work

1-js/2-first-steps/18-function-parameters/article.md

@@ -119,13 +119,16 @@ The `...rest` must always be the last.
 
 ````smart header="The `arguments` variable"
 
-In old times, there were no rest operator. But there was a special variable named `arguments` that contained all arguments by their index. It is still supported and can be used like this:
+In old times, there was no rest operator. But there is a special variable named `arguments` that contains all arguments by their index. It is still supported and can be used like this:
 
 ```js run
 function showName() {
   alert( arguments[0] ); 
   alert( arguments[1] ); 
   alert( arguments.length ); 
+
+  // for..of works too
+  // for(let arg of arguments) alert(arg); 
 }
 
 // shows: Julius, Caesar, 2
@@ -135,7 +138,7 @@ showName("Julius", "Caesar");
 showName("Ilya"); 
 ```
 
-The downside is that `arguments` looks like an array, but it's not. It does not support many useful array features. It mainly exists for backwards compatibility, usually the rest operator is better.
+The downside is that though `arguments` looks like an array, but it's not. It does not support many useful array methods that we'll study later, and if they're needed, then the rest operator should be used.
 ````
 
 ## Destructuring parameters
@@ -158,7 +161,7 @@ Like this?
 showMenu("My Menu", undefined, undefined, ["Item1", "Item2"])
 ```
 
-That's ugly. And becomes unreadable if we deal with more parameters.
+That's ugly. And becomes unreadable when we deal with more parameters.
 
 Destructuring comes to the rescue!
 
@@ -235,13 +238,13 @@ showMenu(); // Menu 100 200
 
 In the code above, the whole arguments object is `{}` by default, so there's always something to destructurize.
 
-## The spread operator
+## The spread operator [#spread-operator]
 
 As we've seen before, the rest operator `...` allows to gather parameters in the array.
 
 But there's a reverse named "the spread operator". It also looks like `...` and works at call-time.
 
-The spread operator allows to "unfurl" an array into a list of parameters, like this:
+The spread operator allows to convert an array into a list of parameters, like this:
 
 ```js run
 let fullName = ["Gaius", "Julius", "Caesar"];
@@ -256,7 +259,6 @@ function showName(firstName, secondName, lastName) {
 showName(...fullName);
 ```
 
-
 Let's see a more real-life example. 
 
 There exist a built-in function [Math.max](mdn:js/Math/max) that takes a list of values and returns the greatest one:
@@ -276,13 +278,14 @@ alert( Math.max(...arr) ); // 7
 
 In short:
 - When `...` occurs in function parameters, it's called a "rest operator" and gathers parameters into the array.
-- When `...` occurs in a function call, it's called a "spread operator" and unfurls an array into the list.
+- When `...` occurs in a function call, it's called a "spread operator" and converts an array into the list.
 
 Together they help to travel between a list and an array of parameters with ease.
 
 
 ## Summary TODO
 
+[todo]
 Основные улучшения в функциях:
 
 - Можно задавать параметры по умолчанию, а также использовать деструктуризацию для чтения приходящего объекта.

1-js/2-first-steps/19-function-expressions-arrows/article.md

@@ -1,4 +1,4 @@
-# Function expressions and more
+# Function expressions and arrows
 
 In JavaScript, a function is a value.
 

1-js/2-first-steps/20-object-methods/2-check-syntax/solution.md

@@ -0,0 +1,43 @@
+**Error**!
+
+Try it:
+
+```js run
+let user = {
+  name: "John",
+  go: function() { alert(this.name) }
+}
+
+(user.go)() // error!
+```
+
+The error message in most browsers does not give understanding what went wrong.
+
+**The error appears because a semicolon is missing after `user = {...}`.**
+
+Javascript does not assume a semicolon before a bracket `(user.go)()`, so it reads the code like:
+
+```js no-beautify
+let user = { go:... }(user.go)()
+```
+
+Then we can also see that such a joint expression is syntactically a call of the object `{ go: ... }` as a function with the argument `(user.go)`. And that also happens on the same line with `let user`, so the `user` object has not yet even been defined, hence the error. 
+
+If we insert the semicolon, all is fine:
+
+```js run
+let user = {
+  name: "John",
+  go: function() { alert(this.name) }
+}*!*;*/!*
+
+(user.go)() // John
+```
+
+Please note that brackets around `(user.go)` do nothing here. Usually they setup the order of operations, but here the dot `.` works first anyway, so there's no effect. Only the semicolon thing matters.
+
+
+
+
+
+

1-js/2-first-steps/20-object-methods/2-check-syntax/task.md

@@ -0,0 +1,19 @@
+importance: 2
+
+---
+
+# Syntax check
+
+What is the resule of this code?
+
+
+```js no-beautify
+let user = {
+  name: "John",
+  go: function() { alert(this.name) }
+}
+
+(user.go)()
+```
+
+P.S. There's a pitfall :)

1-js/2-first-steps/20-object-methods/3-why-this/solution.md

@@ -0,0 +1,22 @@
+
+Here's the explanations.
+
+1. That's a regular object method call.
+
+2. The same, brackets do not change the order of operations here, the dot is first anyway.
+
+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
+    f();        // call what we have
+    ```
+
+    Here `f()` is executed as a function, without `this`.
+
+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.  
+
+Any operation on it except a method call (like assignment `=` or `||`) turns it into an ordinary value, which does not carry the information allowing to set `this`.
+

