // Hello, John ("Hello" argument is passed to say) +say("Bye"); // Bye, John ("Bye" is passed to say) ``` ````smart header="Convenience method: `bindAll`" @@ -247,7 +247,7 @@ The call to `mul.bind(null, 2)` creates a new function `double` that passes call That's called [partial function application](https://en.wikipedia.org/wiki/Partial_application) -- we create a new function by fixing some parameters of the existing one. -Please note that we actually don't use `this` here. But `bind` requires it, so we must put in something like `null`. +Please note that here we actually don't use `this` here. But `bind` requires it, so we must put in something like `null`. The function `triple` in the code below triples the value: diff --git a/1-js/06-advanced-functions/12-arrow-functions/article.md b/1-js/06-advanced-functions/12-arrow-functions/article.md index 8730277a..f5caeaec 100644 --- a/1-js/06-advanced-functions/12-arrow-functions/article.md +++ b/1-js/06-advanced-functions/12-arrow-functions/article.md @@ -52,7 +52,7 @@ let group = { *!* this.students.forEach(function(student) { // Error: Cannot read property 'title' of undefined - alert(this.title + ': ' + student); + alert(this.title + ': ' + student) }); */!* } @@ -87,7 +87,7 @@ For instance, `defer(f, ms)` gets a function and returns a wrapper around it tha ```js run function defer(f, ms) { return function() { - setTimeout(() => f.apply(this, arguments), ms); + setTimeout(() => f.apply(this, arguments), ms) }; } diff --git a/1-js/07-object-properties/01-property-descriptors/article.md b/1-js/07-object-properties/01-property-descriptors/article.md index bdc69341..f8f8d21d 100644 --- a/1-js/07-object-properties/01-property-descriptors/article.md +++ b/1-js/07-object-properties/01-property-descriptors/article.md @@ -19,7 +19,7 @@ We didn't see them yet, because generally they do not show up. When we create a First, let's see how to get those flags. -The method [Object.getOwnPropertyDescriptor](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptor) allows to query the *full* information about a property. +The method [Object.getOwnPropertyDescriptor](mdn:js/Object/getOwnPropertyDescriptor) allows to query the *full* information about a property. The syntax is: ```js @@ -54,7 +54,7 @@ alert( JSON.stringify(descriptor, null, 2 ) ); */ ``` -To change the flags, we can use [Object.defineProperty](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperty). +To change the flags, we can use [Object.defineProperty](mdn:js/Object/defineProperty). The syntax is: @@ -194,7 +194,7 @@ alert(Object.keys(user)); // name The non-configurable flag (`configurable:false`) is sometimes preset for built-in objects and properties. -A non-configurable property can't be deleted, its attributes can't be modified. +A non-configurable property can not be deleted. For instance, `Math.PI` is non-writable, non-enumerable and non-configurable: @@ -214,23 +214,20 @@ alert( JSON.stringify(descriptor, null, 2 ) ); So, a programmer is unable to change the value of `Math.PI` or overwrite it. ```js run -Math.PI = 3; // Error, because it has writable: false +Math.PI = 3; // Error // delete Math.PI won't work either ``` -We also can't change `Math.PI` to be `writable` again: - -```js run -// Error, because of configurable: false -Object.defineProperty(Math, "PI", { writable: true }); -``` - -There's absolutely nothing we can do with `Math.PI`. - Making a property non-configurable is a one-way road. We cannot change it back with `defineProperty`. -**Please note: `configurable: false` prevents changes of property flags and its deletion, while allowing to change its value.** +To be precise, non-configurability imposes several restrictions on `defineProperty`: +1. Can't change `configurable` flag. +2. Can't change `enumerable` flag. +3. Can't change `writable: false` to `true` (the other way round works). +4. Can't change `get/set` for an accessor property (but can assign them if absent). + +**The idea of "configurable: false" is to prevent changes of property flags and its deletion, while allowing to change its value.** Here `user.name` is non-configurable, but we can still change it (as it's writable): @@ -247,7 +244,7 @@ user.name = "Pete"; // works fine delete user.name; // Error ``` -And here we make `user.name` a "forever sealed" constant, just like the built-in `Math.PI`: +And here we make `user.name` a "forever sealed" constant: ```js run let user = { @@ -266,15 +263,10 @@ delete user.name; Object.defineProperty(user, "name", { value: "Pete" }); ``` -```smart header="The only attribute change possible: writable true -> false" -There's a minor exception about changing flags. - -We can change `writable: true` to `false` for a non-configurable property, thus preventing its value modification (to add another layer of protection). Not the other way around though. -``` ## Object.defineProperties -There's a method [Object.defineProperties(obj, descriptors)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/defineProperties) that allows to define many properties at once. +There's a method [Object.defineProperties(obj, descriptors)](mdn:js/Object/defineProperties) that allows to define many properties at once. The syntax is: @@ -300,7 +292,7 @@ So, we can set many properties at once. ## Object.getOwnPropertyDescriptors -To get all property descriptors at once, we can use the method [Object.getOwnPropertyDescriptors(obj)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptors). +To get all property descriptors at once, we can use the method [Object.getOwnPropertyDescriptors(obj)](mdn:js/Object/getOwnPropertyDescriptors). Together with `Object.defineProperties` it can be used as a "flags-aware" way of cloning an object: @@ -318,7 +310,7 @@ for (let key in user) { ...But that does not copy flags. So if we want a "better" clone then `Object.defineProperties` is preferred. -Another difference is that `for..in` ignores symbolic and non-enumerable properties, but `Object.getOwnPropertyDescriptors` returns *all* property descriptors including symbolic and non-enumerable ones. +Another difference is that `for..in` ignores symbolic properties, but `Object.getOwnPropertyDescriptors` returns *all* property descriptors including symbolic ones. ## Sealing an object globally @@ -326,24 +318,24 @@ Property descriptors work at the level of individual properties. There are also methods that limit access to the *whole* object: -[Object.preventExtensions(obj)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/preventExtensions) +[Object.preventExtensions(obj)](mdn:js/Object/preventExtensions) : Forbids the addition of new properties to the object. -[Object.seal(obj)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/seal) +[Object.seal(obj)](mdn:js/Object/seal) : Forbids adding/removing of properties. Sets `configurable: false` for all existing properties. -[Object.freeze(obj)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/freeze) +[Object.freeze(obj)](mdn:js/Object/freeze) : Forbids adding/removing/changing of properties. Sets `configurable: false, writable: false` for all existing properties. And also there are tests for them: -[Object.isExtensible(obj)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isExtensible) +[Object.isExtensible(obj)](mdn:js/Object/isExtensible) : Returns `false` if adding properties is forbidden, otherwise `true`. -[Object.isSealed(obj)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isSealed) +[Object.isSealed(obj)](mdn:js/Object/isSealed) : Returns `true` if adding/removing properties is forbidden, and all existing properties have `configurable: false`. -[Object.isFrozen(obj)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/isFrozen) +[Object.isFrozen(obj)](mdn:js/Object/isFrozen) : Returns `true` if adding/removing/changing properties is forbidden, and all current properties are `configurable: false, writable: false`. These methods are rarely used in practice. diff --git a/1-js/07-object-properties/02-property-accessors/article.md b/1-js/07-object-properties/02-property-accessors/article.md index c2aa35d5..45b9e70e 100644 --- a/1-js/07-object-properties/02-property-accessors/article.md +++ b/1-js/07-object-properties/02-property-accessors/article.md @@ -5,7 +5,7 @@ There are two kinds of object properties. The first kind is *data properties*. We already know how to work with them. All properties that we've been using until now were data properties. -The second type of property is something new. It's an *accessor property*. They are essentially functions that execute on getting and setting a value, but look like regular properties to an external code. +The second type of properties is something new. It's *accessor properties*. They are essentially functions that execute on getting and setting a value, but look like regular properties to an external code. ## Getters and setters diff --git a/1-js/08-prototypes/01-prototype-inheritance/article.md b/1-js/08-prototypes/01-prototype-inheritance/article.md index ef6c7ffe..e5780805 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/article.md +++ b/1-js/08-prototypes/01-prototype-inheritance/article.md @@ -12,7 +12,7 @@ In JavaScript, objects have a special hidden property `[[Prototype]]` (as named ![prototype](object-prototype-empty.svg) -When we read a property from `object`, and it's missing, JavaScript automatically takes it from the prototype. In programming, this is called "prototypal inheritance". And soon we'll study many examples of such inheritance, as well as cooler language features built upon it. +The prototype is a little bit "magical". When we want to read a property from `object`, and it's missing, JavaScript automatically takes it from the prototype. In programming, such thing is called "prototypal inheritance". Many cool language features and programming techniques are based on it. The property `[[Prototype]]` is internal and hidden, but there are many ways to set it. @@ -27,11 +27,19 @@ let rabbit = { }; *!* -rabbit.__proto__ = animal; // sets rabbit.[[Prototype]] = animal +rabbit.__proto__ = animal; */!* ``` -Now if we read a property from `rabbit`, and it's missing, JavaScript will automatically take it from `animal`. +```smart header="`__proto__` is a historical getter/setter for `[[Prototype]]`" +Please note that `__proto__` is *not the same* as `[[Prototype]]`. It's a getter/setter for it. + +It exists for historical reasons. In modern language it is replaced with functions `Object.getPrototypeOf/Object.setPrototypeOf` that also get/set the prototype. We'll study the reasons for that and these functions later. + +By the specification, `__proto__` must only be supported by browsers, but in fact all environments including server-side support it. For now, as `__proto__` notation is a little bit more intuitively obvious, we'll use it in the examples. +``` + +If we look for a property in `rabbit`, and it's missing, JavaScript automatically takes it from `animal`. For instance: @@ -54,7 +62,7 @@ alert( rabbit.eats ); // true (**) alert( rabbit.jumps ); // true ``` -Here the line `(*)` sets `animal` to be the prototype of `rabbit`. +Here the line `(*)` sets `animal` to be a prototype of `rabbit`. Then, when `alert` tries to read property `rabbit.eats` `(**)`, it's not in `rabbit`, so JavaScript follows the `[[Prototype]]` reference and finds it in `animal` (look from the bottom up): @@ -122,8 +130,6 @@ alert(longEar.jumps); // true (from rabbit) ![](proto-animal-rabbit-chain.svg) -Now if we read something from `longEar`, and it's missing, JavaScript will look for it in `rabbit`, and then in `animal`. - There are only two limitations: 1. The references can't go in circles. JavaScript will throw an error if we try to assign `__proto__` in a circle. @@ -131,18 +137,6 @@ There are only two limitations: Also it may be obvious, but still: there can be only one `[[Prototype]]`. An object may not inherit from two others. -```smart header="`__proto__` is a historical getter/setter for `[[Prototype]]`" -It's a common mistake of novice developers not to know the difference between these two. - -Please note that `__proto__` is *not the same* as the internal `[[Prototype]]` property. It's a getter/setter for `[[Prototype]]`. Later we'll see situations where it matters, for now let's just keep it in mind, as we build our understanding of JavaScript language. - -The `__proto__` property is a bit outdated. It exists for historical reasons, modern JavaScript suggests that we should use `Object.getPrototypeOf/Object.setPrototypeOf` functions instead that get/set the prototype. We'll also cover these functions later. - -By the specification, `__proto__` must only be supported by browsers. In fact though, all environments including server-side support `__proto__`, so we're quite safe using it. - -As the `__proto__` notation is a bit more intuitively obvious, we use it in the examples. -``` - ## Writing doesn't use prototype The prototype is only used for reading properties. @@ -204,8 +198,8 @@ alert(admin.fullName); // John Smith (*) // setter triggers! admin.fullName = "Alice Cooper"; // (**) -alert(admin.fullName); // Alice Cooper, state of admin modified -alert(user.fullName); // John Smith, state of user protected +alert(admin.fullName); // Alice Cooper , state of admin modified +alert(user.fullName); // John Smith , state of user protected ``` Here in the line `(*)` the property `admin.fullName` has a getter in the prototype `user`, so it is called. And in the line `(**)` the property has a setter in the prototype, so it is called. @@ -286,7 +280,7 @@ for(let prop in rabbit) alert(prop); // jumps, then eats */!* ``` -If that's not what we want, and we'd like to exclude inherited properties, there's a built-in method [obj.hasOwnProperty(key)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/hasOwnProperty): it returns `true` if `obj` has its own (not inherited) property named `key`. +If that's not what we want, and we'd like to exclude inherited properties, there's a built-in method [obj.hasOwnProperty(key)](mdn:js/Object/hasOwnProperty): it returns `true` if `obj` has its own (not inherited) property named `key`. So we can filter out inherited properties (or do something else with them): diff --git a/1-js/08-prototypes/01-prototype-inheritance/object-prototype-empty.svg b/1-js/08-prototypes/01-prototype-inheritance/object-prototype-empty.svg index eb79c19f..da48a7cc 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/object-prototype-empty.svg +++ b/1-js/08-prototypes/01-prototype-inheritance/object-prototype-empty.svg @@ -1 +1 @@ -prototype objectobject[[Prototype]] \ No newline at end of file +prototype objectobject[[Prototype]] \ No newline at end of file diff --git a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-chain.svg b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-chain.svg index 4bf580ae..520bf87e 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-chain.svg +++ b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-chain.svg @@ -1 +1 @@ -eats: true walk: functionanimaljumps: truerabbit[[Prototype]]earLength: 10longEar[[Prototype]] \ No newline at end of file +eats: true walk: functionanimaljumps: truerabbit[[Prototype]]earLength: 10longEar[[Prototype]] \ No newline at end of file diff --git a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk-2.svg b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk-2.svg index 838c7839..8b657357 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk-2.svg +++ b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk-2.svg @@ -1 +1 @@ -eats: true walk: functionanimalwalk: functionrabbit[[Prototype]] \ No newline at end of file +eats: true walk: functionanimalwalk: functionrabbit[[Prototype]] \ No newline at end of file diff --git a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk-3.svg b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk-3.svg index d791e539..6e3b6f55 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk-3.svg +++ b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk-3.svg @@ -1 +1 @@ -walk: function sleep: functionanimalrabbit[[Prototype]]name: "White Rabbit" isSleeping: true \ No newline at end of file +walk: function sleep: functionanimalrabbit[[Prototype]]name: "White Rabbit" isSleeping: true \ No newline at end of file diff --git a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk.svg b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk.svg index b3247102..b83933a8 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk.svg +++ b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit-walk.svg @@ -1 +1 @@ -eats: true walk: functionanimaljumps: truerabbit[[Prototype]] \ No newline at end of file +eats: true walk: functionanimaljumps: truerabbit[[Prototype]] \ No newline at end of file diff --git a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit.svg b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit.svg index 4f3c1bc0..538f5afb 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit.svg +++ b/1-js/08-prototypes/01-prototype-inheritance/proto-animal-rabbit.svg @@ -1 +1 @@ -eats: trueanimaljumps: truerabbit[[Prototype]] \ No newline at end of file +eats: trueanimaljumps: truerabbit[[Prototype]] \ No newline at end of file diff --git a/1-js/08-prototypes/01-prototype-inheritance/proto-user-admin.svg b/1-js/08-prototypes/01-prototype-inheritance/proto-user-admin.svg index bf0baf01..ed9fea4a 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/proto-user-admin.svg +++ b/1-js/08-prototypes/01-prototype-inheritance/proto-user-admin.svg @@ -1 +1 @@ -name: "John" surname: "Smith" set fullName: functionisAdmin: true name: "Alice" surname: "Cooper"useradmin[[Prototype]] \ No newline at end of file +name: "John" surname: "Smith" set fullName: functionisAdmin: true name: "Alice" surname: "Cooper"useradmin[[Prototype]] \ No newline at end of file diff --git a/1-js/08-prototypes/01-prototype-inheritance/rabbit-animal-object.svg b/1-js/08-prototypes/01-prototype-inheritance/rabbit-animal-object.svg index 32a9858f..782a858b 100644 --- a/1-js/08-prototypes/01-prototype-inheritance/rabbit-animal-object.svg +++ b/1-js/08-prototypes/01-prototype-inheritance/rabbit-animal-object.svg @@ -1 +1 @@ -toString: function hasOwnProperty: function ...Object.prototypeanimal[[Prototype]][[Prototype]][[Prototype]]nulleats: truerabbitjumps: true \ No newline at end of file +toString: function hasOwnProperty: function ...Object.prototypeanimal[[Prototype]][[Prototype]][[Prototype]]nulleats: truerabbitjumps: true \ No newline at end of file diff --git a/1-js/08-prototypes/02-function-prototype/4-new-object-same-constructor/solution.md b/1-js/08-prototypes/02-function-prototype/4-new-object-same-constructor/solution.md index 372d50dd..0073e252 100644 --- a/1-js/08-prototypes/02-function-prototype/4-new-object-same-constructor/solution.md +++ b/1-js/08-prototypes/02-function-prototype/4-new-object-same-constructor/solution.md @@ -38,12 +38,7 @@ Why `user2.name` is `undefined`? Here's how `new user.constructor('Pete')` works: 1. First, it looks for `constructor` in `user`. Nothing. -2. Then it follows the prototype chain. The prototype of `user` is `User.prototype`, and it also has no `constructor` (because we "forgot" to set it right!). -3. Going further up the chain, `User.prototype` is a plain object, its prototype is the built-in `Object.prototype`. -4. Finally, for the built-in `Object.prototype`, there's a built-in `Object.prototype.constructor == Object`. So it is used. +2. Then it follows the prototype chain. The prototype of `user` is `User.prototype`, and it also has nothing. +3. The value of `User.prototype` is a plain object `{}`, its prototype is `Object.prototype`. And there is `Object.prototype.constructor == Object`. So it is used. -Finally, at the end, we have `let user2 = new Object('Pete')`. - -Probably, that's not what we want. We'd like to create `new User`, not `new Object`. That's the outcome of the missing `constructor`. - -(Just in case you're curious, the `new Object(...)` call converts its argument to an object. That's a theoretical thing, in practice no one calls `new Object` with a value, and generally we don't use `new Object` to make objects at all). \ No newline at end of file +At the end, we have `let user2 = new Object('Pete')`. The built-in `Object` constructor ignores arguments, it always creates an empty object, similar to `let user2 = {}`, that's what we have in `user2` after all. diff --git a/1-js/08-prototypes/02-function-prototype/function-prototype-constructor.svg b/1-js/08-prototypes/02-function-prototype/function-prototype-constructor.svg index 59d60b39..187b899e 100644 --- a/1-js/08-prototypes/02-function-prototype/function-prototype-constructor.svg +++ b/1-js/08-prototypes/02-function-prototype/function-prototype-constructor.svg @@ -1 +1 @@ -Rabbitprototypeconstructordefault "prototype" \ No newline at end of file +Rabbitprototypeconstructordefault "prototype" \ No newline at end of file diff --git a/1-js/08-prototypes/02-function-prototype/proto-constructor-animal-rabbit.svg b/1-js/08-prototypes/02-function-prototype/proto-constructor-animal-rabbit.svg index ede4e122..a2c19d85 100644 --- a/1-js/08-prototypes/02-function-prototype/proto-constructor-animal-rabbit.svg +++ b/1-js/08-prototypes/02-function-prototype/proto-constructor-animal-rabbit.svg @@ -1 +1 @@ -eats: truename: "White Rabbit"animalRabbitrabbit[[Prototype]]prototype \ No newline at end of file +eats: truename: "White Rabbit"animalRabbitrabbit[[Prototype]]prototype \ No newline at end of file diff --git a/1-js/08-prototypes/02-function-prototype/rabbit-prototype-constructor.svg b/1-js/08-prototypes/02-function-prototype/rabbit-prototype-constructor.svg index 54b3d798..4d6b10e3 100644 --- a/1-js/08-prototypes/02-function-prototype/rabbit-prototype-constructor.svg +++ b/1-js/08-prototypes/02-function-prototype/rabbit-prototype-constructor.svg @@ -1 +1 @@ -default "prototype"Rabbitrabbit[[Prototype]]prototypeconstructor \ No newline at end of file +default "prototype"Rabbitrabbit[[Prototype]]prototypeconstructor \ No newline at end of file diff --git a/1-js/08-prototypes/03-native-prototypes/article.md b/1-js/08-prototypes/03-native-prototypes/article.md index bdfc86dd..6cf7aebb 100644 --- a/1-js/08-prototypes/03-native-prototypes/article.md +++ b/1-js/08-prototypes/03-native-prototypes/article.md @@ -2,7 +2,7 @@ The `"prototype"` property is widely used by the core of JavaScript itself. All built-in constructor functions use it. -First we'll look at the details, and then how to use it for adding new capabilities to built-in objects. +First we'll see at the details, and then how to use it for adding new capabilities to built-in objects. ## Object.prototype diff --git a/1-js/08-prototypes/03-native-prototypes/function-prototype-constructor.svg b/1-js/08-prototypes/03-native-prototypes/function-prototype-constructor.svg index 59d60b39..187b899e 100644 --- a/1-js/08-prototypes/03-native-prototypes/function-prototype-constructor.svg +++ b/1-js/08-prototypes/03-native-prototypes/function-prototype-constructor.svg @@ -1 +1 @@ -Rabbitprototypeconstructordefault "prototype" \ No newline at end of file +Rabbitprototypeconstructordefault "prototype" \ No newline at end of file diff --git a/1-js/08-prototypes/03-native-prototypes/native-prototypes-array-tostring.svg b/1-js/08-prototypes/03-native-prototypes/native-prototypes-array-tostring.svg index ebb4f320..8475560b 100644 --- a/1-js/08-prototypes/03-native-prototypes/native-prototypes-array-tostring.svg +++ b/1-js/08-prototypes/03-native-prototypes/native-prototypes-array-tostring.svg @@ -1 +1 @@ -toString: function ...Array.prototypetoString: function ...Object.prototype[[Prototype]][[Prototype]][1, 2, 3] \ No newline at end of file +toString: function ...Array.prototypetoString: function ...Object.prototype[[Prototype]][[Prototype]][1, 2, 3] \ No newline at end of file diff --git a/1-js/08-prototypes/03-native-prototypes/native-prototypes-classes.svg b/1-js/08-prototypes/03-native-prototypes/native-prototypes-classes.svg index 4d6129e0..36cc81cd 100644 --- a/1-js/08-prototypes/03-native-prototypes/native-prototypes-classes.svg +++ b/1-js/08-prototypes/03-native-prototypes/native-prototypes-classes.svg @@ -1 +1 @@ -toString: function other object methodsObject.prototypenullslice: function other array methods[[Prototype]][[Prototype]][[Prototype]][[Prototype]][[Prototype]][[Prototype]][[Prototype]]Array.prototypecall: function other function methodsFunction.prototypetoFixed: function other number methodsNumber.prototype[1, 2, 3]function f(args) { ... }5 \ No newline at end of file +toString: function other object methodsObject.prototypenullslice: function other array methods[[Prototype]][[Prototype]][[Prototype]][[Prototype]][[Prototype]][[Prototype]][[Prototype]]Array.prototypecall: function other function methodsFunction.prototypetoFixed: function other number methodsNumber.prototype[1, 2, 3]function f(args) { ... }5 \ No newline at end of file diff --git a/1-js/08-prototypes/03-native-prototypes/object-prototype-1.svg b/1-js/08-prototypes/03-native-prototypes/object-prototype-1.svg index 9630e68e..c111e072 100644 --- a/1-js/08-prototypes/03-native-prototypes/object-prototype-1.svg +++ b/1-js/08-prototypes/03-native-prototypes/object-prototype-1.svg @@ -1 +1 @@ -constructor: Object toString: function ...Object.prototypeObjectobj = new Object()[[Prototype]]prototype \ No newline at end of file +constructor: Object toString: function ...Object.prototypeObjectobj = new Object()[[Prototype]]prototype \ No newline at end of file diff --git a/1-js/08-prototypes/03-native-prototypes/object-prototype-null.svg b/1-js/08-prototypes/03-native-prototypes/object-prototype-null.svg index 9ccb3422..8b802eb4 100644 --- a/1-js/08-prototypes/03-native-prototypes/object-prototype-null.svg +++ b/1-js/08-prototypes/03-native-prototypes/object-prototype-null.svg @@ -1 +1 @@ -obj[[Prototype]]null \ No newline at end of file +obj[[Prototype]]null \ No newline at end of file diff --git a/1-js/08-prototypes/03-native-prototypes/object-prototype.svg b/1-js/08-prototypes/03-native-prototypes/object-prototype.svg index 024dd302..b5014f9f 100644 --- a/1-js/08-prototypes/03-native-prototypes/object-prototype.svg +++ b/1-js/08-prototypes/03-native-prototypes/object-prototype.svg @@ -1 +1 @@ -constructor: Object toString: function ...Object.prototypeObjectprototype \ No newline at end of file +constructor: Object toString: function ...Object.prototypeObjectprototype \ No newline at end of file diff --git a/1-js/08-prototypes/03-native-prototypes/proto-constructor-animal-rabbit.svg b/1-js/08-prototypes/03-native-prototypes/proto-constructor-animal-rabbit.svg new file mode 100644 index 00000000..a2c19d85 --- /dev/null +++ b/1-js/08-prototypes/03-native-prototypes/proto-constructor-animal-rabbit.svg @@ -0,0 +1 @@ +eats: truename: "White Rabbit"animalRabbitrabbit[[Prototype]]prototype \ No newline at end of file diff --git a/1-js/08-prototypes/03-native-prototypes/rabbit-prototype-constructor.svg b/1-js/08-prototypes/03-native-prototypes/rabbit-prototype-constructor.svg index 54b3d798..4d6b10e3 100644 --- a/1-js/08-prototypes/03-native-prototypes/rabbit-prototype-constructor.svg +++ b/1-js/08-prototypes/03-native-prototypes/rabbit-prototype-constructor.svg @@ -1 +1 @@ -default "prototype"Rabbitrabbit[[Prototype]]prototypeconstructor \ No newline at end of file +default "prototype"Rabbitrabbit[[Prototype]]prototypeconstructor \ No newline at end of file diff --git a/1-js/08-prototypes/04-prototype-methods/2-dictionary-tostring/solution.md b/1-js/08-prototypes/04-prototype-methods/2-dictionary-tostring/solution.md index f3c9cf0e..a92e1790 100644 --- a/1-js/08-prototypes/04-prototype-methods/2-dictionary-tostring/solution.md +++ b/1-js/08-prototypes/04-prototype-methods/2-dictionary-tostring/solution.md @@ -28,4 +28,4 @@ alert(dictionary); // "apple,__proto__" When we create a property using a descriptor, its flags are `false` by default. So in the code above, `dictionary.toString` is non-enumerable. -See the chapter [](info:property-descriptors) for review. +See the the chapter [](info:property-descriptors) for review. diff --git a/1-js/08-prototypes/04-prototype-methods/article.md b/1-js/08-prototypes/04-prototype-methods/article.md index 71f118e1..e460ef01 100644 --- a/1-js/08-prototypes/04-prototype-methods/article.md +++ b/1-js/08-prototypes/04-prototype-methods/article.md @@ -3,18 +3,15 @@ In the first chapter of this section, we mentioned that there are modern methods to setup a prototype. -Setting or reading the prototype with `obj.__proto__` is considered outdated and somewhat deprecated (moved to the so-called "Annex B" of the JavaScript standard, meant for browsers only). +The `__proto__` is considered outdated and somewhat deprecated (in browser-only part of the JavaScript standard). -The modern methods to get/set a prototype are: +The modern methods are: +- [Object.create(proto, [descriptors])](mdn:js/Object/create) -- creates an empty object with given `proto` as `[[Prototype]]` and optional property descriptors. - [Object.getPrototypeOf(obj)](mdn:js/Object/getPrototypeOf) -- returns the `[[Prototype]]` of `obj`. - [Object.setPrototypeOf(obj, proto)](mdn:js/Object/setPrototypeOf) -- sets the `[[Prototype]]` of `obj` to `proto`. -The only usage of `__proto__`, that's not frowned upon, is as a property when creating a new object: `{ __proto__: ... }`. - -Although, there's a special method for this too: - -- [Object.create(proto, [descriptors])](mdn:js/Object/create) -- creates an empty object with given `proto` as `[[Prototype]]` and optional property descriptors. +These should be used instead of `__proto__`. For instance: @@ -25,7 +22,7 @@ let animal = { // create a new object with animal as a prototype *!* -let rabbit = Object.create(animal); // same as {__proto__: animal} +let rabbit = Object.create(animal); */!* alert(rabbit.eats); // true @@ -39,9 +36,7 @@ Object.setPrototypeOf(rabbit, {}); // change the prototype of rabbit to {} */!* ``` -The `Object.create` method is a bit more powerful, as it has an optional second argument: property descriptors. - -We can provide additional properties to the new object there, like this: +`Object.create` has an optional second argument: property descriptors. We can provide additional properties to the new object there, like this: ```js run let animal = { @@ -62,34 +57,26 @@ The descriptors are in the same format as described in the chapter ... get __proto__: function set __proto__: functionObject.prototypeObjectobj[[Prototype]]prototype \ No newline at end of file +... get __proto__: function set __proto__: functionObject.prototypeObjectobj[[Prototype]]prototype \ No newline at end of file diff --git a/1-js/08-prototypes/04-prototype-methods/object-prototype-null.svg b/1-js/08-prototypes/04-prototype-methods/object-prototype-null.svg index 9ccb3422..8b802eb4 100644 --- a/1-js/08-prototypes/04-prototype-methods/object-prototype-null.svg +++ b/1-js/08-prototypes/04-prototype-methods/object-prototype-null.svg @@ -1 +1 @@ -obj[[Prototype]]null \ No newline at end of file +obj[[Prototype]]null \ No newline at end of file diff --git a/1-js/09-classes/01-class/1-rewrite-to-class/task.md b/1-js/09-classes/01-class/1-rewrite-to-class/task.md index 4477de67..05365e41 100644 --- a/1-js/09-classes/01-class/1-rewrite-to-class/task.md +++ b/1-js/09-classes/01-class/1-rewrite-to-class/task.md @@ -4,6 +4,6 @@ importance: 5 # Rewrite to class -The `Clock` class (see the sandbox) is written in functional style. Rewrite it in the "class" syntax. +The `Clock` class is written in functional style. Rewrite it the "class" syntax. P.S. The clock ticks in the console, open it to see. diff --git a/1-js/09-classes/01-class/article.md b/1-js/09-classes/01-class/article.md index 135d2492..49a891b7 100644 --- a/1-js/09-classes/01-class/article.md +++ b/1-js/09-classes/01-class/article.md @@ -51,7 +51,7 @@ user.sayHi(); When `new User("John")` is called: 1. A new object is created. -2. The `constructor` runs with the given argument and assigns it to `this.name`. +2. The `constructor` runs with the given argument and assigns `this.name` to it. ...Then we can call object methods, such as `user.sayHi()`. @@ -110,7 +110,7 @@ alert(typeof User); // function alert(User === User.prototype.constructor); // true // The methods are in User.prototype, e.g: -alert(User.prototype.sayHi); // the code of the sayHi method +alert(User.prototype.sayHi); // alert(this.name); // there are exactly two methods in the prototype alert(Object.getOwnPropertyNames(User.prototype)); // constructor, sayHi @@ -118,7 +118,7 @@ alert(Object.getOwnPropertyNames(User.prototype)); // constructor, sayHi ## Not just a syntactic sugar -Sometimes people say that `class` is a "syntactic sugar" (syntax that is designed to make things easier to read, but doesn't introduce anything new), because we could actually declare the same thing without using the `class` keyword at all: +Sometimes people say that `class` is a "syntactic sugar" (syntax that is designed to make things easier to read, but doesn't introduce anything new), because we could actually declare the same without `class` keyword at all: ```js run // rewriting class User in pure functions @@ -144,7 +144,7 @@ The result of this definition is about the same. So, there are indeed reasons wh Still, there are important differences. -1. First, a function created by `class` is labelled by a special internal property `[[IsClassConstructor]]: true`. So it's not entirely the same as creating it manually. +1. First, a function created by `class` is labelled by a special internal property `[[FunctionKind]]:"classConstructor"`. So it's not entirely the same as creating it manually. The language checks for that property in a variety of places. For example, unlike a regular function, it must be called with `new`: @@ -218,7 +218,7 @@ function makeClass(phrase) { return class { sayHi() { alert(phrase); - } + }; }; } diff --git a/1-js/09-classes/01-class/class-user.svg b/1-js/09-classes/01-class/class-user.svg index 418d71d1..95b58179 100644 --- a/1-js/09-classes/01-class/class-user.svg +++ b/1-js/09-classes/01-class/class-user.svg @@ -1 +1 @@ -sayHi: functionUserUser.prototypeprototypeconstructor: User \ No newline at end of file +sayHi: functionUserUser.prototypeprototypeconstructor: User \ No newline at end of file diff --git a/1-js/09-classes/02-class-inheritance/animal-rabbit-extends.svg b/1-js/09-classes/02-class-inheritance/animal-rabbit-extends.svg index 63b5a18a..3471904a 100644 --- a/1-js/09-classes/02-class-inheritance/animal-rabbit-extends.svg +++ b/1-js/09-classes/02-class-inheritance/animal-rabbit-extends.svg @@ -1 +1 @@ -constructor: Animal run: function stop: functionAnimal.prototypeconstructor: Rabbit hide: functionRabbit.prototypeAnimalRabbitnew Rabbit[[Prototype]][[Prototype]]prototypeprototypename: "White Rabbit"constructorconstructorextends \ No newline at end of file +constructor: Animal run: function stop: functionAnimal.prototypeconstructor: Rabbit hide: functionRabbit.prototypeAnimalRabbitnew Rabbit[[Prototype]][[Prototype]]prototypeprototypename: "White Rabbit"constructorconstructorextends \ No newline at end of file diff --git a/1-js/09-classes/02-class-inheritance/article.md b/1-js/09-classes/02-class-inheritance/article.md index 464042d8..69ca4eab 100644 --- a/1-js/09-classes/02-class-inheritance/article.md +++ b/1-js/09-classes/02-class-inheritance/article.md @@ -55,7 +55,7 @@ rabbit.run(5); // White Rabbit runs with speed 5. rabbit.hide(); // White Rabbit hides! ``` -Object of `Rabbit` class have access both to `Rabbit` methods, such as `rabbit.hide()`, and also to `Animal` methods, such as `rabbit.run()`. +Object of `Rabbit` class have access to both `Rabbit` methods, such as `rabbit.hide()`, and also to `Animal` methods, such as `rabbit.run()`. Internally, `extends` keyword works using the good old prototype mechanics. It sets `Rabbit.prototype.