159 lines
5.2 KiB
Markdown
159 lines
5.2 KiB
Markdown
# Code structure
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The first thing we'll study is the building blocks of code.
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## Statements
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Statements are syntax constructs and commands that perform actions.
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We've already seen a statement, `alert('Hello, world!')`, which shows the message "Hello, world!".
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We can have as many statements in our code as we want. Statements can be separated with a semicolon.
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For example, here we split "Hello World" into two alerts:
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```js run no-beautify
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alert('Hello'); alert('World');
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```
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Usually, statements are written on separate lines to make the code more readable:
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```js run no-beautify
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alert('Hello');
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alert('World');
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```
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## Semicolons [#semicolon]
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A semicolon may be omitted in most cases when a line break exists.
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This would also work:
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```js run no-beautify
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alert('Hello')
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alert('World')
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```
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Here, JavaScript interprets the line break as an "implicit" semicolon. This is called an [automatic semicolon insertion](https://tc39.github.io/ecma262/#sec-automatic-semicolon-insertion).
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**In most cases, a newline implies a semicolon. But "in most cases" does not mean "always"!**
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There are cases when a newline does not mean a semicolon. For example:
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```js run no-beautify
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alert(3 +
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1
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+ 2);
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```
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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.
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**But there are situations where JavaScript "fails" to assume a semicolon where it is really needed.**
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Errors which occur in such cases are quite hard to find and fix.
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````smart header="An example of an error"
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If you're curious to see a concrete example of such an error, check this code out:
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```js run
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[1, 2].forEach(alert)
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```
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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`.
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Now, let's add an `alert` before the code and *not* finish it with a semicolon:
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```js run no-beautify
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alert("There will be an error")
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[1, 2].forEach(alert)
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```
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Now if we run the code, only the first `alert` is shown and then we have an error!
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But everything is fine again if we add a semicolon after `alert`:
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```js run
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alert("All fine now");
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[1, 2].forEach(alert)
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```
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Now we have the "All fine now" message followed by `1` and `2`.
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The error in the no-semicolon variant occurs because JavaScript does not assume a semicolon before square brackets `[...]`.
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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:
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```js run no-beautify
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alert("There will be an error")[1, 2].forEach(alert)
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```
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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.
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````
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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.
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## Comments [#code-comments]
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As time goes on, programs become more and more complex. It becomes necessary to add *comments* which describe what the code does and why.
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Comments can be put into any place of a script. They don't affect its execution because the engine simply ignores them.
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**One-line comments start with two forward slash characters `//`.**
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The rest of the line is a comment. It may occupy a full line of its own or follow a statement.
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Like here:
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```js run
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// This comment occupies a line of its own
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alert('Hello');
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alert('World'); // This comment follows the statement
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```
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**Multiline comments start with a forward slash and an asterisk <code>/*</code> and end with an asterisk and a forward slash <code>*/</code>.**
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Like this:
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```js run
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/* An example with two messages.
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This is a multiline comment.
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*/
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alert('Hello');
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alert('World');
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```
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The content of comments is ignored, so if we put code inside <code>/* ... */</code>, it won't execute.
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Sometimes it can be handy to temporarily disable a part of code:
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```js run
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/* Commenting out the code
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alert('Hello');
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*/
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alert('World');
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```
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```smart header="Use hotkeys!"
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In most editors, a line of code can be commented out by pressing the `key:Ctrl+/` hotkey for a single-line comment and something like `key:Ctrl+Shift+/` -- for multiline comments (select a piece of code and press the hotkey). For Mac, try `key:Cmd` instead of `key:Ctrl` and `key:Option` instead of `key:Shift`.
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```
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````warn header="Nested comments are not supported!"
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There may not be `/*...*/` inside another `/*...*/`.
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Such code will die with an error:
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```js run no-beautify
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/*
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/* nested comment ?!? */
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*/
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alert( 'World' );
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```
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````
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Please, don't hesitate to comment your code.
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Comments increase the overall code footprint, but that's not a problem at all. There are many tools which minify code before publishing to a production server. They remove comments, so they don't appear in the working scripts. Therefore, comments do not have negative effects on production at all.
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Later in the tutorial there will be a chapter <info:code-quality> that also explains how to write better comments.
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