1-js/2-first-steps/20-object-methods/3-why-this/task.md

@@ -0,0 +1,26 @@
+importance: 3
+
+---
+
+# Explain the value of "this"
+
+In the code above we intend to call `user.go()` method 4 times in a row.
+
+But calls `(1)` and `(2)` works differently from `(3)` and `(4)`. Why?
+
+```js run no-beautify
+let obj, method;
+
+obj = {
+  go: function() { alert(this); }
+};
+
+obj.go();               // (1) [object Object]
+
+(obj.go)();             // (2) [object Object]
+
+(method = obj.go)();    // (3) undefined
+
+(obj.go || obj.stop)(); // (4) undefined
+```
+

1-js/2-first-steps/20-object-methods/4-object-property-this/solution.md

@@ -0,0 +1,46 @@
+**Answer: an error.**
+
+Try it:
+```js run
+function makeUser() {
+  return {
+    name: "John",
+    ref: this
+  };
+};
+
+let user = makeUser();
+
+alert( user.ref.name ); // Error: Cannot read property 'name' of undefined
+```
+
+That's because rules that set `this` do not look at object literals. 
+
+Here the value of `this` inside `makeUser()` is `undefined`, because it is called as a function, not as a method.
+
+And the object literal itself has no effect on `this`. The value of `this` is one for the whole function, code blocks and object literals do not affect it.
+
+So `ref: this` actually takes current `this` of the function.
+
+Here's the opposite case:
+
+```js run
+function makeUser() {
+  return {
+    name: "John",
+*!*
+    ref() {
+      return this;
+    }
+*/!*
+  };
+};
+
+let user = makeUser();
+
+alert( user.ref().name ); // John
+```
+
+Now it works, because `user.ref()` is a method. And the value of `this` is set to the object before dot `.`.
+
+

1-js/2-first-steps/20-object-methods/4-object-property-this/task.md

@@ -0,0 +1,23 @@
+importance: 5
+
+---
+
+# Using "this" in object literal
+
+Here the function `makeUser` returns an object.
+
+What is the result of accessing its `ref`? Why?
+
+```js
+function makeUser() {
+  return {
+    name: "John",
+    ref: this
+  };
+};
+
+let user = makeUser();
+
+alert( user.ref.name ); // What's the result?
+```
+

1-js/2-first-steps/20-object-methods/7-calculator/_js.view/solution.js

@@ -0,0 +1,14 @@
+let calculator = {
+  sum() {
+    return this.a + this.b;
+  },
+
+  mul() {
+    return this.a * this.b;
+  },
+
+  read() {
+    this.a = +prompt('a?', 0);
+    this.b = +prompt('b?', 0);
+  }
+};

1-js/2-first-steps/20-object-methods/7-calculator/_js.view/test.js

@@ -0,0 +1,28 @@
+
+
+describe("calculator", function() {
+  
+  context("when 2 and 3 entered", function() {
+    beforeEach(function() {
+      sinon.stub(window, "prompt");
+
+      prompt.onCall(0).returns("2");
+      prompt.onCall(1).returns("3");
+
+      calculator.read();
+    });
+
+    afterEach(function() {
+      prompt.restore();
+    });
+
+    it("the sum is 5", function() {
+      assert.equal(calculator.sum(), 5);
+    });
+
+    it("the multiplication product is 6", function() {
+      assert.equal(calculator.mul(), 6);
+    });
+  });
+
+});

1-js/2-first-steps/20-object-methods/7-calculator/solution.md

@@ -0,0 +1,23 @@
+
+
+```js run demo
+let calculator = {
+  sum() {
+    return this.a + this.b;
+  },
+
+  mul() {
+    return this.a * this.b;
+  },
+
+  read() {
+    this.a = +prompt('a?', 0);
+    this.b = +prompt('b?', 0);
+  }
+};
+
+calculator.read();
+alert( calculator.sum() );
+alert( calculator.mul() );
+```
+

1-js/2-first-steps/20-object-methods/7-calculator/task.md

@@ -0,0 +1,24 @@
+importance: 5
+
+---
+
+# Create a calculator
+
+Create an object `calculator` with three methods:
+
+- `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.
+
+```js
+let calculator = {
+  // ... your code ...
+};
+
+calculator.read();
+alert( calculator.sum() );
+alert( calculator.mul() );
+```
+
+[demo]
+

1-js/2-first-steps/20-object-methods/8-chain-calls/solution.md

@@ -0,0 +1,40 @@
+The solution is to return the object itself from every call.
+
+```js run
+let ladder = {
+  step: 0,
+  up() {
+    this.step++;
+*!*
+    return this;
+*/!*
+  },
+  down() {
+    this.step--;
+*!*
+    return this;
+*/!*
+  },
+  showStep() {
+    alert( this.step );
+*!*
+    return this;
+*/!*
+  }
+}
+
+ladder.up().up().down().up().down().showStep(); // 1
+```
+
+We also can write a single call per line. For long chains it's more readable:
+
+```js 
+ladder
+  .up()
+  .up()
+  .down()
+  .up()
+  .down()
+  .showStep(); // 1
+```
+