[[Prototype]]` to `Animal.prototype`. So, if a method is not found in `Rabbit.prototype`, JavaScript takes it from `Animal.prototype`. @@ -76,8 +76,8 @@ For instance, a function call that generates the parent class: ```js run function f(phrase) { return class { - sayHi() { alert(phrase); } - }; + sayHi() { alert(phrase) } + } } *!* @@ -106,7 +106,7 @@ class Rabbit extends Animal { } ``` -Usually, however, we don't want to totally replace a parent method, but rather to build on top of it to tweak or extend its functionality. We do something in our method, but call the parent method before/after it or in the process. +Usually we don't want to totally replace a parent method, but rather to build on top of it to tweak or extend its functionality. We do something in our method, but call the parent method before/after it or in the process. Classes provide `"super"` keyword for that. @@ -151,7 +151,7 @@ class Rabbit extends Animal { let rabbit = new Rabbit("White Rabbit"); rabbit.run(5); // White Rabbit runs with speed 5. -rabbit.stop(); // White Rabbit stands still. White Rabbit hides! +rabbit.stop(); // White Rabbit stands still. White rabbit hides! ``` Now `Rabbit` has the `stop` method that calls the parent `super.stop()` in the process. @@ -160,7 +160,6 @@ Now `Rabbit` has the `stop` method that calls the parent `super.stop()` in the p As was mentioned in the chapter , arrow functions do not have `super`. If accessed, it's taken from the outer function. For instance: - ```js class Rabbit extends Animal { stop() { @@ -177,6 +176,7 @@ setTimeout(function() { super.stop() }, 1000); ``` ```` + ## Overriding constructor With constructors it gets a little bit tricky. @@ -280,6 +280,8 @@ alert(rabbit.earLength); // 10 */!* ``` + + ### Overriding class fields: a tricky note ```warn header="Advanced note" @@ -298,7 +300,7 @@ Consider this example: ```js run class Animal { - name = 'animal'; + name = 'animal' constructor() { alert(this.name); // (*) @@ -315,13 +317,13 @@ new Rabbit(); // animal */!* ``` -Here, class `Rabbit` extends `Animal` and overrides the `name` field with its own value. +Here, class `Rabbit` extends `Animal` and overrides `name` field with its own value. There's no own constructor in `Rabbit`, so `Animal` constructor is called. What's interesting is that in both cases: `new Animal()` and `new Rabbit()`, the `alert` in the line `(*)` shows `animal`. -**In other words, the parent constructor always uses its own field value, not the overridden one.** +**In other words, parent constructor always uses its own field value, not the overridden one.** What's odd about it? @@ -358,9 +360,9 @@ And that's what we naturally expect. When the parent constructor is called in th ...But for class fields it's not so. As said, the parent constructor always uses the parent field. -Why is there a difference? +Why is there the difference? -Well, the reason is the field initialization order. The class field is initialized: +Well, the reason is in the field initialization order. The class field is initialized: - Before constructor for the base class (that doesn't extend anything), - Immediately after `super()` for the derived class. @@ -368,12 +370,13 @@ In our case, `Rabbit` is the derived class. There's no `constructor()` in it. As So, `new Rabbit()` calls `super()`, thus executing the parent constructor, and (per the rule for derived classes) only after that its class fields are initialized. At the time of the parent constructor execution, there are no `Rabbit` class fields yet, that's why `Animal` fields are used. -This subtle difference between fields and methods is specific to JavaScript. +This subtle difference between fields and methods is specific to JavaScript Luckily, this behavior only reveals itself if an overridden field is used in the parent constructor. Then it may be difficult to understand what's going on, so we're explaining it here. If it becomes a problem, one can fix it by using methods or getters/setters instead of fields. + ## Super: internals, [[HomeObject]] ```warn header="Advanced information" diff --git a/1-js/09-classes/02-class-inheritance/class-inheritance-array-object.svg b/1-js/09-classes/02-class-inheritance/class-inheritance-array-object.svg index 5ea9bf29..10af6c4c 100644 --- a/1-js/09-classes/02-class-inheritance/class-inheritance-array-object.svg +++ b/1-js/09-classes/02-class-inheritance/class-inheritance-array-object.svg @@ -1 +1 @@ -slice: function ...Array.prototypearrhasOwnProperty: function ...Object.prototype[1, 2, 3][[Prototype]][[Prototype]] \ No newline at end of file +slice: function ...Array.prototypearrhasOwnProperty: function ...Object.prototype[1, 2, 3][[Prototype]][[Prototype]] \ No newline at end of file diff --git a/1-js/09-classes/02-class-inheritance/class-inheritance-rabbit-animal-2.svg b/1-js/09-classes/02-class-inheritance/class-inheritance-rabbit-animal-2.svg index 72e47e34..a81676e2 100644 --- a/1-js/09-classes/02-class-inheritance/class-inheritance-rabbit-animal-2.svg +++ b/1-js/09-classes/02-class-inheritance/class-inheritance-rabbit-animal-2.svg @@ -1 +1 @@ -jump: functionRabbit.prototyperabbiteat: functionAnimal.prototypename: "White Rabbit"[[Prototype]][[Prototype]]Rabbit.prototype.__proto__ = Animal.prototype sets thistoString: function hasOwnProperty: function ...Object.prototype[[Prototype]][[Prototype]]null \ No newline at end of file +jump: functionRabbit.prototyperabbiteat: functionAnimal.prototypename: "White Rabbit"[[Prototype]][[Prototype]]Rabbit.prototype.__proto__ = Animal.prototype sets thistoString: function hasOwnProperty: function ...Object.prototype[[Prototype]][[Prototype]]null \ No newline at end of file diff --git a/1-js/09-classes/02-class-inheritance/class-inheritance-rabbit-animal.svg b/1-js/09-classes/02-class-inheritance/class-inheritance-rabbit-animal.svg index bced3d35..35529aa4 100644 --- a/1-js/09-classes/02-class-inheritance/class-inheritance-rabbit-animal.svg +++ b/1-js/09-classes/02-class-inheritance/class-inheritance-rabbit-animal.svg @@ -1 +1 @@ -methods of RabbitRabbit.prototyperabbitmethods of AnimalAnimal.prototype[[Prototype]][[Prototype]]properties of rabbit \ No newline at end of file +methods of RabbitRabbit.prototyperabbitmethods of AnimalAnimal.prototype[[Prototype]][[Prototype]]properties of rabbit \ No newline at end of file diff --git a/1-js/09-classes/02-class-inheritance/rabbit-animal-independent-animal.svg b/1-js/09-classes/02-class-inheritance/rabbit-animal-independent-animal.svg index f53fc92d..905efe37 100644 --- a/1-js/09-classes/02-class-inheritance/rabbit-animal-independent-animal.svg +++ b/1-js/09-classes/02-class-inheritance/rabbit-animal-independent-animal.svg @@ -1 +1 @@ - constructor: Animal run: function stop: functionAnimal.prototypeAnimalnew Animal[[Prototype]]prototypename: "My animal" \ No newline at end of file + constructor: Animal run: function stop: functionAnimal.prototypeAnimalnew Animal[[Prototype]]prototypename: "My animal" \ No newline at end of file diff --git a/1-js/09-classes/02-class-inheritance/rabbit-animal-independent-rabbit.svg b/1-js/09-classes/02-class-inheritance/rabbit-animal-independent-rabbit.svg index 2f30a3a9..81bf1850 100644 --- a/1-js/09-classes/02-class-inheritance/rabbit-animal-independent-rabbit.svg +++ b/1-js/09-classes/02-class-inheritance/rabbit-animal-independent-rabbit.svg @@ -1 +1 @@ - constructor: Rabbit hide: functionRabbit.prototypeRabbitnew Rabbit[[Prototype]]prototypename: "My rabbit" \ No newline at end of file + constructor: Rabbit hide: functionRabbit.prototypeRabbitnew Rabbit[[Prototype]]prototypename: "My rabbit" \ No newline at end of file diff --git a/1-js/09-classes/02-class-inheritance/super-homeobject-wrong.svg b/1-js/09-classes/02-class-inheritance/super-homeobject-wrong.svg index f6450ddc..f13d441c 100644 --- a/1-js/09-classes/02-class-inheritance/super-homeobject-wrong.svg +++ b/1-js/09-classes/02-class-inheritance/super-homeobject-wrong.svg @@ -1 +1 @@ -sayHiplantsayHitreesayHianimalrabbit[[HomeObject]]sayHi \ No newline at end of file +sayHiplantsayHitreesayHianimalrabbit[[HomeObject]]sayHi \ No newline at end of file diff --git a/1-js/09-classes/02-class-inheritance/this-super-loop.svg b/1-js/09-classes/02-class-inheritance/this-super-loop.svg index 4f5f4503..bc200fab 100644 --- a/1-js/09-classes/02-class-inheritance/this-super-loop.svg +++ b/1-js/09-classes/02-class-inheritance/this-super-loop.svg @@ -1 +1 @@ -rabbitlongEarrabbitlongEar \ No newline at end of file +rabbitlongEarrabbitlongEar \ No newline at end of file diff --git a/1-js/09-classes/03-static-properties-methods/3-class-extend-object/rabbit-extends-object.svg b/1-js/09-classes/03-static-properties-methods/3-class-extend-object/rabbit-extends-object.svg index 915ab9aa..34d783b4 100644 --- a/1-js/09-classes/03-static-properties-methods/3-class-extend-object/rabbit-extends-object.svg +++ b/1-js/09-classes/03-static-properties-methods/3-class-extend-object/rabbit-extends-object.svg @@ -1 +1 @@ -call: function bind: function ...Function.prototypeconstructorObjectRabbit[[Prototype]][[Prototype]]constructorcall: function bind: function ...Function.prototypeRabbit[[Prototype]]constructorclass Rabbitclass Rabbit extends Object \ No newline at end of file +call: function bind: function ...Function.prototypeconstructorObjectRabbit[[Prototype]][[Prototype]]constructorcall: function bind: function ...Function.prototypeRabbit[[Prototype]]constructorclass Rabbitclass Rabbit extends Object \ No newline at end of file diff --git a/1-js/09-classes/03-static-properties-methods/3-class-extend-object/solution.md b/1-js/09-classes/03-static-properties-methods/3-class-extend-object/solution.md index cb9829ce..ca9e8060 100644 --- a/1-js/09-classes/03-static-properties-methods/3-class-extend-object/solution.md +++ b/1-js/09-classes/03-static-properties-methods/3-class-extend-object/solution.md @@ -21,14 +21,14 @@ alert( rabbit.hasOwnProperty('name') ); // true But that's not all yet. -Even after the fix, there's still an important difference between `"class Rabbit extends Object"` and `class Rabbit`. +Even after the fix, there's still important difference in `"class Rabbit extends Object"` versus `class Rabbit`. As we know, the "extends" syntax sets up two prototypes: 1. Between `"prototype"` of the constructor functions (for methods). 2. Between the constructor functions themselves (for static methods). -In the case of `class Rabbit extends Object` it means: +In our case, for `class Rabbit extends Object` it means: ```js run class Rabbit extends Object {} @@ -37,7 +37,7 @@ alert( Rabbit.prototype.__proto__ === Object.prototype ); // (1) true alert( Rabbit.__proto__ === Object ); // (2) true ``` -So `Rabbit` now provides access to the static methods of `Object` via `Rabbit`, like this: +So `Rabbit` now provides access to static methods of `Object` via `Rabbit`, like this: ```js run class Rabbit extends Object {} @@ -67,7 +67,7 @@ alert ( Rabbit.getOwnPropertyNames({a: 1, b: 2})); // Error So `Rabbit` doesn't provide access to static methods of `Object` in that case. -By the way, `Function.prototype` also has "generic" function methods, like `call`, `bind` etc. They are ultimately available in both cases, because for the built-in `Object` constructor, `Object.__proto__ === Function.prototype`. +By the way, `Function.prototype` has "generic" function methods, like `call`, `bind` etc. They are ultimately available in both cases, because for the built-in `Object` constructor, `Object.__proto__ === Function.prototype`. Here's the picture: diff --git a/1-js/09-classes/03-static-properties-methods/animal-rabbit-static.svg b/1-js/09-classes/03-static-properties-methods/animal-rabbit-static.svg index 3e354b89..18093d7c 100644 --- a/1-js/09-classes/03-static-properties-methods/animal-rabbit-static.svg +++ b/1-js/09-classes/03-static-properties-methods/animal-rabbit-static.svg @@ -1 +1 @@ -constructor: Animal run: functionAnimal.prototypeconstructor: Rabbit hide: functionRabbit.prototypeAnimalRabbitrabbit[[Prototype]][[Prototype]][[Prototype]]prototypeprototypecomparename: "White Rabbit" \ No newline at end of file +constructor: Animal run: functionAnimal.prototypeconstructor: Rabbit hide: functionRabbit.prototypeAnimalRabbitrabbit[[Prototype]][[Prototype]][[Prototype]]prototypeprototypecomparename: "White Rabbit" \ No newline at end of file diff --git a/1-js/09-classes/03-static-properties-methods/article.md b/1-js/09-classes/03-static-properties-methods/article.md index 4b493a5e..ab08f2de 100644 --- a/1-js/09-classes/03-static-properties-methods/article.md +++ b/1-js/09-classes/03-static-properties-methods/article.md @@ -1,9 +1,9 @@ # Static properties and methods -We can also assign a method to the class as a whole. Such methods are called *static*. +We can also assign a method to the class function itself, not to its `"prototype"`. Such methods are called *static*. -In a class declaration, they are prepended by `static` keyword, like this: +In a class, they are prepended by `static` keyword, like this: ```js run class User { @@ -31,11 +31,9 @@ User.staticMethod(); // true The value of `this` in `User.staticMethod()` call is the class constructor `User` itself (the "object before dot" rule). -Usually, static methods are used to implement functions that belong to the class as a whole, but not to any particular object of it. +Usually, static methods are used to implement functions that belong to the class, but not to any particular object of it. -For instance, we have `Article` objects and need a function to compare them. - -A natural solution would be to add `Article.compare` static method: +For instance, we have `Article` objects and need a function to compare them. A natural solution would be to add `Article.compare` method, like this: ```js run class Article { @@ -65,11 +63,9 @@ articles.sort(Article.compare); alert( articles[0].title ); // CSS ``` -Here `Article.compare` method stands "above" articles, as a means to compare them. It's not a method of an article, but rather of the whole class. +Here `Article.compare` stands "above" articles, as a means to compare them. It's not a method of an article, but rather of the whole class. -Another example would be a so-called "factory" method. - -Let's say, we need multiple ways to create an article: +Another example would be a so-called "factory" method. Imagine, we need few ways to create an article: 1. Create by given parameters (`title`, `date` etc). 2. Create an empty article with today's date. @@ -77,7 +73,7 @@ Let's say, we need multiple ways to create an article: The first way can be implemented by the constructor. And for the second one we can make a static method of the class. -Such as `Article.createTodays()` here: +Like `Article.createTodays()` here: ```js run class Article { @@ -105,21 +101,10 @@ Static methods are also used in database-related classes to search/save/remove e ```js // assuming Article is a special class for managing articles -// static method to remove the article by id: +// static method to remove the article: Article.remove({id: 12345}); ``` -````warn header="Static methods aren't available for individual objects" -Static methods are callable on classes, not on individual objects. - -E.g. such code won't work: - -```js -// ... -article.createTodays(); /// Error: article.createTodays is not a function -``` -```` - ## Static properties [recent browser=Chrome] @@ -140,7 +125,7 @@ That is the same as a direct assignment to `Article`: Article.publisher = "Ilya Kantor"; ``` -## Inheritance of static properties and methods [#statics-and-inheritance] +## Inheritance of static properties and methods Static properties and methods are inherited. diff --git a/1-js/09-classes/04-private-protected-properties-methods/article.md b/1-js/09-classes/04-private-protected-properties-methods/article.md index 91efb89e..60ed0ef1 100644 --- a/1-js/09-classes/04-private-protected-properties-methods/article.md +++ b/1-js/09-classes/04-private-protected-properties-methods/article.md @@ -96,9 +96,7 @@ class CoffeeMachine { _waterAmount = 0; set waterAmount(value) { - if (value < 0) { - value = 0; - } + if (value < 0) throw new Error("Negative water"); this._waterAmount = value; } @@ -116,10 +114,10 @@ class CoffeeMachine { let coffeeMachine = new CoffeeMachine(100); // add water -coffeeMachine.waterAmount = -10; // _waterAmount will become 0, not -10 +coffeeMachine.waterAmount = -10; // Error: Negative water ``` -Now the access is under control, so setting the water amount below zero becomes impossible. +Now the access is under control, so setting the water below zero fails. ## Read-only "power" @@ -161,7 +159,7 @@ class CoffeeMachine { _waterAmount = 0; *!*setWaterAmount(value)*/!* { - if (value < 0) value = 0; + if (value < 0) throw new Error("Negative water"); this._waterAmount = value; } @@ -192,7 +190,7 @@ There's a finished JavaScript proposal, almost in the standard, that provides la Privates should start with `#`. They are only accessible from inside the class. -For instance, here's a private `#waterLimit` property and the water-checking private method `#fixWaterAmount`: +For instance, here's a private `#waterLimit` property and the water-checking private method `#checkWater`: ```js run class CoffeeMachine { @@ -201,23 +199,19 @@ class CoffeeMachine { */!* *!* - #fixWaterAmount(value) { - if (value < 0) return 0; - if (value > this.#waterLimit) return this.#waterLimit; + #checkWater(value) { + if (value < 0) throw new Error("Negative water"); + if (value > this.#waterLimit) throw new Error("Too much water"); } */!* - setWaterAmount(value) { - this.#waterLimit = this.#fixWaterAmount(value); - } - } let coffeeMachine = new CoffeeMachine(); *!* // can't access privates from outside of the class -coffeeMachine.#fixWaterAmount(123); // Error +coffeeMachine.#checkWater(); // Error coffeeMachine.#waterLimit = 1000; // Error */!* ``` @@ -238,7 +232,7 @@ class CoffeeMachine { } set waterAmount(value) { - if (value < 0) value = 0; + if (value < 0) throw new Error("Negative water"); this.#waterAmount = value; } } diff --git a/1-js/09-classes/05-extend-natives/object-date-inheritance.svg b/1-js/09-classes/05-extend-natives/object-date-inheritance.svg index be47d7fd..470aabf7 100644 --- a/1-js/09-classes/05-extend-natives/object-date-inheritance.svg +++ b/1-js/09-classes/05-extend-natives/object-date-inheritance.svg @@ -1 +1 @@ -constructor: Object toString: function hasOwnProperty: function ...Object.prototypeconstructor: Date toString: function getDate: function ...Date.prototypeObjectDatenew Date()[[Prototype]][[Prototype]]prototypeprototypedefineProperty keys ...now parse ...1 Jan 2019 \ No newline at end of file +constructor: Object toString: function hasOwnProperty: function ...Object.prototypeconstructor: Date toString: function getDate: function ...Date.prototypeObjectDatenew Date()[[Prototype]][[Prototype]]prototypeprototypedefineProperty keys ...now parse ...1 Jan 2019 \ No newline at end of file diff --git a/1-js/09-classes/06-instanceof/article.md b/1-js/09-classes/06-instanceof/article.md index f9db989c..dd3d61ca 100644 --- a/1-js/09-classes/06-instanceof/article.md +++ b/1-js/09-classes/06-instanceof/article.md @@ -2,7 +2,7 @@ The `instanceof` operator allows to check whether an object belongs to a certain class. It also takes inheritance into account. -Such a check may be necessary in many cases. For example, it can be used for building a *polymorphic* function, the one that treats arguments differently depending on their type. +Such a check may be necessary in many cases. Here we'll use it for building a *polymorphic* function, the one that treats arguments differently depending on their type. ## The instanceof operator [#ref-instanceof] @@ -93,7 +93,7 @@ The algorithm of `obj instanceof Class` works roughly as follows: alert(rabbit instanceof Animal); // true */!* - // rabbit.__proto__ === Animal.prototype (no match) + // rabbit.__proto__ === Rabbit.prototype *!* // rabbit.__proto__.__proto__ === Animal.prototype (match!) */!* diff --git a/1-js/09-classes/06-instanceof/instanceof.svg b/1-js/09-classes/06-instanceof/instanceof.svg index d63b03a8..78bff9f1 100644 --- a/1-js/09-classes/06-instanceof/instanceof.svg +++ b/1-js/09-classes/06-instanceof/instanceof.svg @@ -1 +1 @@ -Animal.prototypeObject.prototypeRabbit.prototype[[Prototype]]rabbit[[Prototype]][[Prototype]]null[[Prototype]]= Animal.prototype? \ No newline at end of file +Animal.prototypeObject.prototypeRabbit.prototype[[Prototype]]rabbit[[Prototype]][[Prototype]]null[[Prototype]]= Animal.prototype? \ No newline at end of file diff --git a/1-js/09-classes/07-mixins/article.md b/1-js/09-classes/07-mixins/article.md index 526b832e..d43b96c9 100644 --- a/1-js/09-classes/07-mixins/article.md +++ b/1-js/09-classes/07-mixins/article.md @@ -69,7 +69,7 @@ let sayMixin = { }; let sayHiMixin = { - __proto__: sayMixin, // (or we could use Object.setPrototypeOf to set the prototype here) + __proto__: sayMixin, // (or we could use Object.create to set the prototype here) sayHi() { *!* @@ -103,7 +103,7 @@ Here's the diagram (see the right part): That's because methods `sayHi` and `sayBye` were initially created in `sayHiMixin`. So even though they got copied, their `[[HomeObject]]` internal property references `sayHiMixin`, as shown in the picture above. -As `super` looks for parent methods in `[[HomeObject]].[[Prototype]]`, that means it searches `sayHiMixin.[[Prototype]]`. +As `super` looks for parent methods in `[[HomeObject]].[[Prototype]]`, that means it searches `sayHiMixin.[[Prototype]]`, not `User.[[Prototype]]`. ## EventMixin diff --git a/1-js/09-classes/07-mixins/mixin-inheritance.svg b/1-js/09-classes/07-mixins/mixin-inheritance.svg index 1fdc2239..aaa8cb7d 100644 --- a/1-js/09-classes/07-mixins/mixin-inheritance.svg +++ b/1-js/09-classes/07-mixins/mixin-inheritance.svg @@ -1 +1 @@ -sayHi: function sayBye: functionsayHiMixinsay: functionsayMixin[[Prototype]]constructor: User sayHi: function sayBye: functionUser.prototype[[Prototype]]name: ...user[[HomeObject] \ No newline at end of file +sayHi: function sayBye: functionsayHiMixinsay: functionsayMixin[[Prototype]]constructor: User sayHi: function sayBye: functionUser.prototype[[Prototype]]name: ...user[[HomeObject] \ No newline at end of file diff --git a/1-js/10-error-handling/1-try-catch/1-finally-or-code-after/solution.md b/1-js/10-error-handling/1-try-catch/1-finally-or-code-after/solution.md index ec0dabc9..303431d6 100644 --- a/1-js/10-error-handling/1-try-catch/1-finally-or-code-after/solution.md +++ b/1-js/10-error-handling/1-try-catch/1-finally-or-code-after/solution.md @@ -1,8 +1,8 @@ The difference becomes obvious when we look at the code inside a function. -The behavior is different if there's a "jump out" of `try...catch`. +The behavior is different if there's a "jump out" of `try..catch`. -For instance, when there's a `return` inside `try...catch`. The `finally` clause works in case of *any* exit from `try...catch`, even via the `return` statement: right after `try...catch` is done, but before the calling code gets the control. +For instance, when there's a `return` inside `try..catch`. The `finally` clause works in case of *any* exit from `try..catch`, even via the `return` statement: right after `try..catch` is done, but before the calling code gets the control. ```js run function f() { @@ -11,7 +11,7 @@ function f() { *!* return "result"; */!* - } catch (err) { + } catch (e) { /// ... } finally { alert('cleanup!'); @@ -28,11 +28,11 @@ function f() { try { alert('start'); throw new Error("an error"); - } catch (err) { + } catch (e) { // ... if("can't handle the error") { *!* - throw err; + throw e; */!* } diff --git a/1-js/10-error-handling/1-try-catch/1-finally-or-code-after/task.md b/1-js/10-error-handling/1-try-catch/1-finally-or-code-after/task.md index b6dc8132..c573cc23 100644 --- a/1-js/10-error-handling/1-try-catch/1-finally-or-code-after/task.md +++ b/1-js/10-error-handling/1-try-catch/1-finally-or-code-after/task.md @@ -6,12 +6,12 @@ importance: 5 Compare the two code fragments. -1. The first one uses `finally` to execute the code after `try...catch`: +1. The first one uses `finally` to execute the code after `try..catch`: ```js try { work work - } catch (err) { + } catch (e) { handle errors } finally { *!* @@ -19,12 +19,12 @@ Compare the two code fragments. */!* } ``` -2. The second fragment puts the cleaning right after `try...catch`: +2. The second fragment puts the cleaning right after `try..catch`: ```js try { work work - } catch (err) { + } catch (e) { handle errors } diff --git a/1-js/10-error-handling/1-try-catch/article.md b/1-js/10-error-handling/1-try-catch/article.md index a928da28..3a2dc4ed 100644 --- a/1-js/10-error-handling/1-try-catch/article.md +++ b/1-js/10-error-handling/1-try-catch/article.md @@ -1,14 +1,14 @@ -# Error handling, "try...catch" +# Error handling, "try..catch" No matter how great we are at programming, sometimes our scripts have errors. They may occur because of our mistakes, an unexpected user input, an erroneous server response, and for a thousand other reasons. Usually, a script "dies" (immediately stops) in case of an error, printing it to console. -But there's a syntax construct `try...catch` that allows us to "catch" errors so the script can, instead of dying, do something more reasonable. +But there's a syntax construct `try..catch` that allows us to "catch" errors so the script can, instead of dying, do something more reasonable. -## The "try...catch" syntax +## The "try..catch" syntax -The `try...catch` construct has two main blocks: `try`, and then `catch`: +The `try..catch` construct has two main blocks: `try`, and then `catch`: ```js try { @@ -25,12 +25,12 @@ try { It works like this: 1. First, the code in `try {...}` is executed. -2. If there were no errors, then `catch (err)` is ignored: the execution reaches the end of `try` and goes on, skipping `catch`. -3. If an error occurs, then the `try` execution is stopped, and control flows to the beginning of `catch (err)`. The `err` variable (we can use any name for it) will contain an error object with details about what happened. +2. If there were no errors, then `catch(err)` is ignored: the execution reaches the end of `try` and goes on, skipping `catch`. +3. If an error occurs, then the `try` execution is stopped, and control flows to the beginning of `catch(err)`. The `err` variable (we can use any name for it) will contain an error object with details about what happened. ![](try-catch-flow.svg) -So, an error inside the `try {...}` block does not kill the script -- we have a chance to handle it in `catch`. +So, an error inside the `try {…}` block does not kill the script -- we have a chance to handle it in `catch`. Let's look at some examples. @@ -45,7 +45,7 @@ Let's look at some examples. alert('End of try runs'); // *!*(2) <--*/!* - } catch (err) { + } catch(err) { alert('Catch is ignored, because there are no errors'); // (3) @@ -64,7 +64,7 @@ Let's look at some examples. alert('End of try (never reached)'); // (2) - } catch (err) { + } catch(err) { alert(`Error has occurred!`); // *!*(3) <--*/!* @@ -72,45 +72,45 @@ Let's look at some examples. ``` -````warn header="`try...catch` only works for runtime errors" -For `try...catch` to work, the code must be runnable. In other words, it should be valid JavaScript. +````warn header="`try..catch` only works for runtime errors" +For `try..catch` to work, the code must be runnable. In other words, it should be valid JavaScript. It won't work if the code is syntactically wrong, for instance it has unmatched curly braces: ```js run try { {{{{{{{{{{{{ -} catch (err) { +} catch(e) { alert("The engine can't understand this code, it's invalid"); } ``` The JavaScript engine first reads the code, and then runs it. The errors that occur on the reading phase are called "parse-time" errors and are unrecoverable (from inside that code). That's because the engine can't understand the code. -So, `try...catch` can only handle errors that occur in valid code. Such errors are called "runtime errors" or, sometimes, "exceptions". +So, `try..catch` can only handle errors that occur in valid code. Such errors are called "runtime errors" or, sometimes, "exceptions". ```` -````warn header="`try...catch` works synchronously" -If an exception happens in "scheduled" code, like in `setTimeout`, then `try...catch` won't catch it: +````warn header="`try..catch` works synchronously" +If an exception happens in "scheduled" code, like in `setTimeout`, then `try..catch` won't catch it: ```js run try { setTimeout(function() { noSuchVariable; // script will die here }, 1000); -} catch (err) { +} catch (e) { alert( "won't work" ); } ``` -That's because the function itself is executed later, when the engine has already left the `try...catch` construct. +That's because the function itself is executed later, when the engine has already left the `try..catch` construct. -To catch an exception inside a scheduled function, `try...catch` must be inside that function: +To catch an exception inside a scheduled function, `try..catch` must be inside that function: ```js run setTimeout(function() { try { - noSuchVariable; // try...catch handles the error! + noSuchVariable; // try..catch handles the error! } catch { alert( "error is caught here!" ); } @@ -125,7 +125,7 @@ When an error occurs, JavaScript generates an object containing the details abou ```js try { // ... -} catch (err) { // <-- the "error object", could use another word instead of err +} catch(err) { // <-- the "error object", could use another word instead of err // ... } ``` @@ -150,7 +150,7 @@ try { *!* lalala; // error, variable is not defined! */!* -} catch (err) { +} catch(err) { alert(err.name); // ReferenceError alert(err.message); // lalala is not defined alert(err.stack); // ReferenceError: lalala is not defined at (...call stack) @@ -175,9 +175,9 @@ try { } ``` -## Using "try...catch" +## Using "try..catch" -Let's explore a real-life use case of `try...catch`. +Let's explore a real-life use case of `try..catch`. As we already know, JavaScript supports the [JSON.parse(str)](mdn:js/JSON/parse) method to read JSON-encoded values. @@ -205,7 +205,7 @@ Should we be satisfied with that? Of course not! This way, if something's wrong with the data, the visitor will never know that (unless they open the developer console). And people really don't like when something "just dies" without any error message. -Let's use `try...catch` to handle the error: +Let's use `try..catch` to handle the error: ```js run let json = "{ bad json }"; @@ -217,12 +217,12 @@ try { */!* alert( user.name ); // doesn't work -} catch (err) { +} catch (e) { *!* // ...the execution jumps here alert( "Our apologies, the data has errors, we'll try to request it one more time." ); - alert( err.name ); - alert( err.message ); + alert( e.name ); + alert( e.message ); */!* } ``` @@ -245,7 +245,7 @@ try { alert( user.name ); // no name! */!* -} catch (err) { +} catch (e) { alert( "doesn't execute" ); } ``` @@ -294,11 +294,11 @@ Let's see what kind of error `JSON.parse` generates: ```js run try { JSON.parse("{ bad json o_O }"); -} catch (err) { +} catch(e) { *!* - alert(err.name); // SyntaxError + alert(e.name); // SyntaxError */!* - alert(err.message); // Unexpected token b in JSON at position 2 + alert(e.message); // Unexpected token b in JSON at position 2 } ``` @@ -323,8 +323,8 @@ try { alert( user.name ); -} catch (err) { - alert( "JSON Error: " + err.message ); // JSON Error: Incomplete data: no name +} catch(e) { + alert( "JSON Error: " + e.message ); // JSON Error: Incomplete data: no name } ``` @@ -334,7 +334,7 @@ Now `catch` became a single place for all error handling: both for `JSON.parse` ## Rethrowing -In the example above we use `try...catch` to handle incorrect data. But is it possible that *another unexpected error* occurs within the `try {...