1-js/2-first-steps/20-object-methods/8-chain-calls/task.md

@@ -0,0 +1,39 @@
+importance: 2
+
+---
+
+# Chaining
+
+There's a `ladder` object that allows to go up and down:
+
+```js
+let ladder = {
+  step: 0,
+  up() { 
+    this.step++;
+  },
+  down() { 
+    this.step--;
+  },
+  showStep: function() { // shows the current step
+    alert( this.step );
+  }
+};
+```
+
+Now, if we need to make several calls in sequence, can do it like this:
+
+```js
+ladder.up();
+ladder.up();
+ladder.down();
+ladder.showStep(); // 1
+```
+
+Modify the code of `up` and `down` to make the calls chainable, like this:
+
+```js
+ladder.up().up().down().showStep(); // 1
+```
+
+Such approach is widely used across Javascript libraries.

1-js/2-first-steps/20-object-methods/article.md

@@ -213,11 +213,9 @@ If you forget `use strict`, then you may see something like "window" in the exam
 That's one of the odd things of the previous standard that `"use strict"` fixes.
 ```
 
-## "this" is for direct calls
+## Reference Type
 
-The value of `this` is only passed the right way if the function is called using a dot `'.'` or square brackets.
-
-A more intricate call would lead to losing `this`, for instance:
+An intricate method call can loose `this`, for instance:
 
 ```js run
 let user = {
@@ -230,21 +228,37 @@ user.hi(); // John (the simple call works)
 
 *!*
 // now let's call user.hi or user.bye depending on the name
-(user.name == "John" ? user.hi : user.bye)(); // undefined
+(user.name == "John" ? user.hi : user.bye)(); // Error!
 */!*
 ```
 
-On the last line the method is retrieved during the execution of the ternary `?`, and immediately called. But `"this"` is lost, the result is not `"John"` how it should be.
+On the last line the method `user.hi` is retrieved during the execution of the ternary `?`, and immediately called with brackets `()`. But that doesn't work right. You can see that the call results in an error, cause the value of `"this"` inside the call becomes `undefined`.
 
-If we want to understand why it happens -- the reason is in the details of how `obj.method()` works.
+If we want to understand why it happens -- the reason is in the details of how `obj.method()` call works.
 
-The method call has two independant operations in it: a dot `'.'` to access the property and brackets `()` to execute it (assuming that's a function).
+The method call has two successive operations in it: 
+- the dot `'.'` retrieves the property
+- brackets `()` execute it (assuming that's a function).
 
-As we've already seen, the function is a value of its own. It does not memorize the object by itself. So to "carry" it to the brackets, Javascript uses a trick -- the dot `'.'` returns not a function, but a value of the special Reference Type.
+So, you might have already asked yourself, why does it work? That is, if we put these operations on separate lines, then `this` is guaranted to be lost:
 
-The Reference Type is a "specification type". It does not exist in real, but used internally to explain how some language features work.
+```js run
+let user = {
+  name: "John",
+  hi() { alert(this.name); }
+}
 
-The value of the Reference Type is a tuple `(base, name, strict)`, where:
+let hi = user.hi;
+hi(); // Error, because this is undefined
+```
+
+...But for a single-line call `user.hi()` all is fine.
+
+That's because a function is a value of its own. It does not carry the object. So to pass it to the brackets, Javascript uses a trick -- the dot `'.'` returns not a function, but a value of the special [Reference Type](https://tc39.github.io/ecma262/#sec-reference-specification-type).
+
+The Reference Type is a "specification type". It does not exist in real, but is used internally. Engines are not required to implement it, just to make sure that code works as described.
+
+The value of the Reference Type is a combination `(base, name, strict)`, where:
 
 - `base` is the object.
 - `name` is the property.
@@ -253,20 +267,25 @@ The value of the Reference Type is a tuple `(base, name, strict)`, where:
 The result of a property access `'.'` is a value of the Reference Type. For `user.sayHi` in strict mode it is:
 