}` block? Like a programming error (variable is not defined) or something else, not just this "incorrect data" thing. +In the example above we use `try..catch` to handle incorrect data. But is it possible that *another unexpected error* occurs within the `try {...}` block? Like a programming error (variable is not defined) or something else, not just this "incorrect data" thing. For example: @@ -345,7 +345,7 @@ try { user = JSON.parse(json); // <-- forgot to put "let" before user // ... -} catch (err) { +} catch(err) { alert("JSON Error: " + err); // JSON Error: ReferenceError: user is not defined // (no JSON Error actually) } @@ -353,7 +353,7 @@ try { Of course, everything's possible! Programmers do make mistakes. Even in open-source utilities used by millions for decades -- suddenly a bug may be discovered that leads to terrible hacks. -In our case, `try...catch` is placed to catch "incorrect data" errors. But by its nature, `catch` gets *all* errors from `try`. Here it gets an unexpected error, but still shows the same `"JSON Error"` message. That's wrong and also makes the code more difficult to debug. +In our case, `try..catch` is placed to catch "incorrect data" errors. But by its nature, `catch` gets *all* errors from `try`. Here it gets an unexpected error, but still shows the same `"JSON Error"` message. That's wrong and also makes the code more difficult to debug. To avoid such problems, we can employ the "rethrowing" technique. The rule is simple: @@ -362,7 +362,7 @@ To avoid such problems, we can employ the "rethrowing" technique. The rule is si The "rethrowing" technique can be explained in more detail as: 1. Catch gets all errors. -2. In the `catch (err) {...}` block we analyze the error object `err`. +2. In the `catch(err) {...}` block we analyze the error object `err`. 3. If we don't know how to handle it, we do `throw err`. Usually, we can check the error type using the `instanceof` operator: @@ -370,7 +370,7 @@ Usually, we can check the error type using the `instanceof` operator: ```js run try { user = { /*...*/ }; -} catch (err) { +} catch(err) { *!* if (err instanceof ReferenceError) { */!* @@ -399,24 +399,24 @@ try { alert( user.name ); -} catch (err) { +} catch(e) { *!* - if (err instanceof SyntaxError) { - alert( "JSON Error: " + err.message ); + if (e instanceof SyntaxError) { + alert( "JSON Error: " + e.message ); } else { - throw err; // rethrow (*) + throw e; // rethrow (*) } */!* } ``` -The error throwing on line `(*)` from inside `catch` block "falls out" of `try...catch` and can be either caught by an outer `try...catch` construct (if it exists), or it kills the script. +The error throwing on line `(*)` from inside `catch` block "falls out" of `try..catch` and can be either caught by an outer `try..catch` construct (if it exists), or it kills the script. So the `catch` block actually handles only errors that it knows how to deal with and "skips" all others. -The example below demonstrates how such errors can be caught by one more level of `try...catch`: +The example below demonstrates how such errors can be caught by one more level of `try..catch`: ```js run function readData() { @@ -427,11 +427,11 @@ function readData() { *!* blabla(); // error! */!* - } catch (err) { + } catch (e) { // ... - if (!(err instanceof SyntaxError)) { + if (!(e instanceof SyntaxError)) { *!* - throw err; // rethrow (don't know how to deal with it) + throw e; // rethrow (don't know how to deal with it) */!* } } @@ -439,20 +439,20 @@ function readData() { try { readData(); -} catch (err) { +} catch (e) { *!* - alert( "External catch got: " + err ); // caught it! + alert( "External catch got: " + e ); // caught it! */!* } ``` -Here `readData` only knows how to handle `SyntaxError`, while the outer `try...catch` knows how to handle everything. +Here `readData` only knows how to handle `SyntaxError`, while the outer `try..catch` knows how to handle everything. -## try...catch...finally +## try..catch..finally Wait, that's not all. -The `try...catch` construct may have one more code clause: `finally`. +The `try..catch` construct may have one more code clause: `finally`. If it exists, it runs in all cases: @@ -464,7 +464,7 @@ The extended syntax looks like this: ```js *!*try*/!* { ... try to execute the code ... -} *!*catch*/!* (err) { +} *!*catch*/!*(e) { ... handle errors ... } *!*finally*/!* { ... execute always ... @@ -477,7 +477,7 @@ Try running this code: try { alert( 'try' ); if (confirm('Make an error?')) BAD_CODE(); -} catch (err) { +} catch (e) { alert( 'catch' ); } finally { alert( 'finally' ); @@ -513,7 +513,7 @@ let start = Date.now(); try { result = fib(num); -} catch (err) { +} catch (e) { result = 0; *!* } finally { @@ -531,14 +531,14 @@ You can check by running the code with entering `35` into `prompt` -- it execute In other words, the function may finish with `return` or `throw`, that doesn't matter. The `finally` clause executes in both cases. -```smart header="Variables are local inside `try...catch...finally`" -Please note that `result` and `diff` variables in the code above are declared *before* `try...catch`. +```smart header="Variables are local inside `try..catch..finally`" +Please note that `result` and `diff` variables in the code above are declared *before* `try..catch`. Otherwise, if we declared `let` in `try` block, it would only be visible inside of it. ``` ````smart header="`finally` and `return`" -The `finally` clause works for *any* exit from `try...catch`. That includes an explicit `return`. +The `finally` clause works for *any* exit from `try..catch`. That includes an explicit `return`. In the example below, there's a `return` in `try`. In this case, `finally` is executed just before the control returns to the outer code. @@ -550,7 +550,7 @@ function func() { return 1; */!* - } catch (err) { + } catch (e) { /* ... */ } finally { *!* @@ -563,9 +563,9 @@ alert( func() ); // first works alert from finally, and then this one ``` ```` -````smart header="`try...finally`" +````smart header="`try..finally`" -The `try...finally` construct, without `catch` clause, is also useful. We apply it when we don't want to handle errors here (let them fall through), but want to be sure that processes that we started are finalized. +The `try..finally` construct, without `catch` clause, is also useful. We apply it when we don't want to handle errors here (let them fall through), but want to be sure that processes that we started are finalized. ```js function func() { @@ -586,7 +586,7 @@ In the code above, an error inside `try` always falls out, because there's no `c The information from this section is not a part of the core JavaScript. ``` -Let's imagine we've got a fatal error outside of `try...catch`, and the script died. Like a programming error or some other terrible thing. +Let's imagine we've got a fatal error outside of `try..catch`, and the script died. Like a programming error or some other terrible thing. Is there a way to react on such occurrences? We may want to log the error, show something to the user (normally they don't see error messages), etc. @@ -643,14 +643,14 @@ They work like this: ## Summary -The `try...catch` construct allows to handle runtime errors. It literally allows to "try" running the code and "catch" errors that may occur in it. +The `try..catch` construct allows to handle runtime errors. It literally allows to "try" running the code and "catch" errors that may occur in it. The syntax is: ```js try { // run this code -} catch (err) { +} catch(err) { // if an error happened, then jump here // err is the error object } finally { @@ -658,7 +658,7 @@ try { } ``` -There may be no `catch` section or no `finally`, so shorter constructs `try...catch` and `try...finally` are also valid. +There may be no `catch` section or no `finally`, so shorter constructs `try..catch` and `try..finally` are also valid. Error objects have following properties: @@ -666,10 +666,10 @@ Error objects have following properties: - `name` -- the string with error name (error constructor name). - `stack` (non-standard, but well-supported) -- the stack at the moment of error creation. -If an error object is not needed, we can omit it by using `catch {` instead of `catch (err) {`. +If an error object is not needed, we can omit it by using `catch {` instead of `catch(err) {`. We can also generate our own errors using the `throw` operator. Technically, the argument of `throw` can be anything, but usually it's an error object inheriting from the built-in `Error` class. More on extending errors in the next chapter. *Rethrowing* is a very important pattern of error handling: a `catch` block usually expects and knows how to handle the particular error type, so it should rethrow errors it doesn't know. -Even if we don't have `try...catch`, most environments allow us to setup a "global" error handler to catch errors that "fall out". In-browser, that's `window.onerror`. +Even if we don't have `try..catch`, most environments allow us to setup a "global" error handler to catch errors that "fall out". In-browser, that's `window.onerror`. diff --git a/1-js/10-error-handling/1-try-catch/try-catch-flow.svg b/1-js/10-error-handling/1-try-catch/try-catch-flow.svg index 2c0d7134..ac816e35 100644 --- a/1-js/10-error-handling/1-try-catch/try-catch-flow.svg +++ b/1-js/10-error-handling/1-try-catch/try-catch-flow.svg @@ -1 +1 @@ -BeginNo ErrorsAn error occured in the codeIgnore catch blockIgnore the rest of tryExecute catch blocktry { }// code... \ No newline at end of file +BeginNo ErrorsAn error occured in the codeIgnore catch blockIgnore the rest of tryExecute catch blocktry { }// code... \ No newline at end of file diff --git a/1-js/10-error-handling/2-custom-errors/article.md b/1-js/10-error-handling/2-custom-errors/article.md index 91828931..ff2e4c52 100644 --- a/1-js/10-error-handling/2-custom-errors/article.md +++ b/1-js/10-error-handling/2-custom-errors/article.md @@ -21,9 +21,9 @@ Internally, we'll use `JSON.parse`. If it receives malformed `json`, then it thr Our function `readUser(json)` will not only read JSON, but check ("validate") the data. If there are no required fields, or the format is wrong, then that's an error. And that's not a `SyntaxError`, because the data is syntactically correct, but another kind of error. We'll call it `ValidationError` and create a class for it. An error of that kind should also carry the information about the offending field. -Our `ValidationError` class should inherit from the `Error` class. +Our `ValidationError` class should inherit from the built-in `Error` class. -The `Error` class is built-in, but here's its approximate code so we can understand what we're extending: +That class is built-in, but here's its approximate code so we can understand what we're extending: ```js // The "pseudocode" for the built-in Error class defined by JavaScript itself @@ -117,15 +117,15 @@ We could also look at `err.name`, like this: // instead of (err instanceof SyntaxError) } else if (err.name == "SyntaxError") { // (*) // ... -``` +``` The `instanceof` version is much better, because in the future we are going to extend `ValidationError`, make subtypes of it, like `PropertyRequiredError`. And `instanceof` check will continue to work for new inheriting classes. So that's future-proof. -Also it's important that if `catch` meets an unknown error, then it rethrows it in the line `(**)`. The `catch` block only knows how to handle validation and syntax errors, other kinds (caused by a typo in the code or other unknown reasons) should fall through. +Also it's important that if `catch` meets an unknown error, then it rethrows it in the line `(**)`. The `catch` block only knows how to handle validation and syntax errors, other kinds (due to a typo in the code or other unknown ones) should fall through. ## Further inheritance -The `ValidationError` class is very generic. Many things may go wrong. The property may be absent or it may be in a wrong format (like a string value for `age` instead of a number). Let's make a more concrete class `PropertyRequiredError`, exactly for absent properties. It will carry additional information about the property that's missing. +The `ValidationError` class is very generic. Many things may go wrong. The property may be absent or it may be in a wrong format (like a string value for `age`). Let's make a more concrete class `PropertyRequiredError`, exactly for absent properties. It will carry additional information about the property that's missing. ```js run class ValidationError extends Error { diff --git a/7-animation/2-css-animations/4-animate-circle-callback/solution.md b/1-js/11-async/01-callbacks/01-animate-circle-callback/solution.md similarity index 100% rename from 7-animation/2-css-animations/4-animate-circle-callback/solution.md rename to 1-js/11-async/01-callbacks/01-animate-circle-callback/solution.md diff --git a/7-animation/2-css-animations/4-animate-circle-callback/solution.view/index.html b/1-js/11-async/01-callbacks/01-animate-circle-callback/solution.view/index.html similarity index 94% rename from 7-animation/2-css-animations/4-animate-circle-callback/solution.view/index.html rename to 1-js/11-async/01-callbacks/01-animate-circle-callback/solution.view/index.html index 64746e85..b2192681 100644 --- a/7-animation/2-css-animations/4-animate-circle-callback/solution.view/index.html +++ b/1-js/11-async/01-callbacks/01-animate-circle-callback/solution.view/index.html @@ -10,7 +10,7 @@ text-align: center; } .circle { - transition-property: width, height; + transition-property: width, height, margin-left, margin-top; transition-duration: 2s; position: fixed; transform: translateX(-50%) translateY(-50%); diff --git a/7-animation/2-css-animations/4-animate-circle-callback/task.md b/1-js/11-async/01-callbacks/01-animate-circle-callback/task.md similarity index 100% rename from 7-animation/2-css-animations/4-animate-circle-callback/task.md rename to 1-js/11-async/01-callbacks/01-animate-circle-callback/task.md diff --git a/1-js/11-async/01-callbacks/article.md b/1-js/11-async/01-callbacks/article.md index 57115a90..9d1a260d 100644 --- a/1-js/11-async/01-callbacks/article.md +++ b/1-js/11-async/01-callbacks/article.md @@ -28,7 +28,7 @@ function loadScript(src) { } ``` -It inserts into the document a new, dynamically created, tag ` ``` -```smart -In the browser, we can make a variable window-level global by explicitly assigning it to a `window` property, e.g. `window.user = "John"`. - -Then all scripts will see it, both with `type="module"` and without it. - -That said, making such global variables is frowned upon. Please try to avoid them. -``` +If we really need to make a window-level global variable, we can explicitly assign it to `window` and access as `window.user`. But that's an exception requiring a good reason. ### A module code is evaluated only the first time when imported -If the same module is imported into multiple other modules, its code is executed only once, upon the first import. Then its exports are given to all further importers. +If the same module is imported into multiple other places, its code is executed only the first time, then exports are given to all importers. -The one-time evaluation has important consequences, that we should be aware of. - -Let's see a couple of examples. +That has important consequences. Let's look at them using examples: First, if executing a module code brings side-effects, like showing a message, then importing it multiple times will trigger it only once -- the first time: @@ -146,11 +133,9 @@ import `./alert.js`; // Module is evaluated! import `./alert.js`; // (shows nothing) ``` -The second import shows nothing, because the module has already been evaluated. +In practice, top-level module code is mostly used for initialization, creation of internal data structures, and if we want something to be reusable -- export it. -There's a rule: top-level module code should be used for initialization, creation of module-specific internal data structures. If we need to make something callable multiple times - we should export it as a function, like we did with `sayHi` above. - -Now, let's consider a deeper example. +Now, a more advanced example. Let's say, a module exports an object: @@ -175,67 +160,54 @@ import {admin} from './admin.js'; alert(admin.name); // Pete *!* -// Both 1.js and 2.js reference the same admin object +// Both 1.js and 2.js imported the same object // Changes made in 1.js are visible in 2.js */!* ``` -As you can see, when `1.js` changes the `name` property in the imported `admin`, then `2.js` can see the new `admin.name`. +So, let's reiterate -- the module is executed only once. Exports are generated, and then they are shared between importers, so if something changes the `admin` object, other modules will see that. -That's exactly because the module is executed only once. Exports are generated, and then they are shared between importers, so if something changes the `admin` object, other importers will see that. +Such behavior allows us to *configure* modules on first import. We can setup its properties once, and then in further imports it's ready. -**Such behavior is actually very convenient, because it allows us to *configure* modules.** - -In other words, a module can provide a generic functionality that needs a setup. E.g. authentication needs credentials. Then it can export a configuration object expecting the outer code to assign to it. - -Here's the classical pattern: -1. A module exports some means of configuration, e.g. a configuration object. -2. On the first import we initialize it, write to its properties. The top-level application script may do that. -3. Further imports use the module. - -For instance, the `admin.js` module may provide certain functionality (e.g. authentication), but expect the credentials to come into the `config` object from outside: +For instance, the `admin.js` module may provide certain functionality, but expect the credentials to come into the `admin` object from outside: ```js // 📁 admin.js -export let config = { }; +export let admin = { }; export function sayHi() { - alert(`Ready to serve, ${config.user}!`); + alert(`Ready to serve, ${admin.name}!`); } ``` -Here, `admin.js` exports the `config` object (initially empty, but may have default properties too). - -Then in `init.js`, the first script of our app, we import `config` from it and set `config.user`: +In `init.js`, the first script of our app, we set `admin.name`. Then everyone will see it, including calls made from inside `admin.js` itself: ```js // 📁 init.js -import {config} from './admin.js'; -config.user = "Pete"; +import {admin} from './admin.js'; +admin.name = "Pete"; ``` -...Now the module `admin.js` is configured. - -Further importers can call it, and it correctly shows the current user: +Another module can also see `admin.name`: ```js -// 📁 another.js -import {sayHi} from './admin.js'; +// 📁 other.js +import {admin, sayHi} from './admin.js'; + +alert(admin.name); // *!*Pete*/!* sayHi(); // Ready to serve, *!*Pete*/!*! ``` - ### import.meta The object `import.meta` contains the information about the current module. -Its content depends on the environment. In the browser, it contains the URL of the script, or a current webpage URL if inside HTML: +Its content depends on the environment. In the browser, it contains the url of the script, or a current webpage url if inside HTML: ```html run height=0 ``` @@ -261,7 +233,7 @@ Compare it to non-module scripts, where `this` is a global object: There are also several browser-specific differences of scripts with `type="module"` compared to regular ones. -You may want to skip this section for now if you're reading for the first time, or if you don't use JavaScript in a browser. +You may want skip this section for now if you're reading for the first time, or if you don't use JavaScript in a browser. ### Module scripts are deferred @@ -272,7 +244,7 @@ In other words: - module scripts wait until the HTML document is fully ready (even if they are tiny and load faster than HTML), and then run. - relative order of scripts is maintained: scripts that go first in the document, execute first. -As a side effect, module scripts always "see" the fully loaded HTML-page, including HTML elements below them. +As a side-effect, module scripts always "see" the fully loaded HTML-page, including HTML elements below them. For instance: @@ -288,7 +260,7 @@ Compare to regular script below: diff --git a/1-js/13-modules/02-import-export/article.md b/1-js/13-modules/02-import-export/article.md index ccbf18cf..4bd41a16 100644 --- a/1-js/13-modules/02-import-export/article.md +++ b/1-js/13-modules/02-import-export/article.md @@ -46,7 +46,7 @@ Also, we can put `export` separately. Here we first declare, and then export: -```js +```js // 📁 say.js function sayHi(user) { alert(`Hello, ${user}!`); @@ -93,14 +93,25 @@ At first sight, "import everything" seems such a cool thing, short to write, why Well, there are few reasons. -1. Explicitly listing what to import gives shorter names: `sayHi()` instead of `say.sayHi()`. -2. Explicit list of imports gives better overview of the code structure: what is used and where. It makes code support and refactoring easier. +1. Modern build tools ([webpack](http://webpack.github.io) and others) bundle modules together and optimize them to speedup loading and remove unused stuff. -```smart header="Don't be afraid to import too much" -Modern build tools, such as [webpack](https://webpack.js.org/) and others, bundle modules together and optimize them to speedup loading. They also removed unused imports. + Let's say, we added a 3rd-party library `say.js` to our project with many functions: + ```js + // 📁 say.js + export function sayHi() { ... } + export function sayBye() { ... } + export function becomeSilent() { ... } + ``` -For instance, if you `import * as library` from a huge code library, and then use only few methods, then unused ones [will not be included](https://github.com/webpack/webpack/tree/main/examples/harmony-unused#examplejs) into the optimzed bundle. -``` + Now if we only use one of `say.js` functions in our project: + ```js + // 📁 main.js + import {sayHi} from './say.js'; + ``` + ...Then the optimizer will see that and remove the other functions from the bundled code, thus making the build smaller. That is called "tree-shaking". + +2. Explicitly listing what to import gives shorter names: `sayHi()` instead of `say.sayHi()`. +3. Explicit list of imports gives better overview of the code structure: what is used and where. It makes code support and refactoring easier. ## Import "as" @@ -213,7 +224,7 @@ Without `default`, such an export would give an error: export class { // Error! (non-default export needs a name) constructor() {} } -``` +``` ### The "default" name @@ -310,12 +321,12 @@ export {default as User} from './user.js'; // re-export default Why would that be needed? Let's see a practical use case. -Imagine, we're writing a "package": a folder with a lot of modules, with some of the functionality exported outside (tools like NPM allow us to publish and distribute such packages, but we don't have to use them), and many modules are just "helpers", for internal use in other package modules. +Imagine, we're writing a "package": a folder with a lot of modules, with some of the functionality exported outside (tools like NPM allow us to publish and distribute such packages), and many modules are just "helpers", for internal use in other package modules. The file structure could be like this: ``` auth/ - index.js + index.js user.js helpers.js tests/ @@ -326,19 +337,13 @@ auth/ ... ``` -We'd like to expose the package functionality via a single entry point. - -In other words, a person who would like to use our package, should import only from the "main file" `auth/index.js`. - -Like this: +We'd like to expose the package functionality via a single entry point, the "main file" `auth/index.js`, to be used like this: ```js import {login, logout} from 'auth/index.js' ``` -The "main file", `auth/index.js` exports all the functionality that we'd like to provide in our package. - -The idea is that outsiders, other programmers who use our package, should not meddle with its internal structure, search for files inside our package folder. We export only what's necessary in `auth/index.js` and keep the rest hidden from prying eyes. +The idea is that outsiders, developers who use our package, should not meddle with its internal structure, search for files inside our package folder. We export only what's necessary in `auth/index.js` and keep the rest hidden from prying eyes. As the actual exported functionality is scattered among the package, we can import it into `auth/index.js` and export from it: @@ -361,21 +366,19 @@ The syntax `export ... from ...` is just a shorter notation for such import-expo ```js // 📁 auth/index.js -// re-export login/logout +// import login/logout and immediately export them export {login, logout} from './helpers.js'; -// re-export the default export as User +// import default as User and export it export {default as User} from './user.js'; ... ``` -The notable difference of `export ... from` compared to `import/export` is that re-exported modules aren't available in the current file. So inside the above example of `auth/index.js` we can't use re-exported `login/logout` functions. - ### Re-exporting the default export The default export needs separate handling when re-exporting. -Let's say we have `user.js` with the `export default class User` and would like to re-export it: +Let's say we have `user.js`, and we'd like to re-export class `User` from it: ```js // 📁 user.js @@ -384,21 +387,19 @@ export default class User { } ``` -We can come across two problems with it: +1. `export User from './user.js'` won't work. What can go wrong?... But that's a syntax error! -1. `export User from './user.js'` won't work. That would lead to a syntax error. - - To re-export the default export, we have to write `export {default as User}`, as in the example above. + To re-export the default export, we have to write `export {default as User}`, as in the example above. 2. `export * from './user.js'` re-exports only named exports, but ignores the default one. - If we'd like to re-export both named and default exports, then two statements are needed: + If we'd like to re-export both named and the default export, then two statements are needed: ```js export * from './user.js'; // to re-export named exports export {default} from './user.js'; // to re-export the default export ``` -Such oddities of re-exporting a default export are one of the reasons why some developers don't like default exports and prefer named ones. +Such oddities of re-exporting the default export are one of the reasons why some developers don't like them. ## Summary @@ -417,14 +418,14 @@ You can check yourself by reading them and recalling what they mean: Import: -- Importing named exports: +- Named exports from module: - `import {x [as y], ...} from "module"` -- Importing the default export: +- Default export: - `import x from "module"` - `import {default as x} from "module"` -- Import all: +- Everything: - `import * as obj from "module"` -- Import the module (its code runs), but do not assign any of its exports to variables: +- Import the module (its code runs), but do not assign it to a variable: - `import "module"` We can put `import/export` statements at the top or at the bottom of a script, that doesn't matter. diff --git a/1-js/99-js-misc/01-proxy/01-error-nonexisting/solution.md b/1-js/99-js-misc/01-proxy/01-error-nonexisting/solution.md index 9db69cb2..357a5731 100644 --- a/1-js/99-js-misc/01-proxy/01-error-nonexisting/solution.md +++ b/1-js/99-js-misc/01-proxy/01-error-nonexisting/solution.md @@ -19,5 +19,5 @@ function wrap(target) { user = wrap(user); alert(user.name); // John -alert(user.age); // ReferenceError: Property doesn't exist: "age" +alert(user.age); // ReferenceError: Property doesn't exist "age" ``` diff --git a/1-js/99-js-misc/01-proxy/01-error-nonexisting/task.md b/1-js/99-js-misc/01-proxy/01-error-nonexisting/task.md index 47985e1a..d7093c0c 100644 --- a/1-js/99-js-misc/01-proxy/01-error-nonexisting/task.md +++ b/1-js/99-js-misc/01-proxy/01-error-nonexisting/task.md @@ -27,6 +27,6 @@ user = wrap(user); alert(user.name); // John *!* -alert(user.age); // ReferenceError: Property doesn't exist: "age" +alert(user.age); // ReferenceError: Property doesn't exist "age" */!* ``` diff --git a/1-js/99-js-misc/01-proxy/article.md b/1-js/99-js-misc/01-proxy/article.md index 1f84912e..0711fd33 100644 --- a/1-js/99-js-misc/01-proxy/article.md +++ b/1-js/99-js-misc/01-proxy/article.md @@ -39,7 +39,7 @@ As there are no traps, all operations on `proxy` are forwarded to `target`. As we can see, without any traps, `proxy` is a transparent wrapper around `target`. -![](proxy.svg) +![](proxy.svg) `Proxy` is a special "exotic object". It doesn't have own properties. With an empty `handler` it transparently forwards operations to `target`. @@ -67,7 +67,7 @@ For every internal method, there's a trap in this table: the name of the method | `[[PreventExtensions]]` | `preventExtensions` | [Object.preventExtensions](mdn:/JavaScript/Reference/Global_Objects/Object/preventExtensions) | | `[[DefineOwnProperty]]` | `defineProperty` | [Object.defineProperty](mdn:/JavaScript/Reference/Global_Objects/Object/defineProperty), [Object.defineProperties](mdn:/JavaScript/Reference/Global_Objects/Object/defineProperties) | | `[[GetOwnProperty]]` | `getOwnPropertyDescriptor` | [Object.getOwnPropertyDescriptor](mdn:/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptor), `for..in`, `Object.keys/values/entries` | -| `[[OwnPropertyKeys]]` | `ownKeys` | [Object.getOwnPropertyNames](mdn:/JavaScript/Reference/Global_Objects/Object/getOwnPropertyNames), [Object.getOwnPropertySymbols](mdn:/JavaScript/Reference/Global_Objects/Object/getOwnPropertySymbols), `for..in`, `Object.keys/values/entries` | +| `[[OwnPropertyKeys]]` | `ownKeys` | [Object.getOwnPropertyNames](mdn:/JavaScript/Reference/Global_Objects/Object/getOwnPropertyNames), [Object.getOwnPropertySymbols](mdn:/JavaScript/Reference/Global_Objects/Object/getOwnPropertySymbols), `for..in`, `Object/keys/values/entries` | ```warn header="Invariants" JavaScript enforces some invariants -- conditions that must be fulfilled by internal methods and traps. @@ -335,7 +335,7 @@ let user = { _password: "secret" }; -alert(user._password); // secret +alert(user._password); // secret ``` Let's use proxies to prevent any access to properties starting with `_`. @@ -376,7 +376,7 @@ user = new Proxy(user, { }, *!* deleteProperty(target, prop) { // to intercept property deletion -*/!* +*/!* if (prop.startsWith('_')) { throw new Error("Access denied"); } else { @@ -437,7 +437,7 @@ user = { ``` -A call to `user.checkPassword()` gets proxied `user` as `this` (the object before dot becomes `this`), so when it tries to access `this._password`, the `get` trap activates (it triggers on any property read) and throws an error. +A call to `user.checkPassword()` call gets proxied `user` as `this` (the object before dot becomes `this`), so when it tries to access `this._password`, the `get` trap activates (it triggers on any property read) and throws an error. So we bind the context of object methods to the original object, `target`, in the line `(*)`. Then their future calls will use `target` as `this`, without any traps. @@ -963,13 +963,9 @@ revoke(); alert(proxy.data); // Error ``` -A call to `revoke()` removes all internal references to the target object from the proxy, so they are no longer connected. +A call to `revoke()` removes all internal references to the target object from the proxy, so they are no longer connected. The target object can be garbage-collected after that. -Initially, `revoke` is separate from `proxy`, so that we can pass `proxy` around while leaving `revoke` in the current scope. - -We can also bind `revoke` method to proxy by setting `proxy.revoke = revoke`. - -Another option is to create a `WeakMap` that has `proxy` as the key and the corresponding `revoke` as the value, that allows to easily find `revoke` for a proxy: +We can also store `revoke` in a `WeakMap`, to be able to easily find it by a proxy object: ```js run *!