 ```js
-// base name   strict
+// Reference Type value
 (user, "sayHi", true)
 ```
 
-Any operation on the Reference Type immediately "resolves" it:
+Then, when brackets `()` are called on the Reference Type, they receive the full information about the object and it's method, and can set the right `this = base`.
 
-- Brackets `()` get the property `base[name]` and execute it with `this = base`.
-- Other operators just get `base[name]` and use it.
+Any other operation just gets `base[name]` value and uses it, discarding the reference type as a whole. 
 
 So any operation on the result of dot `'.'` except a direct call discards `this`. 
 
+
+That's why 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).
+
+
 ## Explicit "this" with "call/apply" [#call-apply]
 
-We can call a function explicitly providing the value of `"this"`.
+The value of `this` does not have to come from the aforementioned rules.
+
+We can explicitly set it to any object using `func.call`.
 
 The syntax is:
 
@@ -291,25 +310,27 @@ sayHi.call( admin ); // Admin
 
 The first parameter of `call` is the intended value of `"this"`, the latter are arguments. 
 
+So `sayHi.call(admin)` runs the function `sayHi` with `this = admin`, hence `this.name` in it becomes `"Admin"`.
+
 These calls are roughly equivalent:
 ```js
 func(1, 2, 3);
 func.call(obj, 1, 2, 3)
 ```
 
-...Except that the `call` sets "this" of course!
+They both call `func` with arguments `1`, `2` and `3`. The only difference is that `call` also sets `"this"`.
 
-That's handy when we want to use a function in the context of different objects, but do not want to actually assign it to them.
+The method `func.call` is used when we'd like to use a function in the context of different objects, but do not want to actually assign it to them. We'll see more examples of it soon.
 
 ### "func.apply"
 
-There's also a similar syntax:
+There's also a similar method `func.apply`:
 
 ```js
 func.apply(context, args)
 ```
 
-It does the same as `call`: executes the function providing `context` as `this`, but where `call` awaits a list of arguments, `apply` awaits a single array of arguments.
+It does the same as `call`: executes the function providing `context` as `this`, but where `call` awaits a list of arguments, `apply` awaits an array.
 
 These two calls do the same:
 
@@ -318,9 +339,9 @@ func.call(obj, 1, 2, 3);
 func.apply(obj, [1, 2, 3]);
 ```
 
-In old times `apply` was more powerful, because it allows to form the array of arguments dynamically. 
+In old times `apply` was more powerful, because it allows to create the array of arguments dynamically. Their number is not hardcoded at code-write time.
 
-But in the modern language, we have the spread operator `'...'` and can use it to unfurl an array into the list of for `call`, so these two are equal:
+But in the modern language, we have the spread operator `'...'` and can use it to convert an array into a list of for `call`, so these two are equal:
 
 ```js
 let args = [1, 2, 3];
@@ -329,7 +350,7 @@ func.call(obj, ...args);
 func.apply(obj, args);
 ```
 
-So the use of `apply` over `call` is mainly a metter of personal preference. And it's somewhat better optimized than the spread operator, because it exists longer.
+Nowadays the use of `apply` or `call` is mainly a metter of personal preference. But `apply` is  somewhat better optimized in engines than the call + spread combination, because it exists longer. So it would execute a little bit faster.
 
 ## Binding "this" with "bind"
 

1-js/4-data-structures/3-string/article.md

@@ -163,6 +163,13 @@ alert( str[1000] ); // undefined
 alert( str.charAt(1000) ); // '' (an empty string)
 ```
 
+Also we can iterate over characters using `for..of`:
+
+```js run
+for(let char of "Hello") {
+  alert(char); // H,e,l,l,o (char becomes "H", then "e", then "l" etc)
+}
+```
 
 ## Strings are immutable
 
@@ -337,9 +344,9 @@ Just remember: `if (~str.indexOf(...))` reads as "if found".
 
 ### includes, startsWith, endsWith
 
-The more modern method [str.includes(substr)](mdn:js/String/includes) returns `true/false` depending on whether `str` has `substr` as its part. 
+The more modern method [str.includes(substr, pos)](mdn:js/String/includes) returns `true/false` depending on whether `str` has `substr` as its part. 
 
-It's the right choice if we need to test for the match, without the position:
+It's the right choice if we need to test for the match, but don't need its position:
 
 ```js run
 alert( "Widget with id".includes("Widget") ); // true
@@ -347,6 +354,13 @@ alert( "Widget with id".includes("Widget") ); // true
 alert( "Hello".includes("Bye") ); // false
 ```
 
+The optional second argument of `str.includes` is the position to start searching from:
+
+```js run
+alert( "Midget".includes("id") ); // true
+alert( "Midget".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
@@ -354,7 +368,6 @@ alert( "Widget".startsWith("Wid") ); // true, "Widget" starts with "Wid"
 alert( "Widget".endsWith("get") );   // true, "Widget" ends with "get"
 ```
 
-
 ## Getting a substring
 
 There are 3 methods in JavaScript to get a substring: `substring`, `substr` and `slice`.
@@ -561,7 +574,7 @@ Note that surrogate pairs did not exist at the time when Javascript was created,
 
 We actually have a single symbol in each of the strings above, but the `length` shows the length of `2`. 
 
-`String.fromCodePoint` and `str.codePointAt` are notable exceptions 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.
+`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.
 
 But, for instance, getting a symbol can be tricky, because surrogate pairs are treated as two characters:
 
@@ -574,7 +587,52 @@ Note that pieces of the surrogate pair have no meaning without each other. So, t
 
 How to solve this problem? First, let's make sure you have it. Not every project deals with surrogate pairs.
 