* @@ -984,13 +980,15 @@ let {proxy, revoke} = Proxy.revocable(object, {}); revokes.set(proxy, revoke); -// ..somewhere else in our code.. +// ..later in our code.. revoke = revokes.get(proxy); revoke(); alert(proxy.data); // Error (revoked) ``` +The benefit of such an approach is that we don't have to carry `revoke` around. We can get it from the map by `proxy` when needed. + We use `WeakMap` instead of `Map` here because it won't block garbage collection. If a proxy object becomes "unreachable" (e.g. no variable references it any more), `WeakMap` allows it to be wiped from memory together with its `revoke` that we won't need any more. ## References diff --git a/1-js/99-js-misc/01-proxy/proxy-inherit-admin.svg b/1-js/99-js-misc/01-proxy/proxy-inherit-admin.svg index 3fba6460..a5e15840 100644 --- a/1-js/99-js-misc/01-proxy/proxy-inherit-admin.svg +++ b/1-js/99-js-misc/01-proxy/proxy-inherit-admin.svg @@ -1 +1 @@ -_name: "Guest" name: getter_name: "Admin"user (proxied)original useradmin[[Prototype]] \ No newline at end of file +_name: "Guest" name: getter_name: "Admin"user (proxied)original useradmin[[Prototype]] \ No newline at end of file diff --git a/1-js/99-js-misc/01-proxy/proxy-inherit.svg b/1-js/99-js-misc/01-proxy/proxy-inherit.svg index 6c34c0f4..510dcef1 100644 --- a/1-js/99-js-misc/01-proxy/proxy-inherit.svg +++ b/1-js/99-js-misc/01-proxy/proxy-inherit.svg @@ -1 +1 @@ -_name: "Guest" name: getteruser (proxied)original user \ No newline at end of file +_name: "Guest" name: getteruser (proxied)original user \ No newline at end of file diff --git a/1-js/99-js-misc/01-proxy/proxy.svg b/1-js/99-js-misc/01-proxy/proxy.svg index 6b2224cf..76a41670 100644 --- a/1-js/99-js-misc/01-proxy/proxy.svg +++ b/1-js/99-js-misc/01-proxy/proxy.svg @@ -1 +1 @@ -test: 5proxytargetget proxy.test5 \ No newline at end of file +test: 5proxytargetget proxy.test5 \ No newline at end of file diff --git a/1-js/99-js-misc/03-currying-partials/article.md b/1-js/99-js-misc/03-currying-partials/article.md index d71ac23f..bb308847 100644 --- a/1-js/99-js-misc/03-currying-partials/article.md +++ b/1-js/99-js-misc/03-currying-partials/article.md @@ -155,7 +155,7 @@ function curried(...args) { if (args.length >= func.length) { // (1) return func.apply(this, args); } else { - return function(...args2) { // (2) + return function pass(...args2) { // (2) return curried.apply(this, args.concat(args2)); } } @@ -164,10 +164,18 @@ function curried(...args) { When we run it, there are two `if` execution branches: -1. If passed `args` count is the same or more than the original function has in its definition (`func.length`) , then just pass the call to it using `func.apply`. -2. Otherwise, get a partial: we don't call `func` just yet. Instead, another wrapper is returned, that will re-apply `curried` providing previous arguments together with the new ones. +1. Call now: if passed `args` count is the same as the original function has in its definition (`func.length`) or longer, then just pass the call to it. +2. Get a partial: otherwise, `func` is not called yet. Instead, another wrapper `pass` is returned, that will re-apply `curried` providing previous arguments together with the new ones. Then on a new call, again, we'll get either a new partial (if not enough arguments) or, finally, the result. -Then, if we call it, again, we'll get either a new partial (if not enough arguments) or, finally, the result. +For instance, let's see what happens in the case of `sum(a, b, c)`. Three arguments, so `sum.length = 3`. + +For the call `curried(1)(2)(3)`: + +1. The first call `curried(1)` remembers `1` in its Lexical Environment, and returns a wrapper `pass`. +2. The wrapper `pass` is called with `(2)`: it takes previous args (`1`), concatenates them with what it got `(2)` and calls `curried(1, 2)` with them together. As the argument count is still less than 3, `curry` returns `pass`. +3. The wrapper `pass` is called again with `(3)`, for the next call `pass(3)` takes previous args (`1`, `2`) and adds `3` to them, making the call `curried(1, 2, 3)` -- there are `3` arguments at last, they are given to the original function. + +If that's still not obvious, just trace the calls sequence in your mind or on paper. ```smart header="Fixed-length functions only" The currying requires the function to have a fixed number of arguments. diff --git a/1-js/99-js-misc/04-reference-type/3-why-this/solution.md b/1-js/99-js-misc/04-reference-type/3-why-this/solution.md index e4ee7874..31ea4ff8 100644 --- a/1-js/99-js-misc/04-reference-type/3-why-this/solution.md +++ b/1-js/99-js-misc/04-reference-type/3-why-this/solution.md @@ -5,7 +5,7 @@ Here's the explanations. 2. The same, parentheses do not change the order of operations here, the dot is first anyway. -3. Here we have a more complex call `(expression)()`. The call works as if it were split into two lines: +3. Here we have a more complex call `(expression).method()`. The call works as if it were split into two lines: ```js no-beautify f = obj.go; // calculate the expression @@ -14,7 +14,7 @@ Here's the explanations. Here `f()` is executed as a function, without `this`. -4. The similar thing as `(3)`, to the left of the parentheses `()` we have an expression. +4. The similar thing as `(3)`, to the left of the dot `.` we have an expression. To explain the behavior of `(3)` and `(4)` we need to recall that property accessors (dot or square brackets) return a value of the Reference Type. diff --git a/1-js/99-js-misc/04-reference-type/article.md b/1-js/99-js-misc/04-reference-type/article.md index 894db8fc..c680c17f 100644 --- a/1-js/99-js-misc/04-reference-type/article.md +++ b/1-js/99-js-misc/04-reference-type/article.md @@ -4,7 +4,7 @@ ```warn header="In-depth language feature" This article covers an advanced topic, to understand certain edge-cases better. -It's not important. Many experienced developers live fine without knowing it. Read on if you want to know how things work under the hood. +It's not important. Many experienced developers live fine without knowing it. Read on if you're want to know how things work under the hood. ``` A dynamically evaluated method call can lose `this`. @@ -59,7 +59,7 @@ If we put these operations on separate lines, then `this` will be lost for sure: let user = { name: "John", hi() { alert(this.name); } -}; +} *!* // split getting and calling the method in two lines @@ -87,13 +87,13 @@ The result of a property access `user.hi` is not a function, but a value of Refe (user, "hi", true) ``` -When parentheses `()` are called on the Reference Type, they receive the full information about the object and its method, and can set the right `this` (`user` in this case). +When parentheses `()` are called on the Reference Type, they receive the full information about the object and its method, and can set the right `this` (`=user` in this case). Reference type is a special "intermediary" internal type, with the purpose to pass information from dot `.` to calling parentheses `()`. Any other operation like assignment `hi = user.hi` discards the reference type as a whole, takes the value of `user.hi` (a function) and passes it on. So any further operation "loses" `this`. -So, as the result, the value of `this` is only passed the right way if the function is called directly using a dot `obj.method()` or square brackets `obj['method']()` syntax (they do the same here). There are various ways to solve this problem such as [func.bind()](/bind#solution-2-bind). +So, as the result, the value of `this` is only passed the right way if the function is called directly using a dot `obj.method()` or square brackets `obj['method']()` syntax (they do the same here). Later in this tutorial, we will learn various ways to solve this problem such as [func.bind()](/bind#solution-2-bind). ## Summary diff --git a/1-js/99-js-misc/06-unicode/article.md b/1-js/99-js-misc/06-unicode/article.md deleted file mode 100644 index c2198989..00000000 --- a/1-js/99-js-misc/06-unicode/article.md +++ /dev/null @@ -1,172 +0,0 @@ - -# Unicode, String internals - -```warn header="Advanced knowledge" -The section goes deeper into string internals. This knowledge will be useful for you if you plan to deal with emoji, rare mathematical or hieroglyphic characters, or other rare symbols. -``` - -As we already know, JavaScript strings are based on [Unicode](https://en.wikipedia.org/wiki/Unicode): each character is represented by a byte sequence of 1-4 bytes. - -JavaScript allows us to insert a character into a string by specifying its hexadecimal Unicode code with one of these three notations: - -- `\xXX` - - `XX` must be two hexadecimal digits with a value between `00` and `FF`, then `\xXX` is the character whose Unicode code is `XX`. - - Because the `\xXX` notation supports only two hexadecimal digits, it can be used only for the first 256 Unicode characters. - - These first 256 characters include the Latin alphabet, most basic syntax characters, and some others. For example, `"\x7A"` is the same as `"z"` (Unicode `U+007A`). - - ```js run - alert( "\x7A" ); // z - alert( "\xA9" ); // ©, the copyright symbol - ``` - -- `\uXXXX` - `XXXX` must be exactly 4 hex digits with the value between `0000` and `FFFF`, then `\uXXXX` is the character whose Unicode code is `XXXX`. - - Characters with Unicode values greater than `U+FFFF` can also be represented with this notation, but in this case, we will need to use a so called surrogate pair (we will talk about surrogate pairs later in this chapter). - - ```js run - alert( "\u00A9" ); // ©, the same as \xA9, using the 4-digit hex notation - alert( "\u044F" ); // я, the Cyrillic alphabet letter - alert( "\u2191" ); // ↑, the arrow up symbol - ``` - -- `\u{X…XXXXXX}` - - `X…XXXXXX` must be a hexadecimal value of 1 to 6 bytes between `0` and `10FFFF` (the highest code point defined by Unicode). This notation allows us to easily represent all existing Unicode characters. - - ```js run - alert( "\u{20331}" ); // 佫, a rare Chinese character (long Unicode) - alert( "\u{1F60D}" ); // 😍, a smiling face symbol (another long Unicode) - ``` - -## Surrogate pairs - -All frequently used characters have 2-byte codes (4 hex digits). Letters in most European languages, numbers, and the basic unified CJK ideographic sets (CJK -- from Chinese, Japanese, and Korean writing systems), have a 2-byte representation. - -Initially, JavaScript was based on UTF-16 encoding that only allowed 2 bytes per character. But 2 bytes only allow 65536 combinations and that's not enough for every possible symbol of Unicode. - -So rare symbols that require more than 2 bytes are encoded with a pair of 2-byte characters called "a surrogate pair". - -As a side effect, the length of such symbols is `2`: - -```js run -alert( '𝒳'.length ); // 2, MATHEMATICAL SCRIPT CAPITAL X -alert( '😂'.length ); // 2, FACE WITH TEARS OF JOY -alert( '𩷶'.length ); // 2, a rare Chinese character -``` - -That's because surrogate pairs did not exist at the time when JavaScript was created, and thus are not correctly processed by the language! - -We actually have a single symbol in each of the strings above, but the `length` property shows a length of `2`. - -Getting a symbol can also be tricky, because most language features treat surrogate pairs as two characters. - -For example, here we can see two odd characters in the output: - -```js run -alert( '𝒳'[0] ); // shows strange symbols... -alert( '𝒳'[1] ); // ...pieces of the surrogate pair -``` - -Pieces of a surrogate pair have no meaning without each other. So the alerts in the example above actually display garbage. - -Technically, surrogate pairs are also detectable by their codes: if a character has the code in the interval of `0xd800..0xdbff`, then it is the first part of the surrogate pair. The next character (second part) must have the code in interval `0xdc00..0xdfff`. These intervals are reserved exclusively for surrogate pairs by the standard. - -So the methods `String.fromCodePoint` and `str.codePointAt` were added in JavaScript to deal with surrogate pairs. - -They are essentially the same as [String.fromCharCode](mdn:js/String/fromCharCode) and [str.charCodeAt](mdn:js/String/charCodeAt), but they treat surrogate pairs correctly. - -One can see the difference here: - -```js run -// charCodeAt is not surrogate-pair aware, so it gives codes for the 1st part of 𝒳: - -alert( '𝒳'.charCodeAt(0).toString(16) ); // d835 - -// codePointAt is surrogate-pair aware -alert( '𝒳'.codePointAt(0).toString(16) ); // 1d4b3, reads both parts of the surrogate pair -``` - -That said, if we take from position 1 (and that's rather incorrect here), then they both return only the 2nd part of the pair: - -```js run -alert( '𝒳'.charCodeAt(1).toString(16) ); // dcb3 -alert( '𝒳'.codePointAt(1).toString(16) ); // dcb3 -// meaningless 2nd half of the pair -``` - -You will find more ways to deal with surrogate pairs later in the chapter . There are probably special libraries for that too, but nothing famous enough to suggest here. - -````warn header="Takeaway: splitting strings at an arbitrary point is dangerous" -We can't just split a string at an arbitrary position, e.g. take `str.slice(0, 4)` and expect it to be a valid string, e.g.: - -```js run -alert( 'hi 😂'.slice(0, 4) ); // hi [?] -``` - -Here we can see a garbage character (first half of the smile surrogate pair) in the output. - -Just be aware of it if you intend to reliably work with surrogate pairs. May not be a big problem, but at least you should understand what happens. -```` - -## Diacritical marks and normalization - -In many languages, there are symbols that are composed of the base character with a mark above/under it. - -For instance, the letter `a` can be the base character for these characters: `àáâäãåā`. - -Most common "composite" characters have their own code in the Unicode table. But not all of them, because there are too many possible combinations. - -To support arbitrary compositions, the Unicode standard allows us to use several Unicode characters: the base character followed by one or many "mark" characters that "decorate" it. - -For instance, if we have `S` followed by the special "dot above" character (code `\u0307`), it is shown as Ṡ. - -```js run -alert( 'S\u0307' ); // Ṡ -``` - -If we need an additional mark above the letter (or below it) -- no problem, just add the necessary mark character. - -For instance, if we append a character "dot below" (code `\u0323`), then we'll have "S with dots above and below": `Ṩ`. - -For example: - -```js run -alert( 'S\u0307\u0323' ); // Ṩ -``` - -This provides great flexibility, but also an interesting problem: two characters may visually look the same, but be represented with different Unicode compositions. - -For instance: - -```js run -let s1 = 'S\u0307\u0323'; // Ṩ, S + dot above + dot below -let s2 = 'S\u0323\u0307'; // Ṩ, S + dot below + dot above - -alert( `s1: ${s1}, s2: ${s2}` ); - -alert( s1 == s2 ); // false though the characters look identical (?!) -``` - -To solve this, there exists a "Unicode normalization" algorithm that brings each string to the single "normal" form. - -It is implemented by [str.normalize()](mdn:js/String/normalize). - -```js run -alert( "S\u0307\u0323".normalize() == "S\u0323\u0307".normalize() ); // true -``` - -It's funny that in our situation `normalize()` actually brings together a sequence of 3 characters to one: `\u1e68` (S with two dots). - -```js run -alert( "S\u0307\u0323".normalize().length ); // 1 - -alert( "S\u0307\u0323".normalize() == "\u1e68" ); // true -``` - -In reality, this is not always the case. The reason is that the symbol `Ṩ` is "common enough", so Unicode creators included it in the main table and gave it the code. - -If you want to learn more about normalization rules and variants -- they are described in the appendix of the Unicode standard: [Unicode Normalization Forms](https://www.unicode.org/reports/tr15/), but for most practical purposes the information from this section is enough. diff --git a/2-ui/1-document/01-browser-environment/article.md b/2-ui/1-document/01-browser-environment/article.md index eedc28fb..56b56883 100644 --- a/2-ui/1-document/01-browser-environment/article.md +++ b/2-ui/1-document/01-browser-environment/article.md @@ -1,10 +1,10 @@ # Browser environment, specs -The JavaScript language was initially created for web browsers. Since then, it has evolved into a language with many uses and platforms. +The JavaScript language was initially created for web browsers. Since then it has evolved and become a language with many uses and platforms. -A platform may be a browser, or a web-server or another *host*, or even a "smart" coffee machine if it can run JavaScript. Each of these provides platform-specific functionality. The JavaScript specification calls that a *host environment*. +A platform may be a browser, or a web-server or another *host*, even a "smart" coffee machine, if it can run JavaScript. Each of them provides platform-specific functionality. The JavaScript specification calls that a *host environment*. -A host environment provides its own objects and functions in addition to the language core. Web browsers give a means to control web pages. Node.js provides server-side features, and so on. +A host environment provides own objects and functions additional to the language core. Web browsers give a means to control web pages. Node.js provides server-side features, and so on. Here's a bird's-eye view of what we have when JavaScript runs in a web browser: @@ -15,9 +15,9 @@ There's a "root" object called `window`. It has two roles: 1. First, it is a global object for JavaScript code, as described in the chapter . 2. Second, it represents the "browser window" and provides methods to control it. -For instance, we can use it as a global object: +For instance, here we use it as a global object: -```js run global +```js run function sayHi() { alert("Hello"); } @@ -26,17 +26,17 @@ function sayHi() { window.sayHi(); ``` -And we can use it as a browser window, to show the window height: +And here we use it as a browser window, to see the window height: ```js run alert(window.innerHeight); // inner window height ``` -There are more window-specific methods and properties, which we'll cover later. +There are more window-specific methods and properties, we'll cover them later. ## DOM (Document Object Model) -The Document Object Model, or DOM for short, represents all page content as objects that can be modified. +Document Object Model, or DOM for short, represents all page content as objects that can be modified. The `document` object is the main "entry point" to the page. We can change or create anything on the page using it. @@ -49,18 +49,18 @@ document.body.style.background = "red"; setTimeout(() => document.body.style.background = "", 1000); ``` -Here, we used `document.body.style`, but there's much, much more. Properties and methods are described in the specification: [DOM Living Standard](https://dom.spec.whatwg.org). +Here we used `document.body.style`, but there's much, much more. Properties and methods are described in the specification: [DOM Living Standard](https://dom.spec.whatwg.org). ```smart header="DOM is not only for browsers" The DOM specification explains the structure of a document and provides objects to manipulate it. There are non-browser instruments that use DOM too. -For instance, server-side scripts that download HTML pages and process them can also use the DOM. They may support only a part of the specification though. +For instance, server-side scripts that download HTML pages and process them can also use DOM. They may support only a part of the specification though. ``` ```smart header="CSSOM for styling" There's also a separate specification, [CSS Object Model (CSSOM)](https://www.w3.org/TR/cssom-1/) for CSS rules and stylesheets, that explains how they are represented as objects, and how to read and write them. -The CSSOM is used together with the DOM when we modify style rules for the document. In practice though, the CSSOM is rarely required, because we rarely need to modify CSS rules from JavaScript (usually we just add/remove CSS classes, not modify their CSS rules), but that's also possible. +CSSOM is used together with DOM when we modify style rules for the document. In practice though, CSSOM is rarely required, because we rarely need to modify CSS rules from JavaScript (usually we just add/remove CSS classes, not modify their CSS rules), but that's also possible. ``` ## BOM (Browser Object Model) @@ -69,7 +69,7 @@ The Browser Object Model (BOM) represents additional objects provided by the bro For instance: -- The [navigator](mdn:api/Window/navigator) object provides background information about the browser and the operating system. There are many properties, but the two most widely known are: `navigator.userAgent` -- about the current browser, and `navigator.platform` -- about the platform (can help to differentiate between Windows/Linux/Mac etc). +- The [navigator](mdn:api/Window/navigator) object provides background information about the browser and the operating system. There are many properties, but the two most widely known are: `navigator.userAgent` -- about the current browser, and `navigator.platform` -- about the platform (can help to differ between Windows/Linux/Mac etc). - The [location](mdn:api/Window/location) object allows us to read the current URL and can redirect the browser to a new one. Here's how we can use the `location` object: @@ -81,12 +81,12 @@ if (confirm("Go to Wikipedia?")) { } ``` -The functions `alert/confirm/prompt` are also a part of the BOM: they are not directly related to the document, but represent pure browser methods for communicating with the user. +Functions `alert/confirm/prompt` are also a part of BOM: they are directly not related to the document, but represent pure browser methods of communicating with the user. ```smart header="Specifications" -The BOM is a part of the general [HTML specification](https://html.spec.whatwg.org). +BOM is the part of the general [HTML specification](https://html.spec.whatwg.org). -Yes, you heard that right. The HTML spec at is not only about the "HTML language" (tags, attributes), but also covers a bunch of objects, methods, and browser-specific DOM extensions. That's "HTML in broad terms". Also, some parts have additional specs listed at . +Yes, you heard that right. The HTML spec at is not only about the "HTML language" (tags, attributes), but also covers a bunch of objects, methods and browser-specific DOM extensions. That's "HTML in broad terms". Also, some parts have additional specs listed at . ``` ## Summary @@ -94,20 +94,20 @@ Yes, you heard that right. The HTML spec at is no Talking about standards, we have: DOM specification -: Describes the document structure, manipulations, and events, see . +: Describes the document structure, manipulations and events, see . CSSOM specification -: Describes stylesheets and style rules, manipulations with them, and their binding to documents, see . +: Describes stylesheets and style rules, manipulations with them and their binding to documents, see . HTML specification : Describes the HTML language (e.g. tags) and also the BOM (browser object model) -- various browser functions: `setTimeout`, `alert`, `location` and so on, see . It takes the DOM specification and extends it with many additional properties and methods. Additionally, some classes are described separately at . -Please note these links, as there's so much to learn that it's impossible to cover everything and remember it all. +Please note these links, as there's so much stuff to learn it's impossible to cover and remember everything. -When you'd like to read about a property or a method, the Mozilla manual at is also a nice resource, but the corresponding spec may be better: it's more complex and longer to read, but will make your fundamental knowledge sound and complete. +When you'd like to read about a property or a method, the Mozilla manual at is also a nice resource, but the corresponding spec may be better: it's more complex and longer to read, but will make your fundamental knowledge sound and complete. To find something, it's often convenient to use an internet search "WHATWG [term]" or "MDN [term]", e.g , . -Now, we'll get down to learning the DOM, because the document plays the central role in the UI. +Now we'll get down to learning DOM, because the document plays the central role in the UI. diff --git a/2-ui/1-document/01-browser-environment/windowObjects.svg b/2-ui/1-document/01-browser-environment/windowObjects.svg index b7e18bb3..d1b280ee 100644 --- a/2-ui/1-document/01-browser-environment/windowObjects.svg +++ b/2-ui/1-document/01-browser-environment/windowObjects.svg @@ -1 +1 @@ -windowdocumentObjectnavigatorscreenlocationframeshistoryArrayFunctionXMLHttpRequestBOMJavaScriptDOM \ No newline at end of file +windowdocumentObjectnavigatorscreenlocationframeshistoryArrayFunctionXMLHttpRequestBOMJavaScriptDOM \ No newline at end of file diff --git a/2-ui/1-document/02-dom-nodes/article.md b/2-ui/1-document/02-dom-nodes/article.md index e18335f3..019398be 100644 --- a/2-ui/1-document/02-dom-nodes/article.md +++ b/2-ui/1-document/02-dom-nodes/article.md @@ -51,7 +51,7 @@ The DOM represents HTML as a tree structure of tags. Here's how it looks:
@@ -143,7 +143,7 @@ drawHtmlTree(node4, 'div.domtree', 690, 360); ````warn header="Tables always have ``" -An interesting "special case" is tables. By DOM specification they must have `` tag, but HTML text may omit it. Then the browser creates `` in the DOM automatically. +An interesting "special case" is tables. By the DOM specification they must have ``, but HTML text may (officially) omit it. Then the browser creates `` in the DOM automatically. For the HTML: @@ -160,7 +160,7 @@ let node5 = {"name":"TABLE","nodeType":1,"children":[{"name":"TBODY","nodeType": drawHtmlTree(node5, 'div.domtree', 600, 200); -You see? The `` appeared out of nowhere. We should keep this in mind while working with tables to avoid surprises. +You see? The `` appeared out of nowhere. You should keep this in mind while working with tables to avoid surprises. ```` ## Other node types @@ -188,7 +188,7 @@ For example, comments:
@@ -199,7 +199,7 @@ We may think -- why is a comment added to the DOM? It doesn't affect the visual **Everything in HTML, even comments, becomes a part of the DOM.** -Even the `` directive at the very beginning of HTML is also a DOM node. It's in the DOM tree right before ``. Few people know about that. We are not going to touch that node, we even don't draw it on diagrams, but it's there. +Even the `` directive at the very beginning of HTML is also a DOM node. It's in the DOM tree right before ``. We are not going to touch that node, we even don't draw it on diagrams for that reason, but it's there. The `document` object that represents the whole document is, formally, a DOM node as well. diff --git a/2-ui/1-document/02-dom-nodes/domconsole0.svg b/2-ui/1-document/02-dom-nodes/domconsole0.svg index eb99f193..c0096060 100644 --- a/2-ui/1-document/02-dom-nodes/domconsole0.svg +++ b/2-ui/1-document/02-dom-nodes/domconsole0.svg @@ -1 +1 @@ - \ No newline at end of file + \ No newline at end of file diff --git a/2-ui/1-document/02-dom-nodes/domconsole1.svg b/2-ui/1-document/02-dom-nodes/domconsole1.svg index 02ef5f0a..db92359d 100644 --- a/2-ui/1-document/02-dom-nodes/domconsole1.svg +++ b/2-ui/1-document/02-dom-nodes/domconsole1.svg @@ -1 +1 @@ - \ No newline at end of file + \ No newline at end of file diff --git a/2-ui/1-document/02-dom-nodes/elk.svg b/2-ui/1-document/02-dom-nodes/elk.svg index 448eea9d..19ea221d 100644 --- a/2-ui/1-document/02-dom-nodes/elk.svg +++ b/2-ui/1-document/02-dom-nodes/elk.svg @@ -1 +1 @@ - \ No newline at end of file + \ No newline at end of file diff --git a/2-ui/1-document/02-dom-nodes/inspect.svg b/2-ui/1-document/02-dom-nodes/inspect.svg index 60696ec0..658ee5ea 100644 --- a/2-ui/1-document/02-dom-nodes/inspect.svg +++ b/2-ui/1-document/02-dom-nodes/inspect.svg @@ -1 +1 @@ - \ No newline at end of file + \ No newline at end of file diff --git a/2-ui/1-document/03-dom-navigation/article.md b/2-ui/1-document/03-dom-navigation/article.md index b5f03098..f7123d70 100644 --- a/2-ui/1-document/03-dom-navigation/article.md +++ b/2-ui/1-document/03-dom-navigation/article.md @@ -214,7 +214,7 @@ alert( document.body.previousSibling ); // HTMLHeadElement ## Element-only navigation -Navigation properties listed above refer to *all* nodes. For instance, in `childNodes` we can see both text nodes, element nodes, and even comment nodes if they exist. +Navigation properties listed above refer to *all* nodes. For instance, in `childNodes` we can see both text nodes, element nodes, and even comment nodes if there exist. But for many tasks we don't want text or comment nodes. We want to manipulate element nodes that represent tags and form the structure of the page. diff --git a/2-ui/1-document/03-dom-navigation/dom-links-elements.svg b/2-ui/1-document/03-dom-navigation/dom-links-elements.svg index fd0b2826..a9ce1fd8 100644 --- a/2-ui/1-document/03-dom-navigation/dom-links-elements.svg +++ b/2-ui/1-document/03-dom-navigation/dom-links-elements.svg @@ -1 +1 @@ -document.documentElement <HTML>document.body (if inside body)parent Element<DIV>next Element Siblingprevious Element Siblingchildrenfirst Element Child last Element Child \ No newline at end of file +document.documentElement <HTML>document.body (if inside body)parent Element<DIV>next Element Siblingprevious Element Siblingchildrenfirst Element Child last Element Child \ No newline at end of file diff --git a/2-ui/1-document/03-dom-navigation/dom-links.svg b/2-ui/1-document/03-dom-navigation/dom-links.svg index 6c34bca4..126530e9 100644 --- a/2-ui/1-document/03-dom-navigation/dom-links.svg +++ b/2-ui/1-document/03-dom-navigation/dom-links.svg @@ -1 +1 @@ -documentdocument.documentElement <HTML>document.body (if inside body)parentNode<DIV>nextSiblingpreviousSiblingchildNodesfirstChild lastChild \ No newline at end of file +documentdocument.documentElement <HTML>document.body (if inside body)parentNode<DIV>nextSiblingpreviousSiblingchildNodesfirstChild lastChild \ No newline at end of file diff --git a/2-ui/1-document/04-searching-elements-dom/article.md b/2-ui/1-document/04-searching-elements-dom/article.md index de47eac9..f5ab0b78 100644 --- a/2-ui/1-document/04-searching-elements-dom/article.md +++ b/2-ui/1-document/04-searching-elements-dom/article.md @@ -55,7 +55,7 @@ Also, there's a global variable named by `id` that references the element: ``` ```warn header="Please don't use id-named global variables to access elements" -This behavior is described [in the specification](http://www.whatwg.org/specs/web-apps/current-work/#dom-window-nameditem), so it's a kind of standard. But it is supported mainly for compatibility. +This behavior is described [in the specification](http://www.whatwg.org/specs/web-apps/current-work/#dom-window-nameditem), so it's kind of standard. But it is supported mainly for compatibility. The browser tries to help us by mixing namespaces of JS and DOM. That's fine for simple scripts, inlined into HTML, but generally isn't a good thing. There may be naming conflicts. Also, when one reads JS code and doesn't have HTML in view, it's not obvious where the variable comes from. @@ -71,7 +71,7 @@ If there are multiple elements with the same `id`, then the behavior of methods ``` ```warn header="Only `document.getElementById`, not `anyElem.getElementById`" -The method `getElementById` can be called only on `document` object. It looks for the given `id` in the whole document. +The method `getElementById` that can be called only on `document` object. It looks for the given `id` in the whole document. ``` ## querySelectorAll [#querySelectorAll] @@ -116,7 +116,7 @@ In other words, the result is the same as `elem.querySelectorAll(css)[0]`, but t Previous methods were searching the DOM. -The [elem.matches(css)](https://dom.spec.whatwg.org/#dom-element-matches) does not look for anything, it merely checks if `elem` matches the given CSS-selector. It returns `true` or `false`. +The [elem.matches(css)](http://dom.spec.whatwg.org/#dom-element-matches) does not look for anything, it merely checks if `elem` matches the given CSS-selector. It returns `true` or `false`. The method comes in handy when we are iterating over elements (like in an array or something) and trying to filter out those that interest us. @@ -142,7 +142,7 @@ For instance: *Ancestors* of an element are: parent, the parent of parent, its parent and so on. The ancestors together form the chain of parents from the element to the top. -The method `elem.closest(css)` looks for the nearest ancestor that matches the CSS-selector. The `elem` itself is also included in the search. +The method `elem.closest(css)` looks the nearest ancestor that matches the CSS-selector. The `elem` itself is also included in the search. In other words, the method `closest` goes up from the element and checks each of parents. If it matches the selector, then the search stops, and the ancestor is returned. @@ -154,7 +154,7 @@ For instance:
  • Chapter 1
  • -
  • Chapter 2
  • +
  • Chapter 1
@@ -363,7 +363,7 @@ There are 6 main methods to search for nodes in DOM: -By far the most used are `querySelector` and `querySelectorAll`, but `getElement(s)By*` can be sporadically helpful or found in the old scripts. +By far the most used are `querySelector` and `querySelectorAll`, but `getElementBy*` can be sporadically helpful or found in the old scripts. Besides that: diff --git a/2-ui/1-document/05-basic-dom-node-properties/article.md b/2-ui/1-document/05-basic-dom-node-properties/article.md index 99dde5bc..76469c18 100644 --- a/2-ui/1-document/05-basic-dom-node-properties/article.md +++ b/2-ui/1-document/05-basic-dom-node-properties/article.md @@ -10,7 +10,7 @@ Different DOM nodes may have different properties. For instance, an element node Each DOM node belongs to the corresponding built-in class. -The root of the hierarchy is [EventTarget](https://dom.spec.whatwg.org/#eventtarget), that is inherited by [Node](https://dom.spec.whatwg.org/#interface-node), and other DOM nodes inherit from it. +The root of the hierarchy is [EventTarget](https://dom.spec.whatwg.org/#eventtarget), that is inherited by [Node](http://dom.spec.whatwg.org/#interface-node), and other DOM nodes inherit from it. Here's the picture, explanations to follow: @@ -18,39 +18,16 @@ Here's the picture, explanations to follow: The classes are: -- [EventTarget](https://dom.spec.whatwg.org/#eventtarget) -- is the root "abstract" class for everything. - - Objects of that class are never created. It serves as a base, so that all DOM nodes support so-called "events", we'll study them later. - -- [Node](https://dom.spec.whatwg.org/#interface-node) -- is also an "abstract" class, serving as a base for DOM nodes. - - It provides the core tree functionality: `parentNode`, `nextSibling`, `childNodes` and so on (they are getters). Objects of `Node` class are never created. But there are other classes that inherit from it (and so inherit the `Node` functionality). - -- [Document](https://dom.spec.whatwg.org/#interface-document), for historical reasons often inherited by `HTMLDocument` (though the latest spec doesn't dictate it) -- is a document as a whole. - - The `document` global object belongs exactly to this class. It serves as an entry point to the DOM. - -- [CharacterData](https://dom.spec.whatwg.org/#interface-characterdata) -- an "abstract" class, inherited by: - - [Text](https://dom.spec.whatwg.org/#interface-text) -- the class corresponding to a text inside elements, e.g. `Hello` in `

Hello

`. - - [Comment](https://dom.spec.whatwg.org/#interface-comment) -- the class for comments. They are not shown, but each comment becomes a member of DOM. - -- [Element](https://dom.spec.whatwg.org/#interface-element) -- is the base class for DOM elements. - - It provides element-level navigation like `nextElementSibling`, `children` and searching methods like `getElementsByTagName`, `querySelector`. - - A browser supports not only HTML, but also XML and SVG. So the `Element` class serves as a base for more specific classes: `SVGElement`, `XMLElement` (we don't need them here) and `HTMLElement`. - -- Finally, [HTMLElement](https://html.spec.whatwg.org/multipage/dom.html#htmlelement) is the basic class for all HTML elements. We'll work with it most of the time. - - It is inherited by concrete HTML elements: +- [EventTarget](https://dom.spec.whatwg.org/#eventtarget) -- is the root "abstract" class. Objects of that class are never created. It serves as a base, so that all DOM nodes support so-called "events", we'll study them later. +- [Node](http://dom.spec.whatwg.org/#interface-node) -- is also an "abstract" class, serving as a base for DOM nodes. It provides the core tree functionality: `parentNode`, `nextSibling`, `childNodes` and so on (they are getters). Objects of `Node` class are never created. But there are concrete node classes that inherit from it, namely: `Text` for text nodes, `Element` for element nodes and more exotic ones like `Comment` for comment nodes. +- [Element](http://dom.spec.whatwg.org/#interface-element) -- is a base class for DOM elements. It provides element-level navigation like `nextElementSibling`, `children` and searching methods like `getElementsByTagName`, `querySelector`. A browser supports not only HTML, but also XML and SVG. The `Element` class serves as a base for more specific classes: `SVGElement`, `XMLElement` and `HTMLElement`. +- [HTMLElement](https://html.spec.whatwg.org/multipage/dom.html#htmlelement) -- is finally the basic class for all HTML elements. It is inherited by concrete HTML elements: - [HTMLInputElement](https://html.spec.whatwg.org/multipage/forms.html#htmlinputelement) -- the class for `` elements, - [HTMLBodyElement](https://html.spec.whatwg.org/multipage/semantics.html#htmlbodyelement) -- the class for `` elements, - [HTMLAnchorElement](https://html.spec.whatwg.org/multipage/semantics.html#htmlanchorelement) -- the class for `` elements, - - ...and so on. + - ...and so on, each tag has its own class that may provide specific properties and methods. -There are many other tags with their own classes that may have specific properties and methods, while some elements, such as ``, `
`, `
` do not have any specific properties, so they are instances of `HTMLElement` class. - -So, the full set of properties and methods of a given node comes as the result of the chain of inheritance. +So, the full set of properties and methods of a given node comes as the result of the inheritance. For example, let's consider the DOM object for an `` element. It belongs to [HTMLInputElement](https://html.spec.whatwg.org/multipage/forms.html#htmlinputelement) class. @@ -151,13 +128,13 @@ For instance: ```html run - ``` -But there are exclusions, for instance `input.value` synchronizes only from attribute -> property, but not back: +But there are exclusions, for instance `input.value` synchronizes only from attribute -> to property, but not back: ```html run @@ -298,7 +298,7 @@ For instance, here for the order state the attribute `order-state` is used: ``` -Why would using an attribute be preferable to having classes like `.order-state-new`, `.order-state-pending`, `.order-state-canceled`? +Why would using an attribute be preferable to having classes like `.order-state-new`, `.order-state-pending`, `order-state-canceled`? Because an attribute is more convenient to manage. The state can be changed as easy as: diff --git a/2-ui/1-document/07-modifying-document/1-createtextnode-vs-innerhtml/task.md b/2-ui/1-document/07-modifying-document/1-createtextnode-vs-innerhtml/task.md index 40c75dff..e127bc0e 100644 --- a/2-ui/1-document/07-modifying-document/1-createtextnode-vs-innerhtml/task.md +++ b/2-ui/1-document/07-modifying-document/1-createtextnode-vs-innerhtml/task.md @@ -6,7 +6,7 @@ importance: 5 We have an empty DOM element `elem` and a string `text`. -Which of these 3 commands will do exactly the same? +Which of these 3 commands do exactly the same? 1. `elem.append(document.createTextNode(text))` 2. `elem.innerHTML = text` diff --git a/2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.md b/2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.md index 1414e90c..15238fcf 100644 --- a/2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.md +++ b/2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.md @@ -39,19 +39,15 @@ The clock-managing functions: ```js let timerId; -function clockStart() { // run the clock - if (!timerId) { // only set a new interval if the clock is not running - timerId = setInterval(update, 1000); - } +function clockStart() { // run the clock + timerId = setInterval(update, 1000); update(); // (*) } function clockStop() { clearInterval(timerId); - timerId = null; // (**) + timerId = null; } ``` Please note that the call to `update()` is not only scheduled in `clockStart()`, but immediately run in the line `(*)`. Otherwise the visitor would have to wait till the first execution of `setInterval`. And the clock would be empty till then. - -Also it is important to set a new interval in `clockStart()` only when the clock is not running. Otherways clicking the start button several times would set multiple concurrent intervals. Even worse - we would only keep the `timerID` of the last interval, losing references to all others. Then we wouldn't be able to stop the clock ever again! Note that we need to clear the `timerID` when the clock is stopped in the line `(**)`, so that it can be started again by running `clockStart()`. diff --git a/2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.view/index.html b/2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.view/index.html index 84ee26f1..1bf642b1 100644 --- a/2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.view/index.html +++ b/2-ui/1-document/07-modifying-document/10-clock-setinterval/solution.view/index.html @@ -43,19 +43,15 @@ } function clockStart() { - // set a new interval only if the clock is stopped - // otherwise we would rewrite the timerID reference to the running interval and wouldn't be able to stop the clock ever again - if (!timerId) { - timerId = setInterval(update, 1000); - } + timerId = setInterval(update, 1000); update(); // <-- start right now, don't wait 1 second till the first setInterval works } function clockStop() { clearInterval(timerId); - timerId = null; // <-- clear timerID to indicate that the clock has been stopped, so that it is possible to start it again in clockStart() } + clockStart(); diff --git a/2-ui/1-document/07-modifying-document/5-why-aaa/solution.md b/2-ui/1-document/07-modifying-document/5-why-aaa/solution.md index 3d1f6698..6b85168b 100644 --- a/2-ui/1-document/07-modifying-document/5-why-aaa/solution.md +++ b/2-ui/1-document/07-modifying-document/5-why-aaa/solution.md @@ -1,9 +1,9 @@ The HTML in the task is incorrect. That's the reason of the odd thing. -The browser has to fix it automatically. But there may be no text inside the ``: according to the spec only table-specific tags are allowed. So the browser shows `"aaa"` *before* the `
`. +The browser has to fix it automatically. But there may be no text inside the `
`: according to the spec only table-specific tags are allowed. So the browser adds `"aaa"` *before* the `
`. Now it's obvious that when we remove the table, it remains. -The question can be easily answered by exploring the DOM using the browser tools. You'll see `"aaa"` before the `
`. +The question can be easily answered by exploring the DOM using the browser tools. It shows `"aaa"` before the `
`. The HTML standard specifies in detail how to process bad HTML, and such behavior of the browser is correct. diff --git a/2-ui/1-document/07-modifying-document/5-why-aaa/task.md b/2-ui/1-document/07-modifying-document/5-why-aaa/task.md index 861f7050..f87074db 100644 --- a/2-ui/1-document/07-modifying-document/5-why-aaa/task.md +++ b/2-ui/1-document/07-modifying-document/5-why-aaa/task.md @@ -22,6 +22,6 @@ Why does that happen? alert(table); // the table, as it should be table.remove(); - // why there's still "aaa" in the document? + // why there's still aaa in the document? ``` diff --git a/2-ui/1-document/07-modifying-document/6-create-list/task.md b/2-ui/1-document/07-modifying-document/6-create-list/task.md index a57e7e2d..43b0a34a 100644 --- a/2-ui/1-document/07-modifying-document/6-create-list/task.md +++ b/2-ui/1-document/07-modifying-document/6-create-list/task.md @@ -10,7 +10,7 @@ For every list item: 1. Ask a user about its content using `prompt`. 2. Create the `
  • ` with it and add it to `
      `. -3. Continue until the user cancels the input (by pressing `key:Esc` or via an empty entry). +3. Continue until the user cancels the input (by pressing `key:Esc` or CANCEL in prompt). All elements should be created dynamically. diff --git a/2-ui/1-document/07-modifying-document/before-prepend-append-after.svg b/2-ui/1-document/07-modifying-document/before-prepend-append-after.svg index 0843713c..6e1fb487 100644 --- a/2-ui/1-document/07-modifying-document/before-prepend-append-after.svg +++ b/2-ui/1-document/07-modifying-document/before-prepend-append-after.svg @@ -1 +1 @@ -ol.afterol.appendol.prependol.before(…nodes or strings) \ No newline at end of file +ol.afterol.appendol.prependol.before(…nodes or strings) \ No newline at end of file diff --git a/2-ui/1-document/07-modifying-document/insert-adjacent.svg b/2-ui/1-document/07-modifying-document/insert-adjacent.svg index e26fd023..64beee03 100644 --- a/2-ui/1-document/07-modifying-document/insert-adjacent.svg +++ b/2-ui/1-document/07-modifying-document/insert-adjacent.svg @@ -1 +1 @@ -ol.insertAdjacentHTML(*, html)afterendbeforeendafterbeginbeforebegin \ No newline at end of file +ol.insertAdjacentHTML(*, html)afterendbeforeendafterbeginbeforebegin \ No newline at end of file diff --git a/2-ui/1-document/08-styles-and-classes/article.md b/2-ui/1-document/08-styles-and-classes/article.md index 46aaa3b0..9154d43d 100644 --- a/2-ui/1-document/08-styles-and-classes/article.md +++ b/2-ui/1-document/08-styles-and-classes/article.md @@ -128,14 +128,6 @@ setTimeout(() => document.body.style.display = "", 1000); // back to normal If we set `style.display` to an empty string, then the browser applies CSS classes and its built-in styles normally, as if there were no such `style.display` property at all. -Also there is a special method for that, `elem.style.removeProperty('style property')`. So, We can remove a property like this: - -```js run -document.body.style.background = 'red'; //set background to red - -setTimeout(() => document.body.style.removeProperty('background'), 1000); // remove background after 1 second -``` - ````smart header="Full rewrite with `style.cssText`" Normally, we use `style.*` to assign individual style properties. We can't set the full style like `div.style="color: red; width: 100px"`, because `div.style` is an object, and it's read-only. @@ -269,6 +261,20 @@ So nowadays `getComputedStyle` actually returns the resolved value of the proper We should always ask for the exact property that we want, like `paddingLeft` or `marginTop` or `borderTopWidth`. Otherwise the correct result is not guaranteed. For instance, if there are properties `paddingLeft/paddingTop`, then what should we get for `getComputedStyle(elem).padding`? Nothing, or maybe a "generated" value from known paddings? There's no standard rule here. + +There are other inconsistencies. As an example, some browsers (Chrome) show `10px` in the document below, and some of them (Firefox) -- do not: + +```html run + + +``` ```` ```smart header="Styles applied to `:visited` links are hidden!" diff --git a/2-ui/1-document/09-size-and-scroll/4-put-ball-in-center/field.svg b/2-ui/1-document/09-size-and-scroll/4-put-ball-in-center/field.svg index f5bd9f4f..ca8bbc3b 100644 --- a/2-ui/1-document/09-size-and-scroll/4-put-ball-in-center/field.svg +++ b/2-ui/1-document/09-size-and-scroll/4-put-ball-in-center/field.svg @@ -1 +1 @@ -(0,0)clientWidth \ No newline at end of file +(0,0)clientWidth \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/article.md b/2-ui/1-document/09-size-and-scroll/article.md index 66f28115..13e245eb 100644 --- a/2-ui/1-document/09-size-and-scroll/article.md +++ b/2-ui/1-document/09-size-and-scroll/article.md @@ -17,8 +17,8 @@ As a sample element to demonstrate properties we'll use the one given below: width: 300px; height: 200px; border: 25px solid #E8C48F; - padding: 20px; - overflow: auto; + padding: 20px; + overflow: auto; } ``` @@ -106,7 +106,7 @@ Geometry properties are calculated only for displayed elements. If an element (or any of its ancestors) has `display:none` or is not in the document, then all geometry properties are zero (or `null` for `offsetParent`). -For example, `offsetParent` is `null`, and `offsetWidth`, `offsetHeight` are `0` when we created an element, but haven't inserted it into the document yet, or it (or its ancestor) has `display:none`. +For example, `offsetParent` is `null`, and `offsetWidth`, `offsetHeight` are `0` when we created an element, but haven't inserted it into the document yet, or it (or it's ancestor) has `display:none`. We can use this to check if an element is hidden, like this: @@ -116,7 +116,7 @@ function isHidden(elem) { } ``` -Please note that such `isHidden` returns `true` for elements that are on-screen, but have zero sizes. +Please note that such `isHidden` returns `true` for elements that are on-screen, but have zero sizes (like an empty `
      `). ```` ## clientTop/Left diff --git a/2-ui/1-document/09-size-and-scroll/metric-all.svg b/2-ui/1-document/09-size-and-scroll/metric-all.svg index 20a59e18..3c77f09a 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-all.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-all.svg @@ -1 +1 @@ -Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/ IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.scrollHeightoffsetHeightscrollTopclientHeightoffsetTopclientLeftclientWidthclientTopoffsetLeftoffsetWidth \ No newline at end of file +Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/ IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.scrollHeightoffsetHeightscrollTopclientHeightoffsetTopclientLeftclientWidthclientTopoffsetLeftoffsetWidth \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-client-left-top-rtl.svg b/2-ui/1-document/09-size-and-scroll/metric-client-left-top-rtl.svg index e8dd3d60..12bc456a 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-client-left-top-rtl.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-client-left-top-rtl.svg @@ -1 +1 @@ -clientTop: 25px = borderclientLeft: 41px \ No newline at end of file +clientTop: 25px = borderclientLeft: 41px \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-client-left-top.svg b/2-ui/1-document/09-size-and-scroll/metric-client-left-top.svg index 8097afa7..5708047d 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-client-left-top.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-client-left-top.svg @@ -1 +1 @@ -Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.clientTop: 25px = borderclientLeft: 25px \ No newline at end of file +Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997.clientTop: 25px = borderclientLeft: 25px \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-client-width-height.svg b/2-ui/1-document/09-size-and-scroll/metric-client-width-height.svg index 2603b05f..fe4b3c69 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-client-width-height.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-client-width-height.svg @@ -1 +1 @@ -border 25pxpadding 20pxcontent width: 284pxborder 25pxpadding 20pxscrollbar 16pxclientWidth = 20+284+20 = 324pxclientHeight: 240pxheight: 200pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with \ No newline at end of file +border 25pxpadding 20pxcontent width: 284pxborder 25pxpadding 20pxscrollbar 16pxclientWidth = 20+284+20 = 324pxclientHeight: 240pxheight: 200pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-client-width-nopadding.svg b/2-ui/1-document/09-size-and-scroll/metric-client-width-nopadding.svg index 330d2a7c..62de5f57 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-client-width-nopadding.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-client-width-nopadding.svg @@ -1 +1 @@ -clientWidth: 284px = content widthCSS width: 300pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with padding: 0; width: 300px; \ No newline at end of file +clientWidth: 284px = content widthCSS width: 300pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with padding: 0; width: 300px; \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-css.svg b/2-ui/1-document/09-size-and-scroll/metric-css.svg index 1f2e5f78..e910a9c8 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-css.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-css.svg @@ -1 +1 @@ -padding: 20pxheight: 200pxpadding: 20pxborder 25pxpadding 20pxcontent width: 284pxborder 25pxpadding 20pxscrollbar 16pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with \ No newline at end of file +padding: 20pxheight: 200pxpadding: 20pxborder 25pxpadding 20pxcontent width: 284pxborder 25pxpadding 20pxscrollbar 16pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-offset-parent.svg b/2-ui/1-document/09-size-and-scroll/metric-offset-parent.svg index 2d108473..d72a2013 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-offset-parent.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-offset-parent.svg @@ -1 +1 @@ -offsetTop: 180pxoffsetLeft: 180pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoftposition: absolute; left: 180px; top: 180px;offsetParent <MAIN> <DIV> \ No newline at end of file +offsetTop: 180pxoffsetLeft: 180pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoftposition: absolute; left: 180px; top: 180px;offsetParent <MAIN> <DIV> \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-offset-width-height.svg b/2-ui/1-document/09-size-and-scroll/metric-offset-width-height.svg index 4d30d90c..76e20b1b 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-offset-width-height.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-offset-width-height.svg @@ -1 +1 @@ -border 25pxpadding 20pxcontent width: 284pxheight: 200pxborder 25pxpadding 20pxscrollbar 16pxoffsetWidth = 25+20+284+20+16+25 = 390pxoffsetHeight: 290pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with \ No newline at end of file +border 25pxpadding 20pxcontent width: 284pxheight: 200pxborder 25pxpadding 20pxscrollbar 16pxoffsetWidth = 25+20+284+20+16+25 = 390pxoffsetHeight: 290pxIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-scroll-top.svg b/2-ui/1-document/09-size-and-scroll/metric-scroll-top.svg index 7f72de42..3c00bed0 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-scroll-top.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-scroll-top.svg @@ -1 +1 @@ -Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/ IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.scrollTopscrollHeight: 723px \ No newline at end of file +Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/ IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.scrollTopscrollHeight: 723px \ No newline at end of file diff --git a/2-ui/1-document/09-size-and-scroll/metric-scroll-width-height.svg b/2-ui/1-document/09-size-and-scroll/metric-scroll-width-height.svg index 75a24e3b..29e25eb6 100644 --- a/2-ui/1-document/09-size-and-scroll/metric-scroll-width-height.svg +++ b/2-ui/1-document/09-size-and-scroll/metric-scroll-width-height.svg @@ -1 +1 @@ -Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.scrollHeight: 723pxscrollWidth = 324px \ No newline at end of file +Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.scrollHeight: 723pxscrollWidth = 324px \ No newline at end of file diff --git a/2-ui/1-document/10-size-and-scroll-window/article.md b/2-ui/1-document/10-size-and-scroll-window/article.md index 08a2f657..95a5cd48 100644 --- a/2-ui/1-document/10-size-and-scroll-window/article.md +++ b/2-ui/1-document/10-size-and-scroll-window/article.md @@ -73,12 +73,6 @@ alert('Current scroll from the left: ' + window.pageXOffset); These properties are read-only. -```smart header="Also available as `window` properties `scrollX` and `scrollY`" -For historical reasons, both properties exist, but they are the same: -- `window.pageXOffset` is an alias of `window.scrollX`. -- `window.pageYOffset` is an alias of `window.scrollY`. -``` - ## Scrolling: scrollTo, scrollBy, scrollIntoView [#window-scroll] ```warn diff --git a/2-ui/1-document/10-size-and-scroll-window/document-client-width-height.svg b/2-ui/1-document/10-size-and-scroll-window/document-client-width-height.svg index 18cd37a7..b0dff4d4 100644 --- a/2-ui/1-document/10-size-and-scroll-window/document-client-width-height.svg +++ b/2-ui/1-document/10-size-and-scroll-window/document-client-width-height.svg @@ -1 +1 @@ -documentElement.clientHeightdocumentElement.clientWidth \ No newline at end of file +documentElement.clientHeightdocumentElement.clientWidth \ No newline at end of file diff --git a/2-ui/1-document/11-coordinates/coordinates-negative.svg b/2-ui/1-document/11-coordinates/coordinates-negative.svg index 4f2e7868..3d338011 100644 --- a/2-ui/1-document/11-coordinates/coordinates-negative.svg +++ b/2-ui/1-document/11-coordinates/coordinates-negative.svg @@ -1 +1 @@ -bottom(x,y)(x,y)leftrightIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at top(width,height) \ No newline at end of file +bottom(x,y)(x,y)leftrightIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at top(width,height) \ No newline at end of file diff --git a/2-ui/1-document/11-coordinates/coordinates.svg b/2-ui/1-document/11-coordinates/coordinates.svg index 261ff669..5e0b6f18 100644 --- a/2-ui/1-document/11-coordinates/coordinates.svg +++ b/2-ui/1-document/11-coordinates/coordinates.svg @@ -1 +1 @@ -heightbottomxleftywidthrightIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at top \ No newline at end of file +heightbottomxleftywidthrightIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at top \ No newline at end of file diff --git a/2-ui/1-document/11-coordinates/document-and-window-coordinates-scrolled.svg b/2-ui/1-document/11-coordinates/document-and-window-coordinates-scrolled.svg index f03317f0..8b4270ff 100644 --- a/2-ui/1-document/11-coordinates/document-and-window-coordinates-scrolled.svg +++ b/2-ui/1-document/11-coordinates/document-and-window-coordinates-scrolled.svg @@ -1 +1 @@ -Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/ IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.😍pageYclientYpageXclientXIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/ IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.pageYclientYpageXclientX😍 \ No newline at end of file +Introduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/ IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.😍pageYclientYpageXclientXIntroduction This Ecma Standard is based on several originating technologies, the most well known being JavaScript (Netscape) and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in that company’s Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in all browsers from Microsoft starting with Internet Explorer 3.0. The development of this Standard started in November 1996. The first edition of this Ecma Standard was adopted by the Ecma General Assembly of June 1997. That Ecma Standard was submitted to ISO/ IEC JTC 1 for adoption under the fast-track procedure, and approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998 approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the first and the second edition are editorial in nature.pageYclientYpageXclientX😍 \ No newline at end of file diff --git a/2-ui/2-events/01-introduction-browser-events/04-move-ball-field/move-ball-coords.svg b/2-ui/2-events/01-introduction-browser-events/04-move-ball-field/move-ball-coords.svg index 2acc6b03..73dcee5f 100644 --- a/2-ui/2-events/01-introduction-browser-events/04-move-ball-field/move-ball-coords.svg +++ b/2-ui/2-events/01-introduction-browser-events/04-move-ball-field/move-ball-coords.svg @@ -1 +1 @@ -ball.style.left?fieldCoords.leftevent.clientX \ No newline at end of file +ball.style.left?fieldCoords.leftevent.clientX \ No newline at end of file diff --git a/2-ui/2-events/01-introduction-browser-events/07-carousel/carousel1.svg b/2-ui/2-events/01-introduction-browser-events/07-carousel/carousel1.svg index 5bb161f6..2c92df87 100644 --- a/2-ui/2-events/01-introduction-browser-events/07-carousel/carousel1.svg +++ b/2-ui/2-events/01-introduction-browser-events/07-carousel/carousel1.svg @@ -1 +1 @@ -div (container)130x130ul (width: 9999px) \ No newline at end of file +div (container)130x130ul (width: 9999px) \ No newline at end of file diff --git a/2-ui/2-events/01-introduction-browser-events/07-carousel/carousel2.svg b/2-ui/2-events/01-introduction-browser-events/07-carousel/carousel2.svg index 81aea5b8..3f53a769 100644 --- a/2-ui/2-events/01-introduction-browser-events/07-carousel/carousel2.svg +++ b/2-ui/2-events/01-introduction-browser-events/07-carousel/carousel2.svg @@ -1 +1 @@ -div (container)130x130ul (margin-left: -350px) \ No newline at end of file +div (container)130x130ul (margin-left: -350px) \ No newline at end of file diff --git a/2-ui/2-events/01-introduction-browser-events/07-carousel/solution.view/index.html b/2-ui/2-events/01-introduction-browser-events/07-carousel/solution.view/index.html index baf86766..2c607331 100644 --- a/2-ui/2-events/01-introduction-browser-events/07-carousel/solution.view/index.html +++ b/2-ui/2-events/01-introduction-browser-events/07-carousel/solution.view/index.html @@ -10,7 +10,7 @@
  • ` inside a table: +Here's the picture of a click on `` inside a table, taken from the specification: ![](eventflow.svg) That is: for a click on `` the event first goes through the ancestors chain down to the element (capturing phase), then it reaches the target and triggers there (target phase), and then it goes up (bubbling phase), calling handlers on its way. -Until now, we only talked about bubbling, because the capturing phase is rarely used. +**Before we only talked about bubbling, because the capturing phase is rarely used. Normally it is invisible to us.** -In fact, the capturing phase was invisible for us, because handlers added using `on`-property or using HTML attributes or using two-argument `addEventListener(event, handler)` don't know anything about capturing, they only run on the 2nd and 3rd phases. +Handlers added using `on`-property or using HTML attributes or using two-argument `addEventListener(event, handler)` don't know anything about capturing, they only run on the 2nd and 3rd phases. To catch an event on the capturing phase, we need to set the handler `capture` option to `true`: ```js elem.addEventListener(..., {capture: true}) - // or, just "true" is an alias to {capture: true} elem.addEventListener(..., true) ``` @@ -181,10 +180,9 @@ The code sets click handlers on *every* element in the document to see which one If you click on `

    `, then the sequence is: -1. `HTML` -> `BODY` -> `FORM` -> `DIV -> P` (capturing phase, the first listener): -2. `P` -> `DIV` -> `FORM` -> `BODY` -> `HTML` (bubbling phase, the second listener). - -Please note, the `P` shows up twice, because we've set two listeners: capturing and bubbling. The target triggers at the end of the first and at the beginning of the second phase. +1. `HTML` -> `BODY` -> `FORM` -> `DIV` (capturing phase, the first listener): +2. `P` (target phase, triggers two times, as we've set two listeners: capturing and bubbling) +3. `DIV` -> `FORM` -> `BODY` -> `HTML` (bubbling phase, the second listener). There's a property `event.eventPhase` that tells us the number of the phase on which the event was caught. But it's rarely used, because we usually know it in the handler. @@ -192,7 +190,7 @@ There's a property `event.eventPhase` that tells us the number of the phase on w If we `addEventListener(..., true)`, then we should mention the same phase in `removeEventListener(..., true)` to correctly remove the handler. ``` -````smart header="Listeners on the same element and same phase run in their set order" +````smart header="Listeners on same element and same phase run in their set order" If we have multiple event handlers on the same phase, assigned to the same element with `addEventListener`, they run in the same order as they are created: ```js @@ -201,12 +199,6 @@ elem.addEventListener("click", e => alert(2)); ``` ```` -```smart header="The `event.stopPropagation()` during the capturing also prevents the bubbling" -The `event.stopPropagation()` method and its sibling `event.stopImmediatePropagation()` can also be called on the capturing phase. Then not only the futher capturing is stopped, but the bubbling as well. - -In other words, normally the event goes first down ("capturing") and then up ("bubbling"). But if `event.stopPropagation()` is called during the capturing phase, then the event travel stops, no bubbling will occur. -``` - ## Summary @@ -214,7 +206,7 @@ When an event happens -- the most nested element where it happens gets labeled a - Then the event moves down from the document root to `event.target`, calling handlers assigned with `addEventListener(..., true)` on the way (`true` is a shorthand for `{capture: true}`). - Then handlers are called on the target element itself. -- Then the event bubbles up from `event.target` to the root, calling handlers assigned using `on`, HTML attributes and `addEventListener` without the 3rd argument or with the 3rd argument `false/{capture:false}`. +- Then the event bubbles up from `event.target` to the root, calling handlers assigned using `on` and `addEventListener` without the 3rd argument or with the 3rd argument `false/{capture:false}`. Each handler can access `event` object properties: @@ -224,7 +216,7 @@ Each handler can access `event` object properties: Any event handler can stop the event by calling `event.stopPropagation()`, but that's not recommended, because we can't really be sure we won't need it above, maybe for completely different things. -The capturing phase is used very rarely, usually we handle events on bubbling. And there's a logical explanation for that. +The capturing phase is used very rarely, usually we handle events on bubbling. And there's a logic behind that. In real world, when an accident happens, local authorities react first. They know best the area where it happened. Then higher-level authorities if needed. diff --git a/2-ui/2-events/02-bubbling-and-capturing/event-order-bubbling.svg b/2-ui/2-events/02-bubbling-and-capturing/event-order-bubbling.svg index 2ea88f08..e3d74989 100644 --- a/2-ui/2-events/02-bubbling-and-capturing/event-order-bubbling.svg +++ b/2-ui/2-events/02-bubbling-and-capturing/event-order-bubbling.svg @@ -1 +1 @@ -123Most deeply nested element \ No newline at end of file +123Most deeply nested element \ No newline at end of file diff --git a/2-ui/2-events/02-bubbling-and-capturing/eventflow.svg b/2-ui/2-events/02-bubbling-and-capturing/eventflow.svg index 566064cd..e7db9bff 100644 --- a/2-ui/2-events/02-bubbling-and-capturing/eventflow.svg +++ b/2-ui/2-events/02-bubbling-and-capturing/eventflow.svg @@ -1 +1 @@ -WindowDocument<html><body><table><tbody><tr><tr><td>Shady Grove<td>Aeolian<td>Over the River,Charlie<td>DorianTargetPhase(2)CapturePhase(1)BubblingPhase(3) \ No newline at end of file +WindowDocument<html><body><table><tbody><tr><tr><td>Shady Grove<td>Aeolian<td>Over the River,Charlie<td>DorianTargetPhase(2)CapturePhase(1)BubblingPhase(3) \ No newline at end of file diff --git a/2-ui/2-events/03-event-delegation/article.md b/2-ui/2-events/03-event-delegation/article.md index 88118374..df086f24 100644 --- a/2-ui/2-events/03-event-delegation/article.md +++ b/2-ui/2-events/03-event-delegation/article.md @@ -1,11 +1,11 @@ # Event delegation -Capturing and bubbling allow us to implement one of the most powerful event handling patterns called *event delegation*. +Capturing and bubbling allow us to implement one of most powerful event handling patterns called *event delegation*. The idea is that if we have a lot of elements handled in a similar way, then instead of assigning a handler to each of them -- we put a single handler on their common ancestor. -In the handler we get `event.target` to see where the event actually happened and handle it. +In the handler we get `event.target`, see where the event actually happened and handle it. Let's see an example -- the [Ba-Gua diagram](http://en.wikipedia.org/wiki/Ba_gua) reflecting the ancient Chinese philosophy. diff --git a/2-ui/2-events/03-event-delegation/bagua-bubble.svg b/2-ui/2-events/03-event-delegation/bagua-bubble.svg index 4ae67102..bb9989b4 100644 --- a/2-ui/2-events/03-event-delegation/bagua-bubble.svg +++ b/2-ui/2-events/03-event-delegation/bagua-bubble.svg @@ -1 +1 @@ -<table><td><strong>event.target \ No newline at end of file +<table><td><strong>event.target \ No newline at end of file diff --git a/2-ui/2-events/04-default-browser-action/3-image-gallery/solution.view/gallery.css b/2-ui/2-events/04-default-browser-action/3-image-gallery/solution.view/gallery.css index 8d6472ee..4522006a 100644 --- a/2-ui/2-events/04-default-browser-action/3-image-gallery/solution.view/gallery.css +++ b/2-ui/2-events/04-default-browser-action/3-image-gallery/solution.view/gallery.css @@ -4,6 +4,16 @@ body { font: 75%/120% sans-serif; } +h2 { + font: bold 190%/100% sans-serif; + margin: 0 0 .2em; +} + +h2 em { + font: normal 80%/100% sans-serif; + color: #999999; +} + #largeImg { border: solid 1px #ccc; width: 550px; diff --git a/2-ui/2-events/04-default-browser-action/3-image-gallery/source.view/gallery.css b/2-ui/2-events/04-default-browser-action/3-image-gallery/source.view/gallery.css index 8d6472ee..b6e52301 100644 --- a/2-ui/2-events/04-default-browser-action/3-image-gallery/source.view/gallery.css +++ b/2-ui/2-events/04-default-browser-action/3-image-gallery/source.view/gallery.css @@ -4,6 +4,16 @@ body { font: 75%/120% sans-serif; } +h2 { + font: bold 190%/100% sans-serif; + margin: 0 0 .2em; +} + +h2 em { + font: normal 80%/100% sans-serif; + color: #999999; +} + #largeImg { border: solid 1px #ccc; width: 550px; @@ -22,13 +32,4 @@ body { #thumbs a:hover { border-color: #FF9900; -} - -#thumbs li { - list-style: none; -} - -#thumbs { - margin: 0; - padding: 0; } \ No newline at end of file diff --git a/2-ui/2-events/04-default-browser-action/article.md b/2-ui/2-events/04-default-browser-action/article.md index cd815654..ceac160c 100644 --- a/2-ui/2-events/04-default-browser-action/article.md +++ b/2-ui/2-events/04-default-browser-action/article.md @@ -17,7 +17,7 @@ There are two ways to tell the browser we don't want it to act: - The main way is to use the `event` object. There's a method `event.preventDefault()`. - If the handler is assigned using `on` (not by `addEventListener`), then returning `false` also works the same. -In this HTML, a click on a link doesn't lead to navigation; the browser doesn't do anything: +In this HTML a click on a link doesn't lead to navigation, browser doesn't do anything: ```html autorun height=60 no-beautify Click here @@ -96,7 +96,7 @@ That's because the browser action is canceled on `mousedown`. The focusing is st The optional `passive: true` option of `addEventListener` signals the browser that the handler is not going to call `preventDefault()`. -Why might that be needed? +Why that may be needed? There are some events like `touchmove` on mobile devices (when the user moves their finger across the screen), that cause scrolling by default, but that scrolling can be prevented using `preventDefault()` in the handler. diff --git a/2-ui/2-events/05-dispatch-events/article.md b/2-ui/2-events/05-dispatch-events/article.md index 77a49490..fa6a0308 100644 --- a/2-ui/2-events/05-dispatch-events/article.md +++ b/2-ui/2-events/05-dispatch-events/article.md @@ -8,7 +8,7 @@ We can generate not only completely new events, that we invent for our own purpo ## Event constructor -Built-in event classes form a hierarchy, similar to DOM element classes. The root is the built-in [Event](http://www.w3.org/TR/dom/#event) class. +Build-in event classes form a hierarchy, similar to DOM element classes. The root is the built-in [Event](http://www.w3.org/TR/dom/#event) class. We can create `Event` objects like this: @@ -210,7 +210,7 @@ Please note: the event must have the flag `cancelable: true`, otherwise the call ## Events-in-events are synchronous -Usually events are processed in a queue. That is: if the browser is processing `onclick` and a new event occurs, e.g. mouse moved, then its handling is queued up, corresponding `mousemove` handlers will be called after `onclick` processing is finished. +Usually events are processed in a queue. That is: if the browser is processing `onclick` and a new event occurs, e.g. mouse moved, then it's handling is queued up, corresponding `mousemove` handlers will be called after `onclick` processing is finished. The notable exception is when one event is initiated from within another one, e.g. using `dispatchEvent`. Such events are processed immediately: the new event handlers are called, and then the current event handling is resumed. @@ -266,7 +266,7 @@ Then we can either put the `dispatchEvent` (or another event-triggering call) at ``` -Now `dispatchEvent` runs asynchronously after the current code execution is finished, including `menu.onclick`, so event handlers are totally separate. +Now `dispatchEvent` runs asynchronously after the current code execution is finished, including `mouse.onclick`, so event handlers are totally separate. The output order becomes: 1 -> 2 -> nested. diff --git a/2-ui/3-event-details/1-mouse-events-basics/article.md b/2-ui/3-event-details/1-mouse-events-basics/article.md index 9574b0c8..651c3273 100644 --- a/2-ui/3-event-details/1-mouse-events-basics/article.md +++ b/2-ui/3-event-details/1-mouse-events-basics/article.md @@ -1,4 +1,3 @@ - # Mouse events In this chapter we'll get into more details about mouse events and their properties. @@ -40,9 +39,9 @@ In cases when a single action initiates multiple events, their order is fixed. T ```online Click the button below and you'll see the events. Try double-click too. -On the teststand below, all mouse events are logged, and if there is more than a 1 second delay between them, they are separated by a horizontal rule. +On the teststand below all mouse events are logged, and if there is more than a 1 second delay between them they are separated by a horizontal ruler. -Also, we can see the `button` property that allows us to detect the mouse button; it's explained below. +Also we can see the `button` property that allows to detect the mouse button, it's explained below.

    ``` @@ -53,7 +52,7 @@ Click-related events always have the `button` property, which allows to get the We usually don't use it for `click` and `contextmenu` events, because the former happens only on left-click, and the latter -- only on right-click. -On the other hand, `mousedown` and `mouseup` handlers may need `event.button`, because these events trigger on any button, so `button` allows to distinguish between "right-mousedown" and "left-mousedown". +From the other hand, `mousedown` and `mouseup` handlers we may need `event.button`, because these events trigger on any button, so `button` allows to distinguish between "right-mousedown" and "left-mousedown". The possible values of `event.button` are: @@ -155,7 +154,7 @@ Move the mouse over the input field to see `clientX/clientY` (the example is in ## Preventing selection on mousedown -Double mouse click has a side effect that may be disturbing in some interfaces: it selects text. +Double mouse click has a side-effect that may be disturbing in some interfaces: it selects text. For instance, double-clicking on the text below selects it in addition to our handler: diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/1-behavior-nested-tooltip/solution.view/index.html b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/1-behavior-nested-tooltip/solution.view/index.html index 84d52b18..e998165f 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/1-behavior-nested-tooltip/solution.view/index.html +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/1-behavior-nested-tooltip/solution.view/index.html @@ -54,7 +54,7 @@

    Once upon a time there was a mother pig who had three little pigs.

    -

    The three little pigs grew so big that their mother said to them, "You are too big to live here any longer. You must go and build houses for yourselves. But take care that the wolf does not catch you."

    +

    The three little pigs grew so big that their mother said to them, "You are too big to live here any longer. You must go and build houses for yourselves. But take care that the wolf does not catch you."

    The three little pigs set off. "We will take care that the wolf does not catch us," they said.

    diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/1-behavior-nested-tooltip/source.view/index.html b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/1-behavior-nested-tooltip/source.view/index.html index 774e24a2..2dc4394e 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/1-behavior-nested-tooltip/source.view/index.html +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/1-behavior-nested-tooltip/source.view/index.html @@ -54,7 +54,7 @@

    Once upon a time there was a mother pig who had three little pigs.

    -

    The three little pigs grew so big that their mother said to them, "You are too big to live here any longer. You must go and build houses for yourselves. But take care that the wolf does not catch you."

    +

    The three little pigs grew so big that their mother said to them, "You are too big to live here any longer. You must go and build houses for yourselves. But take care that the wolf does not catch you."