-But if you do, then search the internet for libraries which implement surrogate-aware versions of `slice`, `indexOf` and other functions. Technically, surrogate pairs are detectable by their codes: the first character has the code in the interval of `0xD800..0xDBFF`, while the second is in `0xDC00..0xDFFF`. So if we see a character with the code, say, `0xD801`, then the next one must be the second part of the surrogate pair. Libraries rely on that to split stirngs right. Unfortunately, there's no single well-known library to advise yet.
+If you do, then there are some more tricks to deal with surrogates:
+
+Use `for..of` to iterate over characters.
+: The `for..of` loop respects surrogate pairs:
+
+    ```js run
+    let str = '𝒳😂';
+    for(let char of str) { // loop over characters of str
+        alert(char); // 𝒳, and then 😂
+    }
+    ```
+
+Create an array of characters using `Array.from(str)`:
+: Here's the trick:
+
+    ```js run
+    let str = '𝒳😂';
+
+    // splits str into array of characters
+    let chars = Array.from(str);
+
+    alert(chars[0]); // 𝒳
+    alert(chars[1]); // 😂
+    alert(chars.length); // 2
+    ```
+
+    The [Array.from](mdn:js/Array/from) takes an array-like object and turns it into a real array. It works with other array-like objects too.
+
+    Technically here it does the same as:
+
+    ```js run
+    let str = '𝒳😂';
+
+    let chars = []; // Array.from internally does the same loop
+    for(let char of str) {
+      chars.push(char); 
+    }
+
+    alert(chars);
+    ```
+
+    ...But is shorter.    
+
+There are probably libraries in the internet that build surrogate-aware versions of other string calls based on that.
+
+Technically, surrogate pairs are also detectable by their codes: the first character has the code in the interval of `0xD800..0xDBFF`, while the second is in `0xDC00..0xDFFF`. So if we see a character with the code, say, `0xD801`, then the next one must be the second part of the surrogate pair. 
 
 ### Diacritical marks
 

1-js/4-data-structures/4-object/article.md

@@ -7,17 +7,17 @@ Objects in JavaScript combine two functionalities.
 
 Here we concentrate on the first part: using objects as a data store, and we will study it in-depth. That's the required base for studying the second part.
 
-An [associative array](https://en.wikipedia.org/wiki/Associative_array), also called "a hash" or "a dictionary" -- is a data structure for storing arbitrary data in the key-value format.
+Let's recap what we know about objects and add a bit more.
+
 
 [cut]
 
+## Object literals
+
 We can imagine it as a cabinet with signed files. Every piece of data is stored in it's file. It's easy to find a file by it's name or add/remove a file.
 
 ![](object.png)
 
-
-## Object literals
-
 An empty object ("empty cabinet") can be created using one of two syntaxes:
 
 ```js
@@ -41,6 +41,44 @@ let user = {
 
 ![](object-user-props.png)
 
+In real code we quite often want to create an object with a property from a variable.
+
+For instance:
+
+```js run
+function makeUser(name, age) {
+  return {
+    name: name,
+    age: age;
+  }
+}
+
+let user = makeUser("John", 30);
+alert(user.name); // John
+```
+
+There's a *property value shorthand* to make it shorter. 
+
+Instead of `name: name` we can just write `name`, like this:
+
+```js
+function makeUser(name, age) {
+  return {
+    name,
+    age;
+  }
+}
+```
+
+We can also combine normal properties and shorthands:
+
+```js
+let user = { 
+  name,  // same as name:name
+  age: 30 
+};
+```
+
 
 ````smart header="Trailing comma"
 The last property may end with a comma:
@@ -221,37 +259,6 @@ In the code above, the property `obj.test` technically exists. So the `in` opera
 Situations like this happen very rarely, because `undefined` is usually not assigned. We mostly use `null` for "unknown" or "empty" values. So the `in` operator is an exotic guest in the code.
 ````
 
-## Property shorthands
-
-There are two more syntax features to write a shorter code.
-
-Property value shorthands
-: To create a property from a variable:
-
-  ```js 
-  let name = "John";
-
-  // same as { name: name }
-  let user = { name }; 
-  ```
-
-  If we have a variable and want to add a same-named property, that's the way to write it shorter.
-
-  ```js
-  // can combine normal properties and shorthands
-  let user = { name, age: 30 };
-  ```
-
-Methods definitions
-: For properties that are functions, we've already seen a shorter syntax.
-
-  ```js 
-  let user = {
-    sayHi() { // same as "sayHi: function()"
-      alert("Hello");
-    }
-  }; 
-  ```
 
 
 ## Loops
@@ -402,9 +409,9 @@ alert(*!*user.name*/!*); // 'Pete', changes are seen from the "user" reference
 
 Quite obvious, if we used one of the keys (`admin`) and changed something inside the cabinet, then if we use another key later (`user`), we find things modified.
 
-### Comparison with objects
+### Comparison by reference
 
-Two objects are equal only when they are one object:
+Two object variabls are equal only when reference the same object:
 