    The three little pigs set off. "We will take care that the wolf does not catch us," they said.

    diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/2-hoverintent/solution.view/hoverIntent.js b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/2-hoverintent/solution.view/hoverIntent.js index 7503ca9c..4e6e2a3e 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/2-hoverintent/solution.view/hoverIntent.js +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/2-hoverintent/solution.view/hoverIntent.js @@ -88,7 +88,7 @@ class HoverIntent { if (speed < this.sensitivity) { clearInterval(this.checkSpeedInterval); this.isHover = true; - this.over.call(this.elem); + this.over.call(this.elem, event); } else { // speed fast, remember new coordinates as the previous ones this.prevX = this.lastX; diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/article.md b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/article.md index d409c3f1..c7ac0d4d 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/article.md +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/article.md @@ -80,7 +80,7 @@ An important feature of `mouseout` -- it triggers, when the pointer moves from a ``` -If we're on `#parent` and then move the pointer deeper into `#child`, we get `mouseout` on `#parent`! +If we're on `#parent` and then move the pointer deeper into `#child`, but we get `mouseout` on `#parent`! ![](mouseover-to-child.svg) diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-bubble-nested.svg b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-bubble-nested.svg index 6044eff1..07830295 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-bubble-nested.svg +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-bubble-nested.svg @@ -1 +1 @@ -mouseoutmouseover#parent#child \ No newline at end of file +mouseoutmouseover#parent#child \ No newline at end of file diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout-from-outside.svg b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout-from-outside.svg index 22335b52..07176ba2 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout-from-outside.svg +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout-from-outside.svg @@ -1 +1 @@ -#TOtargetrelatedTarget = null \ No newline at end of file +#TOtargetrelatedTarget = null \ No newline at end of file diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout-over-elems.svg b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout-over-elems.svg index 437f03b1..262ddf59 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout-over-elems.svg +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout-over-elems.svg @@ -1 +1 @@ -#TO#FROM<DIV><DIV><DIV>mouseovermouseout \ No newline at end of file +#TO#FROM<DIV><DIV><DIV>mouseovermouseout \ No newline at end of file diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout.svg b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout.svg index 1277ddff..784f435d 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout.svg +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-mouseout.svg @@ -1 +1 @@ -<DIV>mouseovermouseout \ No newline at end of file +<DIV>mouseovermouseout \ No newline at end of file diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-to-child.svg b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-to-child.svg index 78210845..b38d76fb 100644 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-to-child.svg +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseover-to-child.svg @@ -1 +1 @@ -mouseoutmouseover#parent#child \ No newline at end of file +mouseoutmouseover#parent#child \ No newline at end of file diff --git a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseoverout-fast.view/script.js b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseoverout-fast.view/script.js index 5752e83a..6d87199c 100755 --- a/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseoverout-fast.view/script.js +++ b/2-ui/3-event-details/3-mousemove-mouseover-mouseout-mouseenter-mouseleave/mouseoverout-fast.view/script.js @@ -3,7 +3,7 @@ parent.onmouseover = parent.onmouseout = parent.onmousemove = handler; function handler(event) { let type = event.type; - while (type.length < 11) type += ' '; + while (type < 11) type += ' '; log(type + " target=" + event.target.id) return false; diff --git a/2-ui/3-event-details/4-mouse-drag-and-drop/2-drag-heroes/solution.view/field.svg b/2-ui/3-event-details/4-mouse-drag-and-drop/2-drag-heroes/solution.view/field.svg index f5bd9f4f..ca8bbc3b 100644 --- a/2-ui/3-event-details/4-mouse-drag-and-drop/2-drag-heroes/solution.view/field.svg +++ b/2-ui/3-event-details/4-mouse-drag-and-drop/2-drag-heroes/solution.view/field.svg @@ -1 +1 @@ -(0,0)clientWidth \ No newline at end of file +(0,0)clientWidth \ No newline at end of file diff --git a/2-ui/3-event-details/4-mouse-drag-and-drop/article.md b/2-ui/3-event-details/4-mouse-drag-and-drop/article.md index 4c928eef..6cb1152c 100644 --- a/2-ui/3-event-details/4-mouse-drag-and-drop/article.md +++ b/2-ui/3-event-details/4-mouse-drag-and-drop/article.md @@ -18,19 +18,19 @@ The basic Drag'n'Drop algorithm looks like this: 2. Then on `mousemove` move it by changing `left/top` with `position:absolute`. 3. On `mouseup` - perform all actions related to finishing the drag'n'drop. -These are the basics. Later we'll see how to add other features, such as highlighting current underlying elements while we drag over them. +These are the basics. Later we'll see how to other features, such as highlighting current underlying elements while we drag over them. Here's the implementation of dragging a ball: ```js -ball.onmousedown = function(event) { +ball.onmousedown = function(event) { // (1) prepare to moving: make absolute and on top by z-index ball.style.position = 'absolute'; ball.style.zIndex = 1000; // move it out of any current parents directly into body // to make it positioned relative to the body - document.body.append(ball); + document.body.append(ball); // centers the ball at (pageX, pageY) coordinates function moveAt(pageX, pageY) { @@ -93,14 +93,14 @@ So we should listen on `document` to catch it. ## Correct positioning -In the examples above the ball is always moved so that its center is under the pointer: +In the examples above the ball is always moved so, that it's center is under the pointer: ```js ball.style.left = pageX - ball.offsetWidth / 2 + 'px'; ball.style.top = pageY - ball.offsetHeight / 2 + 'px'; ``` -Not bad, but there's a side effect. To initiate the drag'n'drop, we can `mousedown` anywhere on the ball. But if "take" it from its edge, then the ball suddenly "jumps" to become centered under the mouse pointer. +Not bad, but there's a side-effect. To initiate the drag'n'drop, we can `mousedown` anywhere on the ball. But if "take" it from its edge, then the ball suddenly "jumps" to become centered under the mouse pointer. It would be better if we keep the initial shift of the element relative to the pointer. @@ -124,7 +124,7 @@ Let's update our algorithm: ```js // onmousemove - // ball has position:absolute + // ball has position:absoute ball.style.left = event.pageX - *!*shiftX*/!* + 'px'; ball.style.top = event.pageY - *!*shiftY*/!* + 'px'; ``` @@ -219,7 +219,7 @@ That's why the initial idea to put handlers on potential droppables doesn't work So, what to do? -There's a method called `document.elementFromPoint(clientX, clientY)`. It returns the most nested element on given window-relative coordinates (or `null` if given coordinates are out of the window). If there are multiple overlapping elements on the same coordinates, then the topmost one is returned. +There's a method called `document.elementFromPoint(clientX, clientY)`. It returns the most nested element on given window-relative coordinates (or `null` if given coordinates are out of the window). We can use it in any of our mouse event handlers to detect the potential droppable under the pointer, like this: diff --git a/2-ui/3-event-details/4-mouse-drag-and-drop/ball_shift.svg b/2-ui/3-event-details/4-mouse-drag-and-drop/ball_shift.svg index 29fdb31e..3aa2541d 100644 --- a/2-ui/3-event-details/4-mouse-drag-and-drop/ball_shift.svg +++ b/2-ui/3-event-details/4-mouse-drag-and-drop/ball_shift.svg @@ -1 +1 @@ -shiftXshiftY \ No newline at end of file +shiftXshiftY \ No newline at end of file diff --git a/2-ui/3-event-details/6-pointer-events/article.md b/2-ui/3-event-details/6-pointer-events/article.md index b8873e9d..3e751a4a 100644 --- a/2-ui/3-event-details/6-pointer-events/article.md +++ b/2-ui/3-event-details/6-pointer-events/article.md @@ -9,16 +9,16 @@ Let's make a small overview, so that you understand the general picture and the - Long ago, in the past, there were only mouse events. Then touch devices became widespread, phones and tablets in particular. For the existing scripts to work, they generated (and still generate) mouse events. For instance, tapping a touchscreen generates `mousedown`. So touch devices worked well with web pages. - + But touch devices have more capabilities than a mouse. For example, it's possible to touch multiple points at once ("multi-touch"). Although, mouse events don't have necessary properties to handle such multi-touches. - So touch events were introduced, such as `touchstart`, `touchend`, `touchmove`, that have touch-specific properties (we don't cover them in detail here, because pointer events are even better). - Still, it wasn't enough, as there are many other devices, such as pens, that have their own features. Also, writing code that listens for both touch and mouse events was cumbersome. + Still, it wasn't enough, as there are many other devices, such as pens, that have their own features. Also, writing code that listens for both touch and mouse events was cumbersome. - To solve these issues, the new standard Pointer Events was introduced. It provides a single set of events for all kinds of pointing devices. -As of now, [Pointer Events Level 2](https://www.w3.org/TR/pointerevents2/) specification is supported in all major browsers, while the newer [Pointer Events Level 3](https://w3c.github.io/pointerevents/) is in the works and is mostly compatible with Pointer Events level 2. +As of now, [Pointer Events Level 2](https://www.w3.org/TR/pointerevents2/) specification is supported in all major browsers, while the newer [Pointer Events Level 3](https://w3c.github.io/pointerevents/) is in the works and is mostly compartible with Pointer Events level 2. Unless you develop for old browsers, such as Internet Explorer 10, or for Safari 12 or below, there's no point in using mouse or touch events any more -- we can switch to pointer events. @@ -43,12 +43,12 @@ Pointer events are named similarly to mouse events: | `gotpointercapture` | - | | `lostpointercapture` | - | -As we can see, for every `mouse`, there's a `pointer` that plays a similar role. Also there are 3 additional pointer events that don't have a corresponding `mouse...` counterpart, we'll explain them soon. +As we can see, for every `mouse`, there's a `pointer` that plays a similar role. Also there are 3 additional pointer events that don't have a corresponding `mouse...` counterpart, we'll explain them soon. ```smart header="Replacing `mouse` with `pointer` in our code" We can replace `mouse` events with `pointer` in our code and expect things to continue working fine with mouse. -The support for touch devices will also "magically" improve. Although, we may need to add `touch-action: none` in some places in CSS. We'll cover it below in the section about `pointercancel`. +The support for touch devices will also "magically" improve. Although, we may need to add `touch-action: none` in some places in CSS. We'll cover it below in the section about `pointercancel`. ``` ## Pointer event properties @@ -56,16 +56,16 @@ The support for touch devices will also "magically" improve. Although, we may ne Pointer events have the same properties as mouse events, such as `clientX/Y`, `target`, etc., plus some others: - `pointerId` - the unique identifier of the pointer causing the event. - + Browser-generated. Allows us to handle multiple pointers, such as a touchscreen with stylus and multi-touch (examples will follow). -- `pointerType` - the pointing device type. Must be a string, one of: "mouse", "pen" or "touch". +- `pointerType` - the pointing device type. Must be a string, one of: "mouse", "pen" or "touch". We can use this property to react differently on various pointer types. - `isPrimary` - is `true` for the primary pointer (the first finger in multi-touch). Some pointer devices measure contact area and pressure, e.g. for a finger on the touchscreen, there are additional properties for that: -- `width` - the width of the area where the pointer (e.g. a finger) touches the device. Where unsupported, e.g. for a mouse, it's always `1`. +- `width` - the width of the area where the pointer (e.g. a finger) touches the device. Where unsupported, e.g. for a mouse, it's always `1`. - `height` - the height of the area where the pointer touches the device. Where unsupported, it's always `1`. - `pressure` - the pressure of the pointer tip, in range from 0 to 1. For devices that don't support pressure must be either `0.5` (pressed) or `0`. - `tangentialPressure` - the normalized tangential pressure. @@ -102,16 +102,16 @@ Please note: you must be using a touchscreen device, such as a phone or a tablet ## Event: pointercancel -The `pointercancel` event fires when there's an ongoing pointer interaction, and then something happens that causes it to be aborted, so that no more pointer events are generated. +The `pointercancel` event fires when there's an ongoing pointer interaction, and then something happens that causes it to be aborted, so that no more pointer events are generated. -Such causes are: +Such causes are: - The pointer device hardware was physically disabled. -- The device orientation changed (tablet rotated). +- The device orientation changed (tablet rotated). - The browser decided to handle the interaction on its own, considering it a mouse gesture or zoom-and-pan action or something else. We'll demonstrate `pointercancel` on a practical example to see how it affects us. -Let's say we're implementing drag'n'drop for a ball, just as in the beginning of the article . +Let's say we're impelementing drag'n'drop for a ball, just as in the beginning of the article . Here is the flow of user actions and the corresponding events: @@ -126,7 +126,7 @@ Here is the flow of user actions and the corresponding events: So the issue is that the browser "hijacks" the interaction: `pointercancel` fires in the beginning of the "drag-and-drop" process, and no more `pointermove` events are generated. ```online -Here's the drag'n'drop demo with loggin of pointer events (only `up/down`, `move` and `cancel`) in the `textarea`: +Here's the drag'n'drop demo with loggin of pointer events (only `up/down`, `move` and `cancel`) in the `textarea`: [iframe src="ball" height=240 edit] ``` @@ -141,7 +141,7 @@ We need to do two things: - We can do this by setting `ball.ondragstart = () => false`, just as described in the article . - That works well for mouse events. 2. For touch devices, there are other touch-related browser actions (besides drag'n'drop). To avoid problems with them too: - - Prevent them by setting `#ball { touch-action: none }` in CSS. + - Prevent them by setting `#ball { touch-action: none }` in CSS. - Then our code will start working on touch devices. After we do that, the events will work as intended, the browser won't hijack the process and doesn't emit `pointercancel`. @@ -163,7 +163,7 @@ Pointer capturing is a special feature of pointer events. The idea is very simple, but may seem quite odd at first, as nothing like that exists for any other event type. The main method is: -- `elem.setPointerCapture(pointerId)` -- binds events with the given `pointerId` to `elem`. After the call all pointer events with the same `pointerId` will have `elem` as the target (as if happened on `elem`), no matter where in document they really happened. +- `elem.setPointerCapture(pointerId)` - binds events with the given `pointerId` to `elem`. After the call all pointer events with the same `pointerId` will have `elem` as the target (as if happened on `elem`), no matter where in document they really happened. In other words, `elem.setPointerCapture(pointerId)` retargets all subsequent events with the given `pointerId` to `elem`. @@ -172,43 +172,29 @@ The binding is removed: - automatically when `elem` is removed from the document, - when `elem.releasePointerCapture(pointerId)` is called. -Now what is it good for? It's time to see a real-life example. - **Pointer capturing can be used to simplify drag'n'drop kind of interactions.** -Let's recall how one can implement a custom slider, described in the . +As an example, let's recall how one can implement a custom slider, described in the . -We can make a `slider` element to represent the strip and the "runner" (`thumb`) inside it: +We make a slider element with the strip and the "runner" (`thumb`) inside it. -```html -
    -
    -
    -``` +Then it works like this: -With styles, it looks like this: +1. The user presses on the slider `thumb` - `pointerdown` triggers. +2. Then they move the pointer - `pointermove` triggers, and we move the `thumb` along. + - ...As the pointer moves, it may leave the slider `thumb`: go above or below it. The `thumb` should move strictly horizontally, remaining aligned with the pointer. -[iframe src="slider-html" height=40 edit] +So, to track all pointer movements, including when it goes above/below the `thumb`, we had to assign `pointermove` event handler on the whole `document`. -

    +That solution looks a bit "dirty". One of the problems is that pointer movements around the document may cause side effects, trigger other event handlers, totally not related to the slider. -And here's the working logic, as it was described, after replacing mouse events with similar pointer events: - -1. The user presses on the slider `thumb` -- `pointerdown` triggers. -2. Then they move the pointer -- `pointermove` triggers, and our code moves the `thumb` element along. - - ...As the pointer moves, it may leave the slider `thumb` element, go above or below it. The `thumb` should move strictly horizontally, remaining aligned with the pointer. - -In the mouse event based solution, to track all pointer movements, including when it goes above/below the `thumb`, we had to assign `mousemove` event handler on the whole `document`. - -That's not a cleanest solution, though. One of the problems is that when a user moves the pointer around the document, it may trigger event handlers (such as `mouseover`) on some other elements, invoke totally unrelated UI functionality, and we don't want that. - -This is the place where `setPointerCapture` comes into play. +Pointer capturing provides a means to bind `pointermove` to `thumb` and avoid any such problems: - We can call `thumb.setPointerCapture(event.pointerId)` in `pointerdown` handler, -- Then future pointer events until `pointerup/cancel` will be retargeted to `thumb`. +- Then future pointer events until `pointerup/cancel` will be retargeted to `thumb`. - When `pointerup` happens (dragging complete), the binding is removed automatically, we don't need to care about it. -So, even if the user moves the pointer around the whole document, events handlers will be called on `thumb`. Nevertheless, coordinate properties of the event objects, such as `clientX/clientY` will still be correct - the capturing only affects `target/currentTarget`. +So, even if the user moves the pointer around the whole document, events handlers will be called on `thumb`. Besides, coordinate properties of the event objects, such as `clientX/clientY` will still be correct - the capturing only affects `target/currentTarget`. Here's the essential code: @@ -216,23 +202,15 @@ Here's the essential code: thumb.onpointerdown = function(event) { // retarget all pointer events (until pointerup) to thumb thumb.setPointerCapture(event.pointerId); - - // start tracking pointer moves - thumb.onpointermove = function(event) { - // moving the slider: listen on the thumb, as all pointer events are retargeted to it - let newLeft = event.clientX - slider.getBoundingClientRect().left; - thumb.style.left = newLeft + 'px'; - }; - - // on pointer up finish tracking pointer moves - thumb.onpointerup = function(event) { - thumb.onpointermove = null; - thumb.onpointerup = null; - // ...also process the "drag end" if needed - }; }; -// note: no need to call thumb.releasePointerCapture, +thumb.onpointermove = function(event) { + // moving the slider: listen on the thumb, as all pointer events are retargeted to it + let newLeft = event.clientX - slider.getBoundingClientRect().left; + thumb.style.left = newLeft + 'px'; +}; + +// note: no need to call thumb.releasePointerCapture, // it happens on pointerup automatically ``` @@ -240,27 +218,15 @@ thumb.onpointerdown = function(event) { The full demo: [iframe src="slider" height=100 edit] - -

    - -In the demo, there's also an additional element with `onmouseover` handler showing the current date. - -Please note: while you're dragging the thumb, you may hover over this element, and its handler *does not* trigger. - -So the dragging is now free of side effects, thanks to `setPointerCapture`. ``` - - At the end, pointer capturing gives us two benefits: 1. The code becomes cleaner as we don't need to add/remove handlers on the whole `document` any more. The binding is released automatically. -2. If there are other pointer event handlers in the document, they won't be accidentally triggered by the pointer while the user is dragging the slider. +2. If there are any `pointermove` handlers in the document, they won't be accidentally triggered by the pointer while the user is dragging the slider. ### Pointer capturing events -There's one more thing to mention here, for the sake of completeness. - -There are two events associated with pointer capturing: +There are two associated pointer events: - `gotpointercapture` fires when an element uses `setPointerCapture` to enable capturing. - `lostpointercapture` fires when the capture is released: either explicitly with `releasePointerCapture` call, or automatically on `pointerup`/`pointercancel`. @@ -271,7 +237,7 @@ Pointer events allow handling mouse, touch and pen events simultaneously, with a Pointer events extend mouse events. We can replace `mouse` with `pointer` in event names and expect our code to continue working for mouse, with better support for other device types. -For drag'n'drops and complex touch interactions that the browser may decide to hijack and handle on its own - remember to cancel the default action on events and set `touch-action: none` in CSS for elements that we engage. +For drag'n'drops and complex touch interactions that the browser may decide to hijack and handle on its own - remember to cancel the default action on events and set `touch-events: none` in CSS for elements that we engage. Additional abilities of pointer events are: diff --git a/2-ui/3-event-details/6-pointer-events/slider-html.view/index.html b/2-ui/3-event-details/6-pointer-events/slider-html.view/index.html deleted file mode 100644 index 781016f5..00000000 --- a/2-ui/3-event-details/6-pointer-events/slider-html.view/index.html +++ /dev/null @@ -1,6 +0,0 @@ - - - -
    -
    -
    diff --git a/2-ui/3-event-details/6-pointer-events/slider-html.view/style.css b/2-ui/3-event-details/6-pointer-events/slider-html.view/style.css deleted file mode 100644 index 9b3d3b82..00000000 --- a/2-ui/3-event-details/6-pointer-events/slider-html.view/style.css +++ /dev/null @@ -1,19 +0,0 @@ -.slider { - border-radius: 5px; - background: #E0E0E0; - background: linear-gradient(left top, #E0E0E0, #EEEEEE); - width: 310px; - height: 15px; - margin: 5px; -} - -.thumb { - width: 10px; - height: 25px; - border-radius: 3px; - position: relative; - left: 10px; - top: -5px; - background: blue; - cursor: pointer; -} diff --git a/2-ui/3-event-details/6-pointer-events/slider.view/index.html b/2-ui/3-event-details/6-pointer-events/slider.view/index.html index b29e646a..2c2a69ec 100644 --- a/2-ui/3-event-details/6-pointer-events/slider.view/index.html +++ b/2-ui/3-event-details/6-pointer-events/slider.view/index.html @@ -5,33 +5,22 @@
    -

    Mouse over here to see the date

    - diff --git a/2-ui/3-event-details/6-pointer-events/slider.view/style.css b/2-ui/3-event-details/6-pointer-events/slider.view/style.css index a84cd5e7..9b3d3b82 100644 --- a/2-ui/3-event-details/6-pointer-events/slider.view/style.css +++ b/2-ui/3-event-details/6-pointer-events/slider.view/style.css @@ -8,7 +8,6 @@ } .thumb { - touch-action: none; width: 10px; height: 25px; border-radius: 3px; diff --git a/2-ui/3-event-details/7-keyboard-events/article.md b/2-ui/3-event-details/7-keyboard-events/article.md index 12fe6320..617852cc 100644 --- a/2-ui/3-event-details/7-keyboard-events/article.md +++ b/2-ui/3-event-details/7-keyboard-events/article.md @@ -107,7 +107,7 @@ So, `event.code` may match a wrong character for unexpected layout. Same letters To reliably track layout-dependent characters, `event.key` may be a better way. -On the other hand, `event.code` has the benefit of staying always the same, bound to the physical key location. So hotkeys that rely on it work well even in case of a language switch. +On the other hand, `event.code` has the benefit of staying always the same, bound to the physical key location, even if the visitor changes languages. So hotkeys that rely on it work well even in case of a language switch. Do we want to handle layout-dependant keys? Then `event.key` is the way to go. @@ -139,25 +139,22 @@ For instance, the `` below expects a phone number, so it does not accept ```html autorun height=60 run ``` -The `onkeydown` handler here uses `checkPhoneKey` to check for the key pressed. If it's valid (from `0..9` or one of `+-()`), then it returns `true`, otherwise `false`. +Please note that special keys, such as `key:Backspace`, `key:Left`, `key:Right`, `key:Ctrl+V`, do not work in the input. That's a side-effect of the strict filter `checkPhoneKey`. -As we know, the `false` value returned from the event handler, assigned using a DOM property or an attribute, such as above, prevents the default action, so nothing appears in the `` for keys that don't pass the test. (The `true` value returned doesn't affect anything, only returning `false` matters) +Let's relax it a little bit: -Please note that special keys, such as `key:Backspace`, `key:Left`, `key:Right`, do not work in the input. That's a side effect of the strict filter `checkPhoneKey`. These keys make it return `false`. - -Let's relax the filter a little bit by allowing arrow keys `key:Left`, `key:Right` and `key:Delete`, `key:Backspace`: ```html autorun height=60 run @@ -165,9 +162,7 @@ function checkPhoneKey(key) { Now arrows and deletion works well. -Even though we have the key filter, one still can enter anything using a mouse and right-click + Paste. Mobile devices provide other means to enter values. So the filter is not 100% reliable. - -The alternative approach would be to track the `oninput` event -- it triggers *after* any modification. There we can check the new `input.value` and modify it/highlight the `` when it's invalid. Or we can use both event handlers together. +...But we still can enter anything by using a mouse and right-click + Paste. So the filter is not 100% reliable. We can just let it be like that, because most of time it works. Or an alternative approach would be to track the `input` event -- it triggers after any modification. There we can check the new value and highlight/modify it when it's invalid. ## Legacy @@ -175,12 +170,6 @@ In the past, there was a `keypress` event, and also `keyCode`, `charCode`, `whic There were so many browser incompatibilities while working with them, that developers of the specification had no way, other than deprecating all of them and creating new, modern events (described above in this chapter). The old code still works, as browsers keep supporting them, but there's totally no need to use those any more. -## Mobile Keyboards - -When using virtual/mobile keyboards, formally known as IME (Input-Method Editor), the W3C standard states that a KeyboardEvent's [`e.keyCode` should be `229`](https://www.w3.org/TR/uievents/#determine-keydown-keyup-keyCode) and [`e.key` should be `"Unidentified"`](https://www.w3.org/TR/uievents-key/#key-attr-values). - -While some of these keyboards might still use the right values for `e.key`, `e.code`, `e.keyCode`... when pressing certain keys such as arrows or backspace, there's no guarantee, so your keyboard logic might not always work on mobile devices. - ## Summary Pressing a key always generates a keyboard event, be it symbol keys or special keys like `key:Shift` or `key:Ctrl` and so on. The only exception is `key:Fn` key that sometimes presents on a laptop keyboard. There's no keyboard event for it, because it's often implemented on lower level than OS. diff --git a/2-ui/3-event-details/7-keyboard-events/german-layout.svg b/2-ui/3-event-details/7-keyboard-events/german-layout.svg index 7ac9a400..8a880e8e 100644 --- a/2-ui/3-event-details/7-keyboard-events/german-layout.svg +++ b/2-ui/3-event-details/7-keyboard-events/german-layout.svg @@ -1 +1 @@ -StrgStrgAl tAlt GrWinWinMenu \ No newline at end of file +StrgStrgAl tAlt GrWinWinMenu \ No newline at end of file diff --git a/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/index.html b/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/index.html index a0d5a4f4..40106283 100644 --- a/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/index.html +++ b/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/index.html @@ -28,7 +28,7 @@ - + diff --git a/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/script.js b/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/script.js index d97f7a7b..5eba24c7 100644 --- a/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/script.js +++ b/2-ui/3-event-details/7-keyboard-events/keyboard-dump.view/script.js @@ -5,8 +5,6 @@ let lastTime = Date.now(); function handle(e) { if (form.elements[e.type + 'Ignore'].checked) return; - area.scrollTop = 1e6; - let text = e.type + ' key=' + e.key + ' code=' + e.code + diff --git a/2-ui/3-event-details/7-keyboard-events/us-layout.svg b/2-ui/3-event-details/7-keyboard-events/us-layout.svg index 353f225f..699277e0 100644 --- a/2-ui/3-event-details/7-keyboard-events/us-layout.svg +++ b/2-ui/3-event-details/7-keyboard-events/us-layout.svg @@ -1 +1 @@ -Caps LockShiftShift \ No newline at end of file +Caps LockShiftShift \ No newline at end of file diff --git a/2-ui/4-forms-controls/1-form-elements/article.md b/2-ui/4-forms-controls/1-form-elements/article.md index f22518d9..01af1f40 100644 --- a/2-ui/4-forms-controls/1-form-elements/article.md +++ b/2-ui/4-forms-controls/1-form-elements/article.md @@ -8,11 +8,11 @@ Working with forms will be much more convenient when we learn them. Document forms are members of the special collection `document.forms`. -That's a so-called *"named collection"*: it's both named and ordered. We can use both the name or the number in the document to get the form. +That's a so-called "named collection": it's both named and ordered. We can use both the name or the number in the document to get the form. ```js no-beautify -document.forms.my; // the form with name="my" -document.forms[0]; // the first form in the document +document.forms.my - the form with name="my" +document.forms[0] - the first form in the document ``` When we have a form, then any element is available in the named collection `form.elements`. @@ -36,9 +36,9 @@ For instance: ``` -There may be multiple elements with the same name. This is typical with radio buttons and checkboxes. +There may be multiple elements with the same name, that's often the case with radio buttons. -In that case, `form.elements[name]` is a *collection*. For instance: +In that case `form.elements[name]` is a collection, for instance: ```html run height=40
    @@ -119,7 +119,7 @@ That's easy to see in an example: ``` -That's usually not a problem, however, because we rarely change names of form elements. +That's usually not a problem, because we rarely change names of form elements. ```` @@ -155,7 +155,7 @@ Let's talk about form controls. ### input and textarea -We can access their value as `input.value` (string) or `input.checked` (boolean) for checkboxes and radio buttons. +We can access their value as `input.value` (string) or `input.checked` (boolean) for checkboxes. Like this: @@ -177,16 +177,18 @@ It stores only the HTML that was initially on the page, not the current value. A ``: -1. Find the corresponding `
    ` is in "edit mode", clicks on other cells are ignored. - The table may have many cells. Use event delegation. diff --git a/2-ui/4-forms-controls/2-focus-blur/5-keyboard-mouse/task.md b/2-ui/4-forms-controls/2-focus-blur/5-keyboard-mouse/task.md index 644d814d..fc48c21f 100644 --- a/2-ui/4-forms-controls/2-focus-blur/5-keyboard-mouse/task.md +++ b/2-ui/4-forms-controls/2-focus-blur/5-keyboard-mouse/task.md @@ -9,5 +9,4 @@ Focus on the mouse. Then use arrow keys to move it: [demo src="solution"] P.S. Don't put event handlers anywhere except the `#mouse` element. - P.P.S. Don't modify HTML/CSS, the approach should be generic and work with any element. diff --git a/2-ui/4-forms-controls/2-focus-blur/article.md b/2-ui/4-forms-controls/2-focus-blur/article.md index c253dc11..d4348d25 100644 --- a/2-ui/4-forms-controls/2-focus-blur/article.md +++ b/2-ui/4-forms-controls/2-focus-blur/article.md @@ -90,8 +90,6 @@ If we enter something into the input and then try to use `key:Tab` or click away Please note that we can't "prevent losing focus" by calling `event.preventDefault()` in `onblur`, because `onblur` works *after* the element lost the focus. -In practice though, one should think well, before implementing something like this, because we generally *should show errors* to the user, but *should not prevent their progress* in filling our form. They may want to fill other fields first. - ```warn header="JavaScript-initiated focus loss" A focus loss can occur for many reasons. @@ -106,7 +104,7 @@ The best recipe is to be careful when using these events. If we want to track us ``` ## Allow focusing on any element: tabindex -By default, many elements do not support focusing. +By default many elements do not support focusing. The list varies a bit between browsers, but one thing is always correct: `focus/blur` support is guaranteed for elements that a visitor can interact with: ` - ## Selection properties -As said, a selection may in theory contain multiple ranges. We can get these range objects using the method: - -- `getRangeAt(i)` -- get i-th range, starting from `0`. In all browsers except Firefox, only `0` is used. - -Also, there exist properties that often provide better convenience. - -Similar to a range, a selection object has a start, called "anchor", and the end, called "focus". +Similar to a range, a selection has a start, called "anchor", and the end, called "focus". The main selection properties are: @@ -352,39 +289,36 @@ The main selection properties are: - `isCollapsed` -- `true` if selection selects nothing (empty range), or doesn't exist. - `rangeCount` -- count of ranges in the selection, maximum `1` in all browsers except Firefox. -```smart header="Selection end/start vs Range" +````smart header="Selection end may be in the document before start" +There are many ways to select the content, depending on the user agent: mouse, hotkeys, taps on a mobile etc. -There's an important differences of a selection anchor/focus compared with a `Range` start/end. +Some of them, such as a mouse, allow the same selection can be created in two directions: "left-to-right" and "right-to-left". -As we know, `Range` objects always have their start before the end. - -For selections, that's not always the case. - -Selecting something with a mouse can be done in both directions: either "left-to-right" or "right-to-left". - -In other words, when the mouse button is pressed, and then it moves forward in the document, then its end (focus) will be after its start (anchor). +If the start (anchor) of the selection goes in the document before the end (focus), this selection is said to have "forward" direction. E.g. if the user starts selecting with mouse and goes from "Example" to "italic": ![](selection-direction-forward.svg) -...But the same selection could be done backwards: starting from "italic" to "Example" (backward direction), then its end (focus) will be before the start (anchor): +Otherwise, if they go from the end of "italic" to "Example", the selection is directed "backward", its focus will be before the anchor: ![](selection-direction-backward.svg) -``` + +That's different from `Range` objects that are always directed forward: the range start can't be after its end. +```` ## Selection events There are events on to keep track of selection: -- `elem.onselectstart` -- when a selection *starts* specifically on element `elem` (or inside it). For instance, when the user presses the mouse button on it and starts to move the pointer. - - Preventing the default action cancels the selection start. So starting a selection from this element becomes impossible, but the element is still selectable. The visitor just needs to start the selection from elsewhere. -- `document.onselectionchange` -- whenever a selection changes or starts. - - Please note: this handler can be set only on `document`, it tracks all selections in it. +- `elem.onselectstart` -- when a selection starts on `elem`, e.g. the user starts moving mouse with pressed button. + - Preventing the default action makes the selection not start. +- `document.onselectionchange` -- whenever a selection changes. + - Please note: this handler can be set only on `document`. ### Selection tracking demo -Here's a small demo. It tracks the current selection on the `document` and shows its boundaries: +Here's a small demo that shows selection boundaries dynamically as it changes: ```html run height=80

    Select me: italic and bold

    @@ -392,25 +326,21 @@ Here's a small demo. It tracks the current selection on the `document` and shows From – To ``` -### Selection copying demo +### Selection getting demo -There are two approaches to copying the selected content: +To get the whole selection: +- As text: just call `document.getSelection().toString()`. +- As DOM nodes: get the underlying ranges and call their `cloneContents()` method (only first range if we don't support Firefox multiselection). -1. We can use `document.getSelection().toString()` to get it as text. -2. Otherwise, to copy the full DOM, e.g. if we need to keep formatting, we can get the underlying ranges with `getRangesAt(...)`. A `Range` object, in turn, has `cloneContents()` method that clones its content and returns as `DocumentFragment` object, that we can insert elsewhere. - -Here's the demo of copying the selected content both as text and as DOM nodes: +And here's the demo of getting the selection both as text and as DOM nodes: ```html run height=100

    Select me: italic and bold

    @@ -438,15 +368,15 @@ As text: ## Selection methods -We can work with the selection by adding/removing ranges: +Selection methods to add/remove ranges: -- `getRangeAt(i)` -- get i-th range, starting from `0`. In all browsers except Firefox, only `0` is used. +- `getRangeAt(i)` -- get i-th range, starting from `0`. In all browsers except firefox, only `0` is used. - `addRange(range)` -- add `range` to selection. All browsers except Firefox ignore the call, if the selection already has an associated range. - `removeRange(range)` -- remove `range` from the selection. - `removeAllRanges()` -- remove all ranges. - `empty()` -- alias to `removeAllRanges`. -There are also convenience methods to manipulate the selection range directly, without intermediate `Range` calls: +Also, there are convenience methods to manipulate the selection range directly, without `Range`: - `collapse(node, offset)` -- replace selected range with a new one that starts and ends at the given `node`, at position `offset`. - `setPosition(node, offset)` -- alias to `collapse`. @@ -458,7 +388,7 @@ There are also convenience methods to manipulate the selection range directly, w - `deleteFromDocument()` -- remove selected content from the document. - `containsNode(node, allowPartialContainment = false)` -- checks whether the selection contains `node` (partially if the second argument is `true`) -For most tasks these methods are just fine, there's no need to access the underlying `Range` object. +So, for many tasks we can call `Selection` methods, no need to access the underlying `Range` object. For example, selecting the whole contents of the paragraph `