 ```js run
 let a = {};
@@ -426,9 +433,9 @@ let b = {}; // two independents object
 alert( a == b ); // false
 ```
 
-For unusual equality checks like: object vs a priimtive, or an object less/greater `< >` than another object, objects are converted to numbers. To say the truth, such comparisons occur very rarely in real code and usually are a result of a mistake.
+For unusual equality checks like: object vs a primitive (`obj == 5`), or an object less/greater than another object (`obj1 > obj2`), objects are converted to numbers. To say the truth, such comparisons occur very rarely in real code and usually are a result of a coding mistake.
 
-## Cloning, Object.assign
+## Cloning and Object.assign
 
 What if we need to duplicate an object? Create an independant copy, a clone?
 

1-js/4-data-structures/6-array/1-item-value/task.md

@@ -9,10 +9,11 @@ What this code is going to show?
 ```js
 let fruits = ["Apples", "Pear", "Orange"];
 
+// push a new value into the "copy"
 let shoppingCart = fruits;
-
 shoppingCart.push("Banana");
 
+// what's in fruits?
 alert( fruits.length ); // ?
 ```
 

1-js/4-data-structures/6-array/3-call-array-this/solution.md

@@ -0,0 +1,15 @@
+The call `arr[2]()` is syntactically the good old `obj[method]()`, in the role of `obj` we have `arr`, and in the role of `method` we have `2`.
+
+So we have a call of the function `arr[2]` as an object method. Naturally, it receives `this` referencing the object `arr` and outputs the array:
+
+```js run
+let arr = ["a", "b"];
+
+arr.push(function() {
+  alert( this );
+})
+
+arr[2](); // "a","b",function
+```
+
+The array has 3 values: initially it had two, plus the function. 

1-js/4-data-structures/6-array/3-call-array-this/task.md

@@ -0,0 +1,18 @@
+importance: 5
+
+---
+
+# Calling in an array context
+
+What is the result? Why?
+
+```js
+let arr = ["a", "b"];
+
+arr.push(function() {
+  alert( this );
+})
+
+arr[2](); // ?
+```
+

1-js/4-data-structures/6-array/article.md

@@ -313,6 +313,24 @@ But that's actually a bad idea. There are potential problems with it:
 
 So we should never use `for..in` for arrays.
 
+
+````smart header="`for..of` over `arr.entries()`"
+For "real" arrays and some array-like structures, there's one more way:
+
+```js run
+let arr = ["Apple", "Orange", "Pear"];
+
+*!*
+for (let [i, item] of arr.entries()) {
+*/!*
+  alert( i + ':' + item ); // 0:Apple, then 1:Orange, then 2:Pear
+}
+```
+
+Here [arr.entries](mdn:js/Array/entries) is a built-in method, most array-like structures do not support that.
+````
+
+
 ## A word about "length"
 
 The `length` property automatically updates when we modify the array. It is actually not the *count* of values in the array, but the greatest numeric index plus one.
@@ -413,8 +431,9 @@ We can use an array as a deque with the following operations:
 - `unshift(...items)` adds items to the beginning.
 
 To loop over the elements of the array:
-  - `for(let item of arr)` -- the modern syntax,
   - `for(let i=0; i<arr.length; i++)` -- works fastest, old-browser-compatible.
+  - `for(let item of arr)` -- the modern syntax for items only,
+  - `for(let [i,item] of arr.entries())` -- the modern syntax for indexes together with items,
   - `for(let i in arr)` -- never use.
 
 That were the "extended basics". There are more methods. In the next chapter we'll study them in detail.

1-js/4-data-structures/8-array-methods/10-filter-anagrams/_js.view/solution.js

@@ -1,16 +0,0 @@
-function aclean(arr) {
-  var obj = {};
-
-  for (var i = 0; i < arr.length; i++) {
-    var sorted = arr[i].toLowerCase().split("").sort().join("");
-    obj[sorted] = arr[i];
-  }
-
-  var result = [];
-
-  for (var key in obj) {
-    result.push(obj[key]);
-  }
-
-  return result;
-}

1-js/4-data-structures/8-array-methods/10-filter-anagrams/_js.view/test.js

@@ -1,26 +0,0 @@
-function intersection(arr1, arr2) {
-  return arr1.filter(function(item) {
-    return arr2.indexOf(item) != -1;
-  });
-}
-
-describe("aclean", function() {
-
-  it("содержит ровно по 1 слову из каждого набора анаграмм", function() {
-    var arr = ["воз", "киборг", "корсет", "зов", "гробик", "костер", "сектор"];
-
-    var result = aclean(arr);
-    assert.equal(result.length, 3);
-
-    assert.equal(intersection(result, ["гробик", "киборг"]).length, 1);
-    assert.equal(intersection(result, ["воз", "зов"]).length, 1);
-    assert.equal(intersection(result, ["корсет", "сектор", "костер"]).length, 1);
-
-  });
-
-  it("не различает регистр символов", function() {
-    var arr = ["воз", "ЗОВ"];
-    assert.equal(aclean(arr).length, 1);
-  });
-
-});

1-js/4-data-structures/8-array-methods/10-filter-anagrams/solution.md

@@ -1,69 +0,0 @@
-# Решение
-
-Чтобы обнаружить анаграммы, разобьём каждое слово на буквы и отсортируем их. В отсортированном по буквам виде все анаграммы одинаковы.
-
-Например:
-
-```
-воз, зов -> взо
-киборг, гробик -> бгикор
-...
-```
-
-По такой последовательности будем делать массив уникальным.