    `: @@ -485,10 +415,10 @@ The same thing using ranges: ``` -```smart header="To select something, remove the existing selection first" -If a document selection already exists, empty it first with `removeAllRanges()`. And then add ranges. Otherwise, all browsers except Firefox ignore new ranges. +```smart header="To select, remove the existing selection first" +If the selection already exists, empty it first with `removeAllRanges()`. And then add ranges. Otherwise, all browsers except Firefox ignore new ranges. -The exception is some selection methods, that replace the existing selection, such as `setBaseAndExtent`. +The exception is some selection methods, that replace the existing selection, like `setBaseAndExtent`. ``` ## Selection in form controls @@ -564,7 +494,7 @@ Focus on me, the cursor will be at position 10. // zero delay setTimeout to run after browser "focus" action finishes setTimeout(() => { // we can set any selection - // if start=end, the cursor is exactly at that place + // if start=end, the cursor it exactly at that place area.selectionStart = area.selectionEnd = 10; }); }; diff --git a/2-ui/99-ui-misc/02-selection-range/range-example-p-0-1.svg b/2-ui/99-ui-misc/02-selection-range/range-example-p-0-1.svg index a97d1b47..4550c373 100644 --- a/2-ui/99-ui-misc/02-selection-range/range-example-p-0-1.svg +++ b/2-ui/99-ui-misc/02-selection-range/range-example-p-0-1.svg @@ -1 +1 @@ -0123 \ No newline at end of file +0123 \ No newline at end of file diff --git a/2-ui/99-ui-misc/02-selection-range/range-example-p-1-3.svg b/2-ui/99-ui-misc/02-selection-range/range-example-p-1-3.svg index 2a8f9aca..dd19679c 100644 --- a/2-ui/99-ui-misc/02-selection-range/range-example-p-1-3.svg +++ b/2-ui/99-ui-misc/02-selection-range/range-example-p-1-3.svg @@ -1 +1 @@ -0123 \ No newline at end of file +0123 \ No newline at end of file diff --git a/2-ui/99-ui-misc/02-selection-range/range-example-p-2-b-3-range.svg b/2-ui/99-ui-misc/02-selection-range/range-example-p-2-b-3-range.svg index 32843436..66397f20 100644 --- a/2-ui/99-ui-misc/02-selection-range/range-example-p-2-b-3-range.svg +++ b/2-ui/99-ui-misc/02-selection-range/range-example-p-2-b-3-range.svg @@ -1 +1 @@ -startContainer (<p>.firstChild)startOffset (=2)commonAncestorContainer (<p>)endContainer (<b>.firstChild)endOffset (=3) \ No newline at end of file +startContainer (<p>.firstChild)startOffset (=2)commonAncestorContainer (<p>)endContainer (<b>.firstChild)endOffset (=3) \ No newline at end of file diff --git a/2-ui/99-ui-misc/02-selection-range/range-example-p-2-b-3.svg b/2-ui/99-ui-misc/02-selection-range/range-example-p-2-b-3.svg index 859f755c..fd058beb 100644 --- a/2-ui/99-ui-misc/02-selection-range/range-example-p-2-b-3.svg +++ b/2-ui/99-ui-misc/02-selection-range/range-example-p-2-b-3.svg @@ -1 +1 @@ -0123 \ No newline at end of file +0123 \ No newline at end of file diff --git a/2-ui/99-ui-misc/02-selection-range/range-hello-1.svg b/2-ui/99-ui-misc/02-selection-range/range-hello-1.svg deleted file mode 100644 index 2951607a..00000000 --- a/2-ui/99-ui-misc/02-selection-range/range-hello-1.svg +++ /dev/null @@ -1 +0,0 @@ -<p>Hello</p>p.firstChild \ No newline at end of file diff --git a/2-ui/99-ui-misc/02-selection-range/selection-direction-backward.svg b/2-ui/99-ui-misc/02-selection-range/selection-direction-backward.svg index 85615d38..a84bdeb5 100644 --- a/2-ui/99-ui-misc/02-selection-range/selection-direction-backward.svg +++ b/2-ui/99-ui-misc/02-selection-range/selection-direction-backward.svg @@ -1 +1 @@ -focusanchormouse move direction \ No newline at end of file +focusanchor \ No newline at end of file diff --git a/2-ui/99-ui-misc/02-selection-range/selection-direction-forward.svg b/2-ui/99-ui-misc/02-selection-range/selection-direction-forward.svg index 511b00a2..fd7716c5 100644 --- a/2-ui/99-ui-misc/02-selection-range/selection-direction-forward.svg +++ b/2-ui/99-ui-misc/02-selection-range/selection-direction-forward.svg @@ -1 +1 @@ -anchorfocusmouse move direction \ No newline at end of file +anchorfocus \ No newline at end of file diff --git a/2-ui/99-ui-misc/02-selection-range/selection-firefox.svg b/2-ui/99-ui-misc/02-selection-range/selection-firefox.svg index aa7ff1eb..879e1cec 100644 --- a/2-ui/99-ui-misc/02-selection-range/selection-firefox.svg +++ b/2-ui/99-ui-misc/02-selection-range/selection-firefox.svg @@ -1 +1 @@ -selection \ No newline at end of file +selection \ No newline at end of file diff --git a/2-ui/99-ui-misc/03-event-loop/2-micro-macro-queue/solution.md b/2-ui/99-ui-misc/03-event-loop/2-micro-macro-queue/solution.md deleted file mode 100644 index 3a0daa97..00000000 --- a/2-ui/99-ui-misc/03-event-loop/2-micro-macro-queue/solution.md +++ /dev/null @@ -1,50 +0,0 @@ -The console output is: 1 7 3 5 2 6 4. - -The task is quite simple, we just need to know how microtask and macrotask queues work. - -Let's see what's going on, step by step. - -```js -console.log(1); -// The first line executes immediately, it outputs `1`. -// Macrotask and microtask queues are empty, as of now. - -setTimeout(() => console.log(2)); -// `setTimeout` appends the callback to the macrotask queue. -// - macrotask queue content: -// `console.log(2)` - -Promise.resolve().then(() => console.log(3)); -// The callback is appended to the microtask queue. -// - microtask queue content: -// `console.log(3)` - -Promise.resolve().then(() => setTimeout(() => console.log(4))); -// The callback with `setTimeout(...4)` is appended to microtasks -// - microtask queue content: -// `console.log(3); setTimeout(...4)` - -Promise.resolve().then(() => console.log(5)); -// The callback is appended to the microtask queue -// - microtask queue content: -// `console.log(3); setTimeout(...4); console.log(5)` - -setTimeout(() => console.log(6)); -// `setTimeout` appends the callback to macrotasks -// - macrotask queue content: -// `console.log(2); console.log(6)` - -console.log(7); -// Outputs 7 immediately. -``` - -To summarize, - -1. Numbers `1` и `7` show up immediately, because simple `console.log` calls don't use any queues. -2. Then, after the main code flow is finished, the microtask queue runs. - - It has commands: `console.log(3); setTimeout(...4); console.log(5)`. - - Numbers `3` и `5` show up, while `setTimeout(() => console.log(4))` adds the `console.log(4)` call to the end of the macrotask queue. - - The macrotask queue is now: `console.log(2); console.log(6); console.log(4)`. -3. After the microtask queue becomes empty, the macrotask queue executes. It outputs `2`, `6`, `4`. - -Finally, we have the output: `1 7 3 5 2 6 4`. \ No newline at end of file diff --git a/2-ui/99-ui-misc/03-event-loop/2-micro-macro-queue/task.md b/2-ui/99-ui-misc/03-event-loop/2-micro-macro-queue/task.md deleted file mode 100644 index ad406b3b..00000000 --- a/2-ui/99-ui-misc/03-event-loop/2-micro-macro-queue/task.md +++ /dev/null @@ -1,21 +0,0 @@ -importance: 5 - ---- - -# What will be the output of this code? - -```js -console.log(1); - -setTimeout(() => console.log(2)); - -Promise.resolve().then(() => console.log(3)); - -Promise.resolve().then(() => setTimeout(() => console.log(4))); - -Promise.resolve().then(() => console.log(5)); - -setTimeout(() => console.log(6)); - -console.log(7); -``` diff --git a/2-ui/99-ui-misc/03-event-loop/eventLoop-full.svg b/2-ui/99-ui-misc/03-event-loop/eventLoop-full.svg index 593cbab9..201d9a14 100644 --- a/2-ui/99-ui-misc/03-event-loop/eventLoop-full.svg +++ b/2-ui/99-ui-misc/03-event-loop/eventLoop-full.svg @@ -1 +1 @@ -...mousemoveevent looprendermicrotasksrendermicrotasksscriptsetTimeout \ No newline at end of file +...mousemoveevent looprendermicrotasksrendermicrotasksscriptsetTimeout \ No newline at end of file diff --git a/2-ui/99-ui-misc/03-event-loop/eventLoop.svg b/2-ui/99-ui-misc/03-event-loop/eventLoop.svg index 6dc459ef..f7de0b6a 100644 --- a/2-ui/99-ui-misc/03-event-loop/eventLoop.svg +++ b/2-ui/99-ui-misc/03-event-loop/eventLoop.svg @@ -1 +1 @@ -...mousemovescriptevent loopmacrotask queuesetTimeout \ No newline at end of file +...mousemovescriptevent loopmacrotask queuesetTimeout \ No newline at end of file diff --git a/3-frames-and-windows/01-popup-windows/article.md b/3-frames-and-windows/01-popup-windows/article.md index 01da2b30..7540b030 100644 --- a/3-frames-and-windows/01-popup-windows/article.md +++ b/3-frames-and-windows/01-popup-windows/article.md @@ -87,7 +87,7 @@ Settings for `params`: There is also a number of less supported browser-specific features, which are usually not used. Check window.open in MDN for examples. -## Example: a minimalistic window +## Example: a minimalistic window Let's open a window with minimal set of features, just to see which of them browser allows to disable: @@ -120,7 +120,7 @@ Rules for omitted settings: ## Accessing popup from window -The `open` call returns a reference to the new window. It can be used to manipulate its properties, change location and even more. +The `open` call returns a reference to the new window. It can be used to manipulate it's properties, change location and even more. In this example, we generate popup content from JavaScript: @@ -192,7 +192,7 @@ newWindow.onload = function() { ``` -## Moving and resizing +## Scrolling and resizing There are methods to move/resize a window: @@ -239,7 +239,7 @@ There's also `window.onscroll` event. Theoretically, there are `window.focus()` and `window.blur()` methods to focus/unfocus on a window. And there are also `focus/blur` events that allow to catch the moment when the visitor focuses on a window and switches elsewhere. -Although, in practice they are severely limited, because in the past evil pages abused them. +Although, in practice they are severely limited, because in the past evil pages abused them. For instance, look at this code: @@ -257,10 +257,10 @@ Still, there are some use cases when such calls do work and can be useful. For instance: -- When we open a popup, it might be a good idea to run `newWindow.focus()` on it. Just in case, for some OS/browser combinations it ensures that the user is in the new window now. +- When we open a popup, it's might be a good idea to run a `newWindow.focus()` on it. Just in case, for some OS/browser combinations it ensures that the user is in the new window now. - If we want to track when a visitor actually uses our web-app, we can track `window.onfocus/onblur`. That allows us to suspend/resume in-page activities, animations etc. But please note that the `blur` event means that the visitor switched out from the window, but they still may observe it. The window is in the background, but still may be visible. -## Summary +## Summary Popup windows are used rarely, as there are alternatives: loading and displaying information in-page, or in iframe. diff --git a/3-frames-and-windows/03-cross-window-communication/article.md b/3-frames-and-windows/03-cross-window-communication/article.md index 4d4e320e..091a0cec 100644 --- a/3-frames-and-windows/03-cross-window-communication/article.md +++ b/3-frames-and-windows/03-cross-window-communication/article.md @@ -116,13 +116,6 @@ document.domain = 'site.com'; That's all. Now they can interact without limitations. Again, that's only possible for pages with the same second-level domain. -```warn header="Deprecated, but still working" -The `document.domain` property is in the process of being removed from the [specification](https://html.spec.whatwg.org/multipage/origin.html#relaxing-the-same-origin-restriction). The cross-window messaging (explained soon below) is the suggested replacement. - -That said, as of now all browsers support it. And the support will be kept for the future, not to break old code that relies on `document.domain`. -``` - - ## Iframe: wrong document pitfall When an iframe comes from the same origin, and we may access its `document`, there's a pitfall. It's not related to cross-origin things, but important to know. @@ -275,7 +268,7 @@ Arguments: `targetOrigin` : Specifies the origin for the target window, so that only a window from the given origin will get the message. -The `targetOrigin` is a safety measure. Remember, if the target window comes from another origin, we can't read its `location` in the sender window. So we can't be sure which site is open in the intended window right now: the user could navigate away, and the sender window has no idea about it. +The `targetOrigin` is a safety measure. Remember, if the target window comes from another origin, we can't read it's `location` in the sender window. So we can't be sure which site is open in the intended window right now: the user could navigate away, and the sender window has no idea about it. Specifying `targetOrigin` ensures that the window only receives the data if it's still at the right site. Important when the data is sensitive. diff --git a/3-frames-and-windows/06-clickjacking/article.md b/3-frames-and-windows/06-clickjacking/article.md index 34d0a91a..1daa87dd 100644 --- a/3-frames-and-windows/06-clickjacking/article.md +++ b/3-frames-and-windows/06-clickjacking/article.md @@ -154,7 +154,7 @@ Depending on your browser, the `iframe` above is either empty or alerting you th ## Showing with disabled functionality -The `X-Frame-Options` header has a side effect. Other sites won't be able to show our page in a frame, even if they have good reasons to do so. +The `X-Frame-Options` header has a side-effect. Other sites won't be able to show our page in a frame, even if they have good reasons to do so. So there are other solutions... For instance, we can "cover" the page with a `

    ` with styles `height: 100%; width: 100%;`, so that it will intercept all clicks. That `
    ` is to be removed if `window == top` or if we figure out that we don't need the protection. diff --git a/4-binary/01-arraybuffer-binary-arrays/01-concat/_js.view/solution.js b/4-binary/01-arraybuffer-binary-arrays/01-concat/_js.view/solution.js index 00c37bb9..2f51384e 100644 --- a/4-binary/01-arraybuffer-binary-arrays/01-concat/_js.view/solution.js +++ b/4-binary/01-arraybuffer-binary-arrays/01-concat/_js.view/solution.js @@ -2,9 +2,9 @@ function concat(arrays) { // sum of individual array lengths let totalLength = arrays.reduce((acc, value) => acc + value.length, 0); + if (!arrays.length) return null; + let result = new Uint8Array(totalLength); - - if (!arrays.length) return result; // for each array - copy it over result // next array is copied right after the previous one diff --git a/4-binary/01-arraybuffer-binary-arrays/8bit-integer-256.svg b/4-binary/01-arraybuffer-binary-arrays/8bit-integer-256.svg index b697d630..89ae9640 100644 --- a/4-binary/01-arraybuffer-binary-arrays/8bit-integer-256.svg +++ b/4-binary/01-arraybuffer-binary-arrays/8bit-integer-256.svg @@ -1 +1 @@ -8-bit integer256 \ No newline at end of file +8-bit integer256 \ No newline at end of file diff --git a/4-binary/01-arraybuffer-binary-arrays/8bit-integer-257.svg b/4-binary/01-arraybuffer-binary-arrays/8bit-integer-257.svg index 8e3074fd..c7b74cd6 100644 --- a/4-binary/01-arraybuffer-binary-arrays/8bit-integer-257.svg +++ b/4-binary/01-arraybuffer-binary-arrays/8bit-integer-257.svg @@ -1 +1 @@ -8-bit integer257 \ No newline at end of file +8-bit integer257 \ No newline at end of file diff --git a/4-binary/01-arraybuffer-binary-arrays/arraybuffer-view-buffersource.svg b/4-binary/01-arraybuffer-binary-arrays/arraybuffer-view-buffersource.svg index b9de47de..12daeeae 100644 --- a/4-binary/01-arraybuffer-binary-arrays/arraybuffer-view-buffersource.svg +++ b/4-binary/01-arraybuffer-binary-arrays/arraybuffer-view-buffersource.svg @@ -1 +1 @@ -02134567012301new ArrayBuffer(16)ArrayBufferViewUint16Array Int16ArrayUint8Array Int8Array Uint8ClampedArrayUint32Array Int32Array Float32ArrayFloat64ArrayDataViewget/setUint8(offset) get/setFloat32(offset)...BufferSource1023456789101112131415 \ No newline at end of file +02134567012301new ArrayBuffer(16)ArrayBufferViewUint16Array Int16ArrayUint8Array Int8Array Uint8ClampedArrayUint32Array Int32Array Float32ArrayFloat64ArrayDataViewget/setUint8(offset) get/setFloat32(offset)...BufferSource1023456789101112131415 \ No newline at end of file diff --git a/4-binary/01-arraybuffer-binary-arrays/arraybuffer-views.svg b/4-binary/01-arraybuffer-binary-arrays/arraybuffer-views.svg index b022796a..02160e31 100644 --- a/4-binary/01-arraybuffer-binary-arrays/arraybuffer-views.svg +++ b/4-binary/01-arraybuffer-binary-arrays/arraybuffer-views.svg @@ -1 +1 @@ -100213234567891011121314154567012301new ArrayBuffer(16)Uint16ArrayUint8ArrayUint32ArrayFloat64Array \ No newline at end of file +100213234567891011121314154567012301new ArrayBuffer(16)Uint16ArrayUint8ArrayUint32ArrayFloat64Array \ No newline at end of file diff --git a/4-binary/01-arraybuffer-binary-arrays/article.md b/4-binary/01-arraybuffer-binary-arrays/article.md index 2827e277..4beb6f5c 100644 --- a/4-binary/01-arraybuffer-binary-arrays/article.md +++ b/4-binary/01-arraybuffer-binary-arrays/article.md @@ -30,11 +30,11 @@ Let's eliminate a possible source of confusion. `ArrayBuffer` has nothing in com **To manipulate an `ArrayBuffer`, we need to use a "view" object.** -A view object does not store anything on its own. It's the "eyeglasses" that give an interpretation of the bytes stored in the `ArrayBuffer`. +A view object does not store anything on it's own. It's the "eyeglasses" that give an interpretation of the bytes stored in the `ArrayBuffer`. For instance: -- **`Uint8Array`** -- treats each byte in `ArrayBuffer` as a separate number, with possible values from 0 to 255 (a byte is 8-bit, so it can hold only that much). Such value is called a "8-bit unsigned integer". +- **`Uint8Array`** -- treats each byte in `ArrayBuffer` as a separate number, with possible values are from 0 to 255 (a byte is 8-bit, so it can hold only that much). Such value is called a "8-bit unsigned integer". - **`Uint16Array`** -- treats every 2 bytes as an integer, with possible values from 0 to 65535. That's called a "16-bit unsigned integer". - **`Uint32Array`** -- treats every 4 bytes as an integer, with possible values from 0 to 4294967295. That's called a "32-bit unsigned integer". - **`Float64Array`** -- treats every 8 bytes as a floating point number with possible values from 5.0x10-324 to 1.8x10308. @@ -71,13 +71,13 @@ for(let num of view) { ## TypedArray -The common term for all these views (`Uint8Array`, `Uint32Array`, etc) is [TypedArray](https://tc39.github.io/ecma262/#sec-typedarray-objects). They share the same set of methods and properties. +The common term for all these views (`Uint8Array`, `Uint32Array`, etc) is [TypedArray](https://tc39.github.io/ecma262/#sec-typedarray-objects). They share the same set of methods and properities. Please note, there's no constructor called `TypedArray`, it's just a common "umbrella" term to represent one of views over `ArrayBuffer`: `Int8Array`, `Uint8Array` and so on, the full list will soon follow. When you see something like `new TypedArray`, it means any of `new Int8Array`, `new Uint8Array`, etc. -Typed arrays behave like regular arrays: have indexes and are iterable. +Typed array behave like regular arrays: have indexes and iterable. A typed array constructor (be it `Int8Array` or `Float64Array`, doesn't matter) behaves differently depending on argument types. @@ -126,9 +126,9 @@ new TypedArray(); We can create a `TypedArray` directly, without mentioning `ArrayBuffer`. But a view cannot exist without an underlying `ArrayBuffer`, so gets created automatically in all these cases except the first one (when provided). -To access the underlying `ArrayBuffer`, there are following properties in `TypedArray`: -- `buffer` -- references the `ArrayBuffer`. -- `byteLength` -- the length of the `ArrayBuffer`. +To access the `ArrayBuffer`, there are properties: +- `arr.buffer` -- references the `ArrayBuffer`. +- `arr.byteLength` -- the length of the `ArrayBuffer`. So, we can always move from one view to another: ```js @@ -259,7 +259,7 @@ To do almost any operation on `ArrayBuffer`, we need a view. - `Float32Array`, `Float64Array` -- for signed floating-point numbers of 32 and 64 bits. - Or a `DataView` -- the view that uses methods to specify a format, e.g. `getUint8(offset)`. -In most cases we create and operate directly on typed arrays, leaving `ArrayBuffer` under cover, as a "common denominator". We can access it as `.buffer` and make another view if needed. +In most cases we create and operate directly on typed arrays, leaving `ArrayBuffer` under cover, as a "common discriminator". We can access it as `.buffer` and make another view if needed. There are also two additional terms, that are used in descriptions of methods that operate on binary data: - `ArrayBufferView` is an umbrella term for all these kinds of views. diff --git a/4-binary/02-text-decoder/article.md b/4-binary/02-text-decoder/article.md index a0c80145..fe9b8c04 100644 --- a/4-binary/02-text-decoder/article.md +++ b/4-binary/02-text-decoder/article.md @@ -2,7 +2,7 @@ What if the binary data is actually a string? For instance, we received a file with textual data. -The built-in [TextDecoder](https://encoding.spec.whatwg.org/#interface-textdecoder) object allows one to read the value into an actual JavaScript string, given the buffer and the encoding. +The build-in [TextDecoder](https://encoding.spec.whatwg.org/#interface-textdecoder) object allows to read the value into an actual JavaScript string, given the buffer and the encoding. We first need to create it: ```js @@ -12,7 +12,7 @@ let decoder = new TextDecoder([label], [options]); - **`label`** -- the encoding, `utf-8` by default, but `big5`, `windows-1251` and many other are also supported. - **`options`** -- optional object: - **`fatal`** -- boolean, if `true` then throw an exception for invalid (non-decodable) characters, otherwise (default) replace them with character `\uFFFD`. - - **`ignoreBOM`** -- boolean, if `true` then ignore BOM (an optional byte-order Unicode mark), rarely needed. + - **`ignoreBOM`** -- boolean, if `true` then ignore BOM (an optional byte-order unicode mark), rarely needed. ...And then decode: diff --git a/4-binary/03-blob/article.md b/4-binary/03-blob/article.md index fc015057..bb475bc7 100644 --- a/4-binary/03-blob/article.md +++ b/4-binary/03-blob/article.md @@ -55,7 +55,7 @@ This behavior is similar to JavaScript strings: we can't change a character in a ## Blob as URL -A Blob can be easily used as a URL for ``, `` or other tags, to show its contents. +A Blob can be easily used as an URL for ``, `` or other tags, to show its contents. Thanks to `type`, we can also download/upload `Blob` objects, and the `type` naturally becomes `Content-Type` in network requests. @@ -74,7 +74,7 @@ link.href = URL.createObjectURL(blob); We can also create a link dynamically in JavaScript and simulate a click by `link.click()`, then download starts automatically. -Here's the similar code that causes user to download the dynamically created `Blob`, without any HTML: +Here's the similar code that causes user to download the dynamicallly created `Blob`, without any HTML: ```js run let link = document.createElement('a'); @@ -99,9 +99,9 @@ blob:https://javascript.info/1e67e00e-860d-40a5-89ae-6ab0cbee6273 For each URL generated by `URL.createObjectURL` the browser stores a URL -> `Blob` mapping internally. So such URLs are short, but allow to access the `Blob`. -A generated URL (and hence the link with it) is only valid within the current document, while it's open. And it allows to reference the `Blob` in ``, ``, basically any other object that expects a URL. +A generated URL (and hence the link with it) is only valid within the current document, while it's open. And it allows to reference the `Blob` in ``, ``, basically any other object that expects an url. -There's a side effect though. While there's a mapping for a `Blob`, the `Blob` itself resides in the memory. The browser can't free it. +There's a side-effect though. While there's a mapping for a `Blob`, the `Blob` itself resides in the memory. The browser can't free it. The mapping is automatically cleared on document unload, so `Blob` objects are freed then. But if an app is long-living, then that doesn't happen soon. @@ -151,7 +151,7 @@ reader.onload = function() { }; ``` -Both ways of making a URL of a `Blob` are usable. But usually `URL.createObjectURL(blob)` is simpler and faster. +Both ways of making an URL of a `Blob` are usable. But usually `URL.createObjectURL(blob)` is simpler and faster. ```compare title-plus="URL.createObjectURL(blob)" title-minus="Blob to data url" + We need to revoke them if care about memory. @@ -186,7 +186,7 @@ let context = canvas.getContext('2d'); context.drawImage(img, 0, 0); // we can context.rotate(), and do many other things on canvas -// toBlob is async operation, callback is called when done +// toBlob is async opereation, callback is called when done canvas.toBlob(function(blob) { // blob ready, download it let link = document.createElement('a'); @@ -211,44 +211,21 @@ For screenshotting a page, we can use a library such as /* process the ArrayBuffer */); +*!* +fileReader.readAsArrayBuffer(blob); +*/!* + +fileReader.onload = function(event) { + let arrayBuffer = fileReader.result; +}; ``` -## From Blob to stream - -When we read and write to a blob of more than `2 GB`, the use of `arrayBuffer` becomes more memory intensive for us. At this point, we can directly convert the blob to a stream. - -A stream is a special object that allows to read from it (or write into it) portion by portion. It's outside of our scope here, but here's an example, and you can read more at . Streams are convenient for data that is suitable for processing piece-by-piece. - -The `Blob` interface's `stream()` method returns a `ReadableStream` which upon reading returns the data contained within the `Blob`. - -Then we can read from it, like this: - -```js -// get readableStream from blob -const readableStream = blob.stream(); -const stream = readableStream.getReader(); - -while (true) { - // for each iteration: value is the next blob fragment - let { done, value } = await stream.read(); - if (done) { - // no more data in the stream - console.log('all blob processed.'); - break; - } - - // do something with the data portion we've just read from the blob - console.log(value); -} -``` ## Summary @@ -258,9 +235,7 @@ That makes Blobs convenient for upload/download operations, that are so common i Methods that perform web-requests, such as [XMLHttpRequest](info:xmlhttprequest), [fetch](info:fetch) and so on, can work with `Blob` natively, as well as with other binary types. -We can easily convert between `Blob` and low-level binary data types: +We can easily convert betweeen `Blob` and low-level binary data types: -- We can make a `Blob` from a typed array using `new Blob(...)` constructor. -- We can get back `ArrayBuffer` from a Blob using `blob.arrayBuffer()`, and then create a view over it for low-level binary processing. - -Conversion streams are very useful when we need to handle large blob. You can easily create a `ReadableStream` from a blob. The `Blob` interface's `stream()` method returns a `ReadableStream` which upon reading returns the data contained within the blob. +- We can make a Blob from a typed array using `new Blob(...)` constructor. +- We can get back `ArrayBuffer` from a Blob using `FileReader`, and then create a view over it for low-level binary processing. diff --git a/4-binary/03-blob/blob.svg b/4-binary/03-blob/blob.svg index 8f424545..b1cf6fcc 100644 --- a/4-binary/03-blob/blob.svg +++ b/4-binary/03-blob/blob.svg @@ -1 +1 @@ -image/pngblob1blob2strbuffer...typeBlobblobParts+= \ No newline at end of file +image/pngblob1blob2strbuffer...typeBlobblobParts+= \ No newline at end of file diff --git a/5-network/01-fetch/article.md b/5-network/01-fetch/article.md index 4669fc45..8db2243b 100644 --- a/5-network/01-fetch/article.md +++ b/5-network/01-fetch/article.md @@ -27,7 +27,7 @@ let promise = fetch(url, [options]) - **`url`** -- the URL to access. - **`options`** -- optional parameters: method, headers etc. -Without `options`, this is a simple GET request, downloading the contents of the `url`. +Without `options`, that is a simple GET request, downloading the contents of the `url`. The browser starts the request right away and returns a promise that the calling code should use to get the result. @@ -195,7 +195,7 @@ To make a `POST` request, or a request with another method, we need to use `fetc - **`method`** -- HTTP-method, e.g. `POST`, - **`body`** -- the request body, one of: - a string (e.g. JSON-encoded), - - `FormData` object, to submit the data as `multipart/form-data`, + - `FormData` object, to submit the data as `form/multipart`, - `Blob`/`BufferSource` to send binary data, - [URLSearchParams](info:url), to submit the data in `x-www-form-urlencoded` encoding, rarely used. @@ -298,13 +298,13 @@ fetch(url, options) Response properties: - `response.status` -- HTTP code of the response, -- `response.ok` -- `true` if the status is 200-299. +- `response.ok` -- `true` is the status is 200-299. - `response.headers` -- Map-like object with HTTP headers. Methods to get response body: - **`response.text()`** -- return the response as text, - **`response.json()`** -- parse the response as JSON object, -- **`response.formData()`** -- return the response as `FormData` object (`multipart/form-data` encoding, see the next chapter), +- **`response.formData()`** -- return the response as `FormData` object (form/multipart encoding, see the next chapter), - **`response.blob()`** -- return the response as [Blob](info:blob) (binary data with type), - **`response.arrayBuffer()`** -- return the response as [ArrayBuffer](info:arraybuffer-binary-arrays) (low-level binary data), diff --git a/5-network/02-formdata/article.md b/5-network/02-formdata/article.md index a73d554b..d281d075 100644 --- a/5-network/02-formdata/article.md +++ b/5-network/02-formdata/article.md @@ -47,7 +47,7 @@ As you can see, that's almost one-liner: ``` -In this example, the server code is not presented, as it's beyond our scope. The server accepts the POST request and replies "User saved". +In this example, the server code is not presented, as it's beyound our scope. The server accepts the POST request and replies "User saved". ## FormData Methods @@ -75,7 +75,7 @@ formData.append('key2', 'value2'); // List key/value pairs for(let [name, value] of formData) { - alert(`${name} = ${value}`); // key1 = value1, then key2 = value2 + alert(`${name} = ${value}`); // key1=value1, then key2=value2 } ``` @@ -168,7 +168,7 @@ The server reads form data and the file, as if it were a regular form submission [FormData](https://xhr.spec.whatwg.org/#interface-formdata) objects are used to capture HTML form and submit it using `fetch` or another network method. -We can either create `new FormData(form)` from an HTML form, or create an object without a form at all, and then append fields with methods: +We can either create `new FormData(form)` from an HTML form, or create a object without a form at all, and then append fields with methods: - `formData.append(name, value)` - `formData.append(name, blob, fileName)` diff --git a/5-network/03-fetch-progress/article.md b/5-network/03-fetch-progress/article.md index 76b05d51..2d003157 100644 --- a/5-network/03-fetch-progress/article.md +++ b/5-network/03-fetch-progress/article.md @@ -5,11 +5,11 @@ The `fetch` method allows to track *download* progress. Please note: there's currently no way for `fetch` to track *upload* progress. For that purpose, please use [XMLHttpRequest](info:xmlhttprequest), we'll cover it later. -To track download progress, we can use `response.body` property. It's a `ReadableStream` -- a special object that provides body chunk-by-chunk, as it comes. Readable streams are described in the [Streams API](https://streams.spec.whatwg.org/#rs-class) specification. +To track download progress, we can use `response.body` property. It's `ReadableStream` -- a special object that provides body chunk-by-chunk, as it comes. Readable streams are described in the [Streams API](https://streams.spec.whatwg.org/#rs-class) specification. Unlike `response.text()`, `response.json()` and other methods, `response.body` gives full control over the reading process, and we can count how much is consumed at any moment. -Here's the sketch of code that reads the response from `response.body`: +Here's the sketch of code that reads the reponse from `response.body`: ```js // instead of response.json() and other methods @@ -110,5 +110,3 @@ Let's explain that step-by-step: At the end we have the result (as a string or a blob, whatever is convenient), and progress-tracking in the process. Once again, please note, that's not for *upload* progress (no way now with `fetch`), only for *download* progress. - -Also, if the size is unknown, we should check `receivedLength` in the loop and break it once it reaches a certain limit. So that the `chunks` won't overflow the memory. diff --git a/5-network/04-fetch-abort/article.md b/5-network/04-fetch-abort/article.md index eadc5aac..6548f81d 100644 --- a/5-network/04-fetch-abort/article.md +++ b/5-network/04-fetch-abort/article.md @@ -18,15 +18,15 @@ let controller = new AbortController(); A controller is an extremely simple object. - It has a single method `abort()`, -- And a single property `signal` that allows to set event listeners on it. +- And a single property `signal` that allows to set event liseners on it. When `abort()` is called: - `controller.signal` emits the `"abort"` event. - `controller.signal.aborted` property becomes `true`. -Generally, we have two parties in the process: -1. The one that performs a cancelable operation, it sets a listener on `controller.signal`. -2. The one that cancels: it calls `controller.abort()` when needed. +Generally, we have two parties in the process: +1. The one that performs an cancelable operation, it sets a listener on `controller.signal`. +2. The one one that cancels: it calls `controller.abort()` when needed. Here's the full example (without `fetch` yet): @@ -34,8 +34,8 @@ Here's the full example (without `fetch` yet): let controller = new AbortController(); let signal = controller.signal; -// The party that performs a cancelable operation -// gets the "signal" object +// The party that performs a cancelable operation +// gets "signal" object // and sets the listener to trigger when controller.abort() is called signal.addEventListener('abort', () => alert("abort!")); @@ -46,15 +46,15 @@ controller.abort(); // abort! alert(signal.aborted); // true ``` -As we can see, `AbortController` is just a mean to pass `abort` events when `abort()` is called on it. +As we can see, `AbortController` is just a means to pass `abort` events when `abort()` is called on it. -We could implement the same kind of event listening in our code on our own, without the `AbortController` object. +We could implement same kind of event listening in our code on our own, without `AbortController` object at all. -But what's valuable is that `fetch` knows how to work with the `AbortController` object. It's integrated in it. +But what's valuable is that `fetch` knows how to work with `AbortController` object, it's integrated with it. ## Using with fetch -To be able to cancel `fetch`, pass the `signal` property of an `AbortController` as a `fetch` option: +To become able to cancel `fetch`, pass the `signal` property of an `AbortController` as a `fetch` option: ```js let controller = new AbortController(); @@ -65,7 +65,7 @@ fetch(url, { The `fetch` method knows how to work with `AbortController`. It will listen to `abort` events on `signal`. -Now, to abort, call `controller.abort()`: +Now, to to abort, call `controller.abort()`: ```js controller.abort(); @@ -97,7 +97,7 @@ try { ## AbortController is scalable -`AbortController` is scalable. It allows to cancel multiple fetches at once. +`AbortController` is scalable, it allows to cancel multiple fetches at once. Here's a sketch of code that fetches many `urls` in parallel, and uses a single controller to abort them all: @@ -113,7 +113,7 @@ let fetchJobs = urls.map(url => fetch(url, { let results = await Promise.all(fetchJobs); -// if controller.abort() is called from anywhere, +// if controller.abort() is called from elsewhere, // it aborts all fetches ``` @@ -137,12 +137,12 @@ let fetchJobs = urls.map(url => fetch(url, { // fetches // Wait for fetches and our task in parallel let results = await Promise.all([...fetchJobs, ourJob]); -// if controller.abort() is called from anywhere, +// if controller.abort() is called from elsewhere, // it aborts all fetches and ourJob ``` ## Summary -- `AbortController` is a simple object that generates an `abort` event on its `signal` property when the `abort()` method is called (and also sets `signal.aborted` to `true`). -- `fetch` integrates with it: we pass the `signal` property as the option, and then `fetch` listens to it, so it's possible to abort the `fetch`. +- `AbortController` is a simple object that generates `abort` event on it's `signal` property when `abort()` method is called (and also sets `signal.aborted` to `true`). +- `fetch` integrates with it: we pass `signal` property as the option, and then `fetch` listens to it, so it becomes possible to abort the `fetch`. - We can use `AbortController` in our code. The "call `abort()`" -> "listen to `abort` event" interaction is simple and universal. We can use it even without `fetch`. diff --git a/5-network/05-fetch-crossorigin/article.md b/5-network/05-fetch-crossorigin/article.md index d45ee391..0c142969 100644 --- a/5-network/05-fetch-crossorigin/article.md +++ b/5-network/05-fetch-crossorigin/article.md @@ -28,7 +28,7 @@ Seriously. Let's make a very brief historical digression. **For many years a script from one site could not access the content of another site.** -That simple, yet powerful rule was a foundation of the internet security. E.g. an evil script from website `hacker.com` could not access the user's mailbox at website `gmail.com`. People felt safe. +That simple, yet powerful rule was a foundation of the internet security. E.g. an evil script from website `hacker.com` could not access user's mailbox at website `gmail.com`. People felt safe. JavaScript also did not have any special methods to perform network requests at that time. It was a toy language to decorate a web page. @@ -97,43 +97,43 @@ After a while, networking methods appeared in browser JavaScript. At first, cross-origin requests were forbidden. But as a result of long discussions, cross-origin requests were allowed, but with any new capabilities requiring an explicit allowance by the server, expressed in special headers. -## Safe requests +## Simple requests There are two types of cross-origin requests: -1. Safe requests. +1. Simple requests. 2. All the others. -Safe Requests are simpler to make, so let's start with them. +Simple Requests are, well, simpler to make, so let's start with them. -A request is safe if it satisfies two conditions: +A [simple request](http://www.w3.org/TR/cors/#terminology) is a request that satisfies two conditions: -1. [Safe method](https://fetch.spec.whatwg.org/#cors-safelisted-method): GET, POST or HEAD -2. [Safe headers](https://fetch.spec.whatwg.org/#cors-safelisted-request-header) -- the only allowed custom headers are: +1. [Simple method](http://www.w3.org/TR/cors/#simple-method): GET, POST or HEAD +2. [Simple headers](http://www.w3.org/TR/cors/#simple-header) -- the only allowed custom headers are: - `Accept`, - `Accept-Language`, - `Content-Language`, - `Content-Type` with the value `application/x-www-form-urlencoded`, `multipart/form-data` or `text/plain`. -Any other request is considered "unsafe". For instance, a request with `PUT` method or with an `API-Key` HTTP-header does not fit the limitations. +Any other request is considered "non-simple". For instance, a request with `PUT` method or with an `API-Key` HTTP-header does not fit the limitations. -**The essential difference is that a safe request can be made with a `` or a ` ``` -Now, let's cover animation properties one by one. +Now let's cover animation properties one by one. ## transition-property -In `transition-property`, we write a list of properties to animate, for instance: `left`, `margin-left`, `height`, `color`. Or we could write `all`, which means "animate all properties". +In `transition-property` we write a list of property to animate, for instance: `left`, `margin-left`, `height`, `color`. -Do note that, there are properties which can not be animated. However, [most of the generally used properties are animatable](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_animated_properties). +Not all properties can be animated, but [many of them](http://www.w3.org/TR/css3-transitions/#animatable-properties-). The value `all` means "animate all properties". ## transition-duration @@ -84,11 +84,11 @@ In `transition-duration` we can specify how long the animation should take. The ## transition-delay -In `transition-delay` we can specify the delay *before* the animation. For instance, if `transition-delay` is `1s` and `transition-duration` is `2s`, then the animation starts 1 second after the property change and the total duration will be 2 seconds. +In `transition-delay` we can specify the delay *before* the animation. For instance, if `transition-delay: 1s`, then animation starts after 1 second after the change. -Negative values are also possible. Then the animation is shown immediately, but the starting point of the animation will be after given value (time). For example, if `transition-delay` is `-1s` and `transition-duration` is `2s`, then animation starts from the halfway point and total duration will be 1 second. +Negative values are also possible. Then the animation starts from the middle. For instance, if `transition-duration` is `2s`, and the delay is `-1s`, then the animation takes 1 second and starts from the half. -Here the animation shifts numbers from `0` to `9` using CSS `translate` property: +Here's the animation shifts numbers from `0` to `9` using CSS `translate` property: [codetabs src="digits"] @@ -108,13 +108,13 @@ In the example above JavaScript adds the class `.animate` to the element -- and stripe.classList.add('animate'); ``` -We could also start it from somewhere in the middle of the transition, from an exact number, e.g. corresponding to the current second, using a negative `transition-delay`. +We can also start it "from the middle", from the exact number, e.g. corresponding to the current second, using the negative `transition-delay`. Here if you click the digit -- it starts the animation from the current second: [codetabs src="digits-negative-delay"] -JavaScript does it with an extra line: +JavaScript does it by an extra line: ```js stripe.onclick = function() { @@ -129,25 +129,25 @@ stripe.onclick = function() { ## transition-timing-function -The timing function describes how the animation process is distributed along its timeline. Will it start slowly and then go fast, or vice versa. +Timing function describes how the animation process is distributed along the time. Will it start slowly and then go fast or vise versa. -It appears to be the most complicated property at first. But it becomes very simple if we devote a bit time to it. +That's the most complicated property from the first sight. But it becomes very simple if we devote a bit time to it. -That property accepts two kinds of values: a Bezier curve or steps. Let's start with the curve, as it's used more often. +That property accepts two kinds of values: a Bezier curve or steps. Let's start from the curve, as it's used more often. ### Bezier curve -The timing function can be set as a [Bezier curve](/bezier-curve) with 4 control points that satisfy the conditions: +The timing function can be set as a [Bezier curve](/bezier-curve) with 4 control points that satisfies the conditions: 1. First control point: `(0,0)`. 2. Last control point: `(1,1)`. -3. For intermediate points, the values of `x` must be in the interval `0..1`, `y` can be anything. +3. For intermediate points values of `x` must be in the interval `0..1`, `y` can be anything. The syntax for a Bezier curve in CSS: `cubic-bezier(x2, y2, x3, y3)`. Here we need to specify only 2nd and 3rd control points, because the 1st one is fixed to `(0,0)` and the 4th one is `(1,1)`. -The timing function describes how fast the animation process goes. +The timing function describes how fast the animation process goes in time. -- The `x` axis is the time: `0` -- the start, `1` -- the end of `transition-duration`. +- The `x` axis is the time: `0` -- the starting moment, `1` -- the last moment of `transition-duration`. - The `y` axis specifies the completion of the process: `0` -- the starting value of the property, `1` -- the final value. The simplest variant is when the animation goes uniformly, with the same linear speed. That can be specified by the curve `cubic-bezier(0, 0, 1, 1)`. @@ -168,7 +168,7 @@ The CSS `transition` is based on that curve: .train { left: 0; transition: left 5s cubic-bezier(0, 0, 1, 1); - /* click on a train sets left to 450px, thus triggering the animation */ + /* JavaScript sets left to 450px */ } ``` @@ -191,13 +191,13 @@ CSS: .train { left: 0; transition: left 5s cubic-bezier(0, .5, .5, 1); - /* click on a train sets left to 450px, thus triggering the animation */ + /* JavaScript sets left to 450px */ } ``` There are several built-in curves: `linear`, `ease`, `ease-in`, `ease-out` and `ease-in-out`. -The `linear` is a shorthand for `cubic-bezier(0, 0, 1, 1)` -- a straight line, which we described above. +The `linear` is a shorthand for `cubic-bezier(0, 0, 1, 1)` -- a straight line, we saw it already. Other names are shorthands for the following `cubic-bezier`: @@ -210,27 +210,27 @@ Other names are shorthands for the following `cubic-bezier`: So we could use `ease-out` for our slowing down train: + ```css .train { left: 0; transition: left 5s ease-out; - /* same as transition: left 5s cubic-bezier(0, .5, .5, 1); */ + /* transition: left 5s cubic-bezier(0, .5, .5, 1); */ } ``` But it looks a bit differently. -**A Bezier curve can make the animation exceed its range.** +**A Bezier curve can make the animation "jump out" of its range.** -The control points on the curve can have any `y` coordinates: even negative or huge ones. Then the Bezier curve would also extend very low or high, making the animation go beyond its normal range. +The control points on the curve can have any `y` coordinates: even negative or huge. Then the Bezier curve would also jump very low or high, making the animation go beyond its normal range. In the example below the animation code is: - ```css .train { left: 100px; transition: left 5s cubic-bezier(.5, -1, .5, 2); - /* click on a train sets left to 450px */ + /* JavaScript sets left to 400px */ } ``` @@ -244,29 +244,21 @@ But if you click the train, you'll see that: [codetabs src="train-over"] -Why it happens is pretty obvious if we look at the graph of the given Bezier curve: +Why it happens -- pretty obvious if we look at the graph of the given Bezier curve: ![](bezier-train-over.svg) -We moved the `y` coordinate of the 2nd point below zero, and for the 3rd point we made it over `1`, so the curve goes out of the "regular" quadrant. The `y` is out of the "standard" range `0..1`. +We moved the `y` coordinate of the 2nd point below zero, and for the 3rd point we made put it over `1`, so the curve goes out of the "regular" quadrant. The `y` is out of the "standard" range `0..1`. -As we know, `y` measures "the completion of the animation process". The value `y = 0` corresponds to the starting property value and `y = 1` -- the ending value. So values `y<0` move the property beyond the starting `left` and `y>1` -- past the final `left`. +As we know, `y` measures "the completion of the animation process". The value `y = 0` corresponds to the starting property value and `y = 1` -- the ending value. So values `y<0` move the property lower than the starting `left` and `y>1` -- over the final `left`. That's a "soft" variant for sure. If we put `y` values like `-99` and `99` then the train would jump out of the range much more. -But how do we make a Bezier curve for a specific task? There are many tools. - -- For instance, we can do it on the site . -- Browser developer tools also have special support for Bezier curves in CSS: - 1. Open the developer tools with `key:F12` (Mac: `key:Cmd+Opt+I`). - 2. Select the `Elements` tab, then pay attention to the `Styles` sub-panel at the right side. - 3. CSS properties with a word `cubic-bezier` will have an icon before this word. - 4. Click this icon to edit the curve. - +But how to make the Bezier curve for a specific task? There are many tools. For instance, we can do it on the site . ### Steps -The timing function `steps(number of steps[, start/end])` allows splitting an transition into multiple steps. +Timing function `steps(number of steps[, start/end])` allows to split animation into steps. Let's see that in an example with digits. @@ -274,19 +266,7 @@ Here's a list of digits, without any animations, just as a source: [codetabs src="step-list"] -In the HTML, a stripe of digits is enclosed into a fixed-length `
    `: - -```html -
    -
    0123456789
    -
    -``` - -The `#digit` div has a fixed width and a border, so it looks like a red window. - -We'll make a timer: the digits will appear one by one, in a discrete way. - -To achieve that, we'll hide the `#stripe` outside of `#digit` using `overflow: hidden`, and then shift the `#stripe` to the left step-by-step. +We'll make the digits appear in a discrete way by making the part of the list outside of the red "window" invisible and shifting the list to the left with each step. There will be 9 steps, a step-move for each digit: @@ -297,60 +277,58 @@ There will be 9 steps, a step-move for each digit: } ``` -The first argument of `steps(9, start)` is the number of steps. The transform will be split into 9 parts (10% each). The time interval is automatically divided into 9 parts as well, so `transition: 9s` gives us 9 seconds for the whole animation – 1 second per digit. - -The second argument is one of two words: `start` or `end`. - -The `start` means that in the beginning of animation we need to make the first step immediately. - In action: [codetabs src="step"] -A click on the digit changes it to `1` (the first step) immediately, and then changes in the beginning of the next second. +The first argument of `steps(9, start)` is the number of steps. The transform will be split into 9 parts (10% each). The time interval is automatically divided into 9 parts as well, so `transition: 9s` gives us 9 seconds for the whole animation – 1 second per digit. + +The second argument is one of two words: `start` or `end`. + +The `start` means that in the beginning of animation we need to do make the first step immediately. + +We can observe that during the animation: when we click on the digit it changes to `1` (the first step) immediately, and then changes in the beginning of the next second. The process is progressing like this: - `0s` -- `-10%` (first change in the beginning of the 1st second, immediately) - `1s` -- `-20%` - ... -- `8s` -- `-90%` +- `8s` -- `-80%` - (the last second shows the final value). -Here, the first change was immediate because of `start` in the `steps`. - The alternative value `end` would mean that the change should be applied not in the beginning, but at the end of each second. -So the process for `steps(9, end)` would go like this: +So the process would go like this: -- `0s` -- `0` (during the first second nothing changes) +- `0s` -- `0` - `1s` -- `-10%` (first change at the end of the 1st second) - `2s` -- `-20%` - ... - `9s` -- `-90%` -Here's `steps(9, end)` in action (note the pause before the first digit change): +Here's `steps(9, end)` in action (note the pause between the first digit change): [codetabs src="step-end"] -There are also some pre-defined shorthands for `steps(...)`: +There are also shorthand values: - `step-start` -- is the same as `steps(1, start)`. That is, the animation starts immediately and takes 1 step. So it starts and finishes immediately, as if there were no animation. - `step-end` -- the same as `steps(1, end)`: make the animation in a single step at the end of `transition-duration`. -These values are rarely used, as they represent not a real animation, but rather a single-step change. We mention them here for completeness. +These values are rarely used, because that's not really animation, but rather a single-step change. -## Event: "transitionend" +## Event transitionend -When the CSS animation finishes, the `transitionend` event triggers. +When the CSS animation finishes the `transitionend` event triggers. It is widely used to do an action after the animation is done. Also we can join animations. -For instance, the ship in the example below starts to sail there and back when clicked, each time farther and farther to the right: +For instance, the ship in the example below starts to swim there and back on click, each time farther and farther to the right: [iframe src="boat" height=300 edit link] -The animation is initiated by the function `go` that re-runs each time the transition finishes, and flips the direction: +The animation is initiated by the function `go` that re-runs each time when the transition finishes and flips the direction: ```js boat.onclick = function() { @@ -359,11 +337,11 @@ boat.onclick = function() { function go() { if (times % 2) { - // sail to the right + // swim to the right boat.classList.remove('back'); boat.style.marginLeft = 100 * times + 200 + 'px'; } else { - // sail to the left + // swim to the left boat.classList.add('back'); boat.style.marginLeft = 100 * times - 200 + 'px'; } @@ -379,7 +357,7 @@ boat.onclick = function() { }; ``` -The event object for `transitionend` has a few specific properties: +The event object for `transitionend` has few specific properties: `event.propertyName` : The property that has finished animating. Can be good if we animate multiple properties simultaneously. @@ -391,7 +369,7 @@ The event object for `transitionend` has a few specific properties: We can join multiple simple animations together using the `@keyframes` CSS rule. -It specifies the "name" of the animation and rules - what, when and where to animate. Then using the `animation` property, we can attach the animation to the element and specify additional parameters for it. +It specifies the "name" of the animation and rules: what, when and where to animate. Then using the `animation` property we attach the animation to the element and specify additional parameters for it. Here's an example with explanations: @@ -427,92 +405,11 @@ Here's an example with explanations: There are many articles about `@keyframes` and a [detailed specification](https://drafts.csswg.org/css-animations/). -You probably won't need `@keyframes` often, unless everything is in constant motion on your sites. - -## Performance - -Most CSS properties can be animated, because most of them are numeric values. For instance, `width`, `color`, `font-size` are all numbers. When you animate them, the browser gradually changes these numbers frame by frame, creating a smooth effect. - -However, not all animations will look as smooth as you'd like, because different CSS properties cost differently to change. - -In more technical details, when there's a style change, the browser goes through 3 steps to render the new look: - -1. **Layout**: re-compute the geometry and position of each element, then -2. **Paint**: re-compute how everything should look like at their places, including background, colors, -3. **Composite**: render the final results into pixels on screen, apply CSS transforms if they exist. - -During a CSS animation, this process repeats every frame. However, CSS properties that never affect geometry or position, such as `color`, may skip the Layout step. If a `color` changes, the browser doesn't calculate any new geometry, it goes to Paint -> Composite. And there are few properties that directly go to Composite. You can find a longer list of CSS properties and which stages they trigger at . - -The calculations may take time, especially on pages with many elements and a complex layout. And the delays are actually visible on most devices, leading to "jittery", less fluid animations. - -Animations of properties that skip the Layout step are faster. It's even better if Paint is skipped too. - -The `transform` property is a great choice, because: -- CSS transforms affect the target element box as a whole (rotate, flip, stretch, shift it). -- CSS transforms never affect neighbour elements. - -...So browsers apply `transform` "on top" of existing Layout and Paint calculations, in the Composite stage. - -In other words, the browser calculates the Layout (sizes, positions), paints it with colors, backgrounds, etc at the Paint stage, and then applies `transform` to element boxes that need it. - -Changes (animations) of the `transform` property never trigger Layout and Paint steps. More than that, the browser leverages the graphics accelerator (a special chip on the CPU or graphics card) for CSS transforms, thus making them very efficient. - -Luckily, the `transform` property is very powerful. By using `transform` on an element, you could rotate and flip it, stretch and shrink it, move it around, and [much more](https://developer.mozilla.org/docs/Web/CSS/transform#syntax). So instead of `left/margin-left` properties we can use `transform: translateX(…)`, use `transform: scale` for increasing element size, etc. - -The `opacity` property also never triggers Layout (also skips Paint in Mozilla Gecko). We can use it for show/hide or fade-in/fade-out effects. - -Paring `transform` with `opacity` can usually solve most of our needs, providing fluid, good-looking animations. - -For example, here clicking on the `#boat` element adds the class with `transform: translateX(300)` and `opacity: 0`, thus making it move `300px` to the right and disappear: - -```html run height=260 autorun no-beautify - - - - -``` - -Here's a more complex example, with `@keyframes`: - -```html run height=80 autorun no-beautify -

    click me to start / stop

    - -``` +Probably you won't need `@keyframes` often, unless everything is in the constant move on your sites. ## Summary -CSS animations allow smoothly (or step-by-step) animated changes of one or multiple CSS properties. +CSS animations allow to smoothly (or not) animate changes of one or multiple CSS properties. They are good for most animation tasks. We're also able to use JavaScript for animations, the next chapter is devoted to that. @@ -522,11 +419,9 @@ Limitations of CSS animations compared to JavaScript animations: + Simple things done simply. + Fast and lightweight for CPU. - JavaScript animations are flexible. They can implement any animation logic, like an "explosion" of an element. -- Not just property changes. We can create new elements in JavaScript as part of the animation. +- Not just property changes. We can create new elements in JavaScript for purposes of animation. ``` -In early examples in this chapter, we animate `font-size`, `left`, `width`, `height`, etc. In real life projects, we should use `transform: scale()` and `transform: translate()` for better performance. - -The majority of animations can be implemented using CSS as described in this chapter. And the `transitionend` event allows JavaScript to be run after the animation, so it integrates fine with the code. +The majority of animations can be implemented using CSS as described in this chapter. And `transitionend` event allows to run JavaScript after the animation, so it integrates fine with the code. But in the next chapter we'll do some JavaScript animations to cover more complex cases. diff --git a/7-animation/2-css-animations/bezier-linear.svg b/7-animation/2-css-animations/bezier-linear.svg index 0c2e970f..34949d61 100644 --- a/7-animation/2-css-animations/bezier-linear.svg +++ b/7-animation/2-css-animations/bezier-linear.svg @@ -1 +1 @@ -12 \ No newline at end of file +12 \ No newline at end of file diff --git a/7-animation/2-css-animations/bezier-train-over.svg b/7-animation/2-css-animations/bezier-train-over.svg index d12d0922..ff5501c4 100644 --- a/7-animation/2-css-animations/bezier-train-over.svg +++ b/7-animation/2-css-animations/bezier-train-over.svg @@ -1 +1 @@ -(1,1)(0,0)(0,1)(1,0)1243 \ No newline at end of file +(1,1)(0,0)(0,1)(1,0)1243 \ No newline at end of file diff --git a/7-animation/2-css-animations/ease-in-out.svg b/7-animation/2-css-animations/ease-in-out.svg index d5c8809d..29ef69a1 100644 --- a/7-animation/2-css-animations/ease-in-out.svg +++ b/7-animation/2-css-animations/ease-in-out.svg @@ -1 +1 @@ -1234 \ No newline at end of file +1234 \ No newline at end of file diff --git a/7-animation/2-css-animations/ease-in.svg b/7-animation/2-css-animations/ease-in.svg index 38c98ecb..0e8b797a 100644 --- a/7-animation/2-css-animations/ease-in.svg +++ b/7-animation/2-css-animations/ease-in.svg @@ -1 +1 @@ -1234 \ No newline at end of file +1234 \ No newline at end of file diff --git a/7-animation/2-css-animations/ease-out.svg b/7-animation/2-css-animations/ease-out.svg index 9d22eeaf..c2837287 100644 --- a/7-animation/2-css-animations/ease-out.svg +++ b/7-animation/2-css-animations/ease-out.svg @@ -1 +1 @@ -1234 \ No newline at end of file +1234 \ No newline at end of file diff --git a/7-animation/2-css-animations/ease.svg b/7-animation/2-css-animations/ease.svg index 8f9d41fe..a3a1feec 100644 --- a/7-animation/2-css-animations/ease.svg +++ b/7-animation/2-css-animations/ease.svg @@ -1 +1 @@ -1234 \ No newline at end of file +1234 \ No newline at end of file diff --git a/7-animation/2-css-animations/train-curve.svg b/7-animation/2-css-animations/train-curve.svg index 298dacd4..f1523599 100644 --- a/7-animation/2-css-animations/train-curve.svg +++ b/7-animation/2-css-animations/train-curve.svg @@ -1 +1 @@ -1243 \ No newline at end of file +1243 \ No newline at end of file diff --git a/7-animation/3-js-animation/1-animate-ball/solution.md b/7-animation/3-js-animation/1-animate-ball/solution.md index 0dc67b8b..5d3f08ee 100644 --- a/7-animation/3-js-animation/1-animate-ball/solution.md +++ b/7-animation/3-js-animation/1-animate-ball/solution.md @@ -2,7 +2,7 @@ To bounce we can use CSS property `top` and `position:absolute` for the ball ins The bottom coordinate of the field is `field.clientHeight`. The CSS `top` property refers to the upper edge of the ball. So it should go from `0` till `field.clientHeight - ball.clientHeight`, that's the final lowest position of the upper edge of the ball. -To get the "bouncing" effect we can use the timing function `bounce` in `easeOut` mode. +To to get the "bouncing" effect we can use the timing function `bounce` in `easeOut` mode. Here's the final code for the animation: diff --git a/7-animation/3-js-animation/1-animate-ball/solution.view/index.html b/7-animation/3-js-animation/1-animate-ball/solution.view/index.html index 146033cf..7e031e8d 100644 --- a/7-animation/3-js-animation/1-animate-ball/solution.view/index.html +++ b/7-animation/3-js-animation/1-animate-ball/solution.view/index.html @@ -21,7 +21,7 @@ } function bounce(timeFraction) { - for (let a = 0, b = 1; 1; a += b, b /= 2) { + for (let a = 0, b = 1, result; 1; a += b, b /= 2) { if (timeFraction >= (7 - 4 * a) / 11) { return -Math.pow((11 - 6 * a - 11 * timeFraction) / 4, 2) + Math.pow(b, 2) } diff --git a/7-animation/3-js-animation/2-animate-ball-hops/solution.view/index.html b/7-animation/3-js-animation/2-animate-ball-hops/solution.view/index.html index f587ff60..b246f422 100644 --- a/7-animation/3-js-animation/2-animate-ball-hops/solution.view/index.html +++ b/7-animation/3-js-animation/2-animate-ball-hops/solution.view/index.html @@ -21,7 +21,7 @@ } function bounce(timeFraction) { - for (let a = 0, b = 1; 1; a += b, b /= 2) { + for (let a = 0, b = 1, result; 1; a += b, b /= 2) { if (timeFraction >= (7 - 4 * a) / 11) { return -Math.pow((11 - 6 * a - 11 * timeFraction) / 4, 2) + Math.pow(b, 2) } diff --git a/7-animation/3-js-animation/article.md b/7-animation/3-js-animation/article.md index be648314..00495434 100644 --- a/7-animation/3-js-animation/article.md +++ b/7-animation/3-js-animation/article.md @@ -77,7 +77,7 @@ setInterval(animate3, 20); These several independent redraws should be grouped together, to make the redraw easier for the browser and hence load less CPU load and look smoother. -There's one more thing to keep in mind. Sometimes CPU is overloaded, or there are other reasons to redraw less often (like when the browser tab is hidden), so we really shouldn't run it every `20ms`. +There's one more thing to keep in mind. Sometimes when CPU is overloaded, or there are other reasons to redraw less often (like when the browser tab is hidden), so we really shouldn't run it every `20ms`. But how do we know about that in JavaScript? There's a specification [Animation timing](http://www.w3.org/TR/animation-timing/) that provides the function `requestAnimationFrame`. It addresses all these issues and even more. @@ -96,7 +96,7 @@ The returned value `requestId` can be used to cancel the call: cancelAnimationFrame(requestId); ``` -The `callback` gets one argument -- the time passed from the beginning of the page load in milliseconds. This time can also be obtained by calling [performance.now()](mdn:api/Performance/now). +The `callback` gets one argument -- the time passed from the beginning of the page load in microseconds. This time can also be obtained by calling [performance.now()](mdn:api/Performance/now). Usually `callback` runs very soon, unless the CPU is overloaded or the laptop battery is almost discharged, or there's another reason. @@ -159,7 +159,7 @@ Function `animate` accepts 3 parameters that essentially describes the animation } ``` - Its graph: + It's graph: ![](linear.svg) That's just like `transition-timing-function: linear`. There are more interesting variants shown below. @@ -227,7 +227,7 @@ See in action (click to activate): [iframe height=40 src="quad" link] -...Or the cubic curve or even greater `n`. Increasing the power makes it speed up faster. +...Or the cubic curve or event greater `n`. Increasing the power makes it speed up faster. Here's the graph for `progress` in the power `5`: @@ -283,7 +283,7 @@ The `bounce` function does the same, but in the reverse order: "bouncing" starts ```js function bounce(timeFraction) { - for (let a = 0, b = 1; 1; a += b, b /= 2) { + for (let a = 0, b = 1, result; 1; a += b, b /= 2) { if (timeFraction >= (7 - 4 * a) / 11) { return -Math.pow((11 - 6 * a - 11 * timeFraction) / 4, 2) + Math.pow(b, 2) } @@ -397,7 +397,7 @@ The effect is clearly seen if we compare the graphs of `easeIn`, `easeOut` and ` ![](circ-ease.svg) -- Red is the regular variant of `circ` (`easeIn`). +- Red is the regular variantof `circ` (`easeIn`). - Green -- `easeOut`. - Blue -- `easeInOut`. @@ -405,7 +405,7 @@ As we can see, the graph of the first half of the animation is the scaled down ` ## More interesting "draw" -Instead of moving the element we can do something else. All we need is to write the proper `draw`. +Instead of moving the element we can do something else. All we need is to write the write the proper `draw`. Here's the animated "bouncing" text typing: diff --git a/7-animation/3-js-animation/back.svg b/7-animation/3-js-animation/back.svg index fcef09ad..836a72cc 100644 --- a/7-animation/3-js-animation/back.svg +++ b/7-animation/3-js-animation/back.svg @@ -1 +1 @@ -011 \ No newline at end of file +011 \ No newline at end of file diff --git a/7-animation/3-js-animation/bezier-linear.svg b/7-animation/3-js-animation/bezier-linear.svg index 0c2e970f..34949d61 100644 --- a/7-animation/3-js-animation/bezier-linear.svg +++ b/7-animation/3-js-animation/bezier-linear.svg @@ -1 +1 @@ -12 \ No newline at end of file +12 \ No newline at end of file diff --git a/7-animation/3-js-animation/bounce-easeinout.view/index.html b/7-animation/3-js-animation/bounce-easeinout.view/index.html index aed3d9d0..837c50db 100644 --- a/7-animation/3-js-animation/bounce-easeinout.view/index.html +++ b/7-animation/3-js-animation/bounce-easeinout.view/index.html @@ -26,7 +26,7 @@ function bounce(timeFraction) { - for (let a = 0, b = 1; 1; a += b, b /= 2) { + for (let a = 0, b = 1, result; 1; a += b, b /= 2) { if (timeFraction >= (7 - 4 * a) / 11) { return -Math.pow((11 - 6 * a - 11 * timeFraction) / 4, 2) + Math.pow(b, 2) } diff --git a/7-animation/3-js-animation/bounce-easeout.view/index.html b/7-animation/3-js-animation/bounce-easeout.view/index.html index 69dbb7ce..e52eae8d 100644 --- a/7-animation/3-js-animation/bounce-easeout.view/index.html +++ b/7-animation/3-js-animation/bounce-easeout.view/index.html @@ -22,7 +22,7 @@ } function bounce(timeFraction) { - for (let a = 0, b = 1; 1; a += b, b /= 2) { + for (let a = 0, b = 1, result; 1; a += b, b /= 2) { if (timeFraction >= (7 - 4 * a) / 11) { return -Math.pow((11 - 6 * a - 11 * timeFraction) / 4, 2) + Math.pow(b, 2) } diff --git a/7-animation/3-js-animation/bounce-inout.svg b/7-animation/3-js-animation/bounce-inout.svg index 363633ab..7274d715 100644 --- a/7-animation/3-js-animation/bounce-inout.svg +++ b/7-animation/3-js-animation/bounce-inout.svg @@ -1 +1 @@ -011 \ No newline at end of file +011 \ No newline at end of file diff --git a/7-animation/3-js-animation/bounce.view/index.html b/7-animation/3-js-animation/bounce.view/index.html index 3575ed82..1be2580d 100644 --- a/7-animation/3-js-animation/bounce.view/index.html +++ b/7-animation/3-js-animation/bounce.view/index.html @@ -19,7 +19,7 @@ animate({ duration: 3000, timing: function bounce(timeFraction) { - for (let a = 0, b = 1; 1; a += b, b /= 2) { + for (let a = 0, b = 1, result; 1; a += b, b /= 2) { if (timeFraction >= (7 - 4 * a) / 11) { return -Math.pow((11 - 6 * a - 11 * timeFraction) / 4, 2) + Math.pow(b, 2) } diff --git a/7-animation/3-js-animation/circ-ease.svg b/7-animation/3-js-animation/circ-ease.svg index a7db9abc..cf2ed8d9 100644 --- a/7-animation/3-js-animation/circ-ease.svg +++ b/7-animation/3-js-animation/circ-ease.svg @@ -1 +1 @@ -011 \ No newline at end of file +011 \ No newline at end of file diff --git a/7-animation/3-js-animation/circ.svg b/7-animation/3-js-animation/circ.svg index 3595dd62..1c2beade 100644 --- a/7-animation/3-js-animation/circ.svg +++ b/7-animation/3-js-animation/circ.svg @@ -1 +1 @@ -011 \ No newline at end of file +011 \ No newline at end of file diff --git a/7-animation/3-js-animation/elastic.svg b/7-animation/3-js-animation/elastic.svg index 17f04ccd..851da406 100644 --- a/7-animation/3-js-animation/elastic.svg +++ b/7-animation/3-js-animation/elastic.svg @@ -1 +1 @@ -011 \ No newline at end of file +011 \ No newline at end of file diff --git a/7-animation/3-js-animation/linear.svg b/7-animation/3-js-animation/linear.svg index daa753f0..7a5bd71a 100644 --- a/7-animation/3-js-animation/linear.svg +++ b/7-animation/3-js-animation/linear.svg @@ -1 +1 @@ -011 \ No newline at end of file +011 \ No newline at end of file diff --git a/7-animation/3-js-animation/quad.svg b/7-animation/3-js-animation/quad.svg index 25a4d000..e9bc6ac9 100644 --- a/7-animation/3-js-animation/quad.svg +++ b/7-animation/3-js-animation/quad.svg @@ -1 +1 @@ -011 \ No newline at end of file +011 \ No newline at end of file diff --git a/7-animation/3-js-animation/quint.svg b/7-animation/3-js-animation/quint.svg index c879ef93..ad8ece28 100644 --- a/7-animation/3-js-animation/quint.svg +++ b/7-animation/3-js-animation/quint.svg @@ -1 +1 @@ -011 \ No newline at end of file +011 \ No newline at end of file diff --git a/7-animation/3-js-animation/text.view/index.html b/7-animation/3-js-animation/text.view/index.html index 4947e4cd..e404fe5c 100644 --- a/7-animation/3-js-animation/text.view/index.html +++ b/7-animation/3-js-animation/text.view/index.html @@ -29,14 +29,14 @@ And stood awhile in thought. timing: bounce, draw: function(progress) { let result = (to - from) * progress + from; - textArea.value = text.slice(0, Math.ceil(result)) + textArea.value = text.substr(0, Math.ceil(result)) } }); } function bounce(timeFraction) { - for (let a = 0, b = 1; 1; a += b, b /= 2) { + for (let a = 0, b = 1, result; 1; a += b, b /= 2) { if (timeFraction >= (7 - 4 * a) / 11) { return -Math.pow((11 - 6 * a - 11 * timeFraction) / 4, 2) + Math.pow(b, 2) } diff --git a/8-web-components/1-webcomponents-intro/article.md b/8-web-components/1-webcomponents-intro/article.md index c3522dea..3279cb13 100644 --- a/8-web-components/1-webcomponents-intro/article.md +++ b/8-web-components/1-webcomponents-intro/article.md @@ -26,9 +26,9 @@ The International Space Station: ...And this thing flies, keeps humans alive in space! -How are such complex devices created? +How such complex devices are created? -Which principles could we borrow to make our development same-level reliable and scalable? Or, at least, close to it? +Which principles we could borrow to make our development same-level reliable and scalable? Or, at least, close to it. ## Component architecture diff --git a/8-web-components/1-webcomponents-intro/web-components-twitter.svg b/8-web-components/1-webcomponents-intro/web-components-twitter.svg index 8f59f789..534e629b 100644 --- a/8-web-components/1-webcomponents-intro/web-components-twitter.svg +++ b/8-web-components/1-webcomponents-intro/web-components-twitter.svg @@ -1 +1 @@ -1243567 \ No newline at end of file +1243567 \ No newline at end of file diff --git a/8-web-components/2-custom-elements/article.md b/8-web-components/2-custom-elements/article.md index a84ed119..5a07cc67 100644 --- a/8-web-components/2-custom-elements/article.md +++ b/8-web-components/2-custom-elements/article.md @@ -149,7 +149,7 @@ The `connectedCallback` triggers when the element is added to the document. Not In the current implementation of ``, after the element is rendered, further attribute changes don't have any effect. That's strange for an HTML element. Usually, when we change an attribute, like `a.href`, we expect the change to be immediately visible. So let's fix this. -We can observe attributes by providing their list in `observedAttributes()` static getter. For such attributes, `attributeChangedCallback` is called when they are modified. It doesn't trigger for other, unlisted attributes (that's for performance reasons). +We can observe attributes by providing their list in `observedAttributes()` static getter. For such attributes, `attributeChangedCallback` is called when they are modified. It doesn't trigger for an attribute for performance reasons. Here's a new ``, that auto-updates when attributes change: @@ -320,7 +320,7 @@ For example, buttons are instances of `HTMLButtonElement`, let's build upon it. class HelloButton extends HTMLButtonElement { /* custom element methods */ } ``` -2. Provide the third argument to `customElements.define`, that specifies the tag: +2. Provide an third argument to `customElements.define`, that specifies the tag: ```js customElements.define('hello-button', HelloButton, *!*{extends: 'button'}*/!*); ``` @@ -365,7 +365,7 @@ Our new button extends the built-in one. So it keeps the same styles and standar ## References - HTML Living Standard: . -- Compatiblity: . +- Compatiblity: . ## Summary @@ -397,4 +397,4 @@ Custom elements can be of two types: /*