-
-Для этого воспользуемся вспомогательным объектом, в который будем записывать слова по отсортированному ключу:
-
-```js run
-function aclean(arr) {
-  // этот объект будем использовать для уникальности
-  var obj = {};
-
-  for (var i = 0; i < arr.length; i++) {
-    // разбить строку на буквы, отсортировать и слить обратно
-*!*
-    var sorted = arr[i].toLowerCase().split('').sort().join(''); // (*)
-*/!*
-
-    obj[sorted] = arr[i]; // сохраняет только одно значение с таким ключом
-  }
-
-  var result = [];
-
-  // теперь в obj находится для каждого ключа ровно одно значение
-  for (var key in obj) result.push(obj[key]);
-
-  return result;
-}
-
-var arr = ["воз", "киборг", "корсет", "ЗОВ", "гробик", "костер", "сектор"];
-
-alert( aclean(arr) );
-```
-
-Приведение слова к  сортированному по буквам виду осуществляется цепочкой вызовов в строке `(*)`.
-
-Для удобства комментирования разобьём её на несколько строк (JavaScript это позволяет):
-
-```js
-var sorted = arr[i] // ЗОВ
-  .toLowerCase() // зов
-  .split('') // ['з','о','в']
-  .sort() // ['в','з','о']
-  .join(''); // взо
-```
-
-Получится, что два разных слова `'ЗОВ'` и `'воз'` получат одинаковую отсортированную форму `'взо'`.
-
-Следующая строка:
-
-```js
-obj[sorted] = arr[i];
-```
-
-В объект `obj` будет записано сначала первое из слов `obj['взо'] = "воз"`, а затем `obj['взо'] = 'ЗОВ'`.
-
-Обратите внимание, ключ -- отсортирован, а само слово -- в исходной форме, чтобы можно было потом получить его из объекта.
-
-Вторая запись по тому же ключу перезапишет первую, то есть в объекте останется ровно одно слово с таким набором букв.
-

1-js/4-data-structures/8-array-methods/10-filter-anagrams/task.md

@@ -1,27 +0,0 @@
-importance: 3
-
----
-
-# Отфильтровать анаграммы
-
-*Анаграммы* -- слова, состоящие из одинакового количества одинаковых букв, но в разном порядке.
-Например:
-
-```
-воз - зов
-киборг - гробик
-корсет - костер - сектор
-```
-
-Напишите функцию `aclean(arr)`, которая возвращает массив слов, очищенный от анаграмм.
-
-Например:
-
-```js
-var arr = ["воз", "киборг", "корсет", "ЗОВ", "гробик", "костер", "сектор"];
-
-alert( aclean(arr) ); // "воз,киборг,корсет" или "ЗОВ,гробик,сектор"
-```
-
-Из каждой группы анаграмм должно остаться только одно слово, не важно какое.
-

1-js/4-data-structures/8-iterable/article.md

@@ -0,0 +1,227 @@
+
+# Iterables
+
+*Iterable* objects is a general concept that allows to make any object to be useable in a `for..of` loop.
+
+Many built-ins are partial cases of this concept. For instance, arrays are iterable. But not only arrays. Strings are iterable too. 
+
+Iterables come from the very core of Javascript and are widely used both in built-in methods and those provided by the environment. For instance, in-browser lists of DOM-nodes are iterable.
+
+[cut]
+
+## Symbol.iterator
+
+We can easily grasp the concept of iterables by making one of our own.
+
+For instance, we have an object, that is not an array, but looks a suitable for `for..of`.
+
+Like a `range` object that represents an interval of numbers:
+
+```js
+let range = {
+  from: 1,
+  to: 5
+};
+
+// We want for..of to work:
+// for(let num of range) ... num=1,2,3,4,5
+```
+
+To make the `range` iterable (and enable `for..of`) we need to add a method to the object with the name `Symbol.iterator` (a special built-in symbol just for that).
+
+- When `for..of` starts, it calls that method (or errors if none found). 
+- The method must return an *iterator* -- an object with the method `next`.
+- When `for..of` wants the next value, it calls `next()` on that object.
+- The result of `next()` must have the form `{done: Boolean, value: any}`: `done:true` means that the iteration is finished, otherwise `value` must be the new value.
+
+Let's see how it can work for `range`:
+
+```js run
+let range = {
+  from: 1,
+  to: 5
+}
+
+// call to for..of initially calls this
+range[Symbol.iterator] = function() {
+
+  // ...it returns the iterator:
+  return {
+    current: this.from,
+    last: this.to,
+
+    // ...whose next() is called on each iteration of the loop
+    next() {
+      if (this.current <= this.last) {
+        return { done: false, value: this.current++ };
+      } else {
+        return { done: true};
+      }
+    }
+  };
+};
+
+for (let num of range) {
+  alert(num); // 1, then 2, 3, 4, 5
+}
+
+alert(Math.max(...range)); // 5 (*)
+```
+
+There is an important separation of concerns in this code.
+
+- The `range` itself does not have the `next()` method.
+- Instead, another object, a so-called "iterator" is created by the call to `range[Symbol.iterator]()`.
+- It keeps the iteration state in its `current` property. That's good, because the original object is not modified by iterations. Also multiple `for..of` loops over the same object can run simultaneously, because they create separate iterators. 
+
+The fact that the object itself does not do the iteration also adds flexibility, because `range[Symbol.iterator]` can create iterators the smart way, depending on other object properties or external conditions. It's a full-fledged function that may be more complex than just a `return {...}`.
+
+Please note that the internal mechanics is not seen from outside. Here `for..of` calls `range[Symbol.iterator]()` and then the `next()` until `done: false`, but the external code doesn't see that. It only gets values.
+
+```smart header="Spread operator `...` and iterables"
+At the last line `(*)` of the example above, we can see that an iterable object can be expanded to a list through a spread operator.
+
+The call to `Math.max(...range)` internally does a full `for..of`-like iteration over `range`, and its results are used as a list of arguments for `Math.max`, in our case `Math.max(1,2,3,4,5)`.
+```
+
+```smart header="Infinite iterators"
+Infinite iterators are also doable. For instance, the `range` becomes infinite for `range.to = Infinity`. Or we can make an iterable object that generates an infinite sequence of pseudorandom numbers. Also can be useful.
+
+There are no limitations on `next`, it can return more and more values, that's normal.
+
+Of course, the `for..of` loop over such an iterable would be endless, we'll need to stop if, for instance, using `break`.
+```
+
+````smart header="`Symbol.iterator` in a literal"
+We could also write `Symbol.iterator` directly in the object literal, via computed properties syntax:
+
+```js
+let range = {
+  from: 1,
+  to: 5,
+  [Symbol.iterator]() {
+    return {...};
+  }
+};
+```
+````
+
+## Built-in iterables
+
+Iterators can also be created explicitly, without `for..of`, with a direct call of `Symbol.iterator`. For built-in objects too.
+
+For instance, this code gets a string iterator and calls it "manually":
+
+```js run
+let str = "Hello";
+
+// does the same as
+// for (let char of str) alert(char);
+
+let iterator = str[Symbol.iterator]();
+
+while(true) {
+  let result = iterator.next();
+  if (result.done) break;
+  alert(result.value); // outputs characters one by one
+}
+```
+
+The same works for an array.
+
+## Iterables VS array-likes
+
+There are two official terms that are similar, but actually very different. Please take care 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. 
+
+Sometimes they can both be applied. For instance, strings are both iterable and array-like.
+
+But an iterable may be not array-like and vise versa.
+
+For example, the `range` in the example above is not array-like, because it does not have `length`.
+
+And here's the object that is array-like, but not iterable:
+
+```js run
+let arrayLike = { // has indexes and length => array-like
+  0: "Hello",
+  1: "World",
+  length: 2
+};
+
+*!*
+// Error (not iterable)
+for(let item of arrayLike) {}
+*/!*
+```
+
+...But what they share in common -- both iterables and array-likes are usually not arrays, they don't have `join`, `slice` etc. Or maybe have other methods that have same names, but behave differently from their `Array` counterparts.
+
+Remember the special `arguments` object for a function call? It is both array-like and iterable object that has all function arguments:
+
+```js run
+function f() {
+  // array-like demo
+  alert( arguments[0] ); // 1
+  alert( arguments.length ); // 2
+
+  // iterable demo
+  for(let arg of arguments) alert(arg); // 1, then 2
+}
+
+f(1, 2);
+```
+
+...But as it's not an array, the following would fail:
+
+```js run
+function f() {
+*!*
+  alert( arguments.join() ); // Error: arguments.join is not a function
+*/!*
+}
+
+f(1, 2);
+```
+
+There's a universal method [Array.from](mdn:js/Array/from) that brings them together. It takes an iterable *or* an array-like value and makes a "real" `Array` from it.
+
+For instance:
+
+```js run
+let arrayLike = {
+  0: "Hello",
+  1: "World",
+  length: 2
+};
+
+alert( Array.from(arrayLike).join() ); // Hello,World
+```
+
+```js
+// assuming range is taken from the example above
+alert( Array.from(range) ); // 1,2,3,4,5
+```
+
+This method is really handy when we want to use array methods like `map`, `forEach` and others on array-likes or iterables.
+
+
+## Summary
+
+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 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 iteration end, 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`.
+
+The modern specification mostly uses iterables instead of arrays where an ordered collection is required, because they are more abstract. For instance, the spread operator `"..."` does it.
+
+Objects that have indexed properties and `length` are called *array-like*. Such objects may also have other properties and methods, but lack built-in methods of arrays.
+
+`Array.from(obj)` makes a real `Array` of an iterable or array-like `obj`, and then we can use array methods on it.
+
+

1-js/4-data-structures/9-map-set-weakmap-weakset/1-filter-anagrams/_js.view/solution.js

@@ -0,0 +1,11 @@
+
+function aclean(arr) {
+  let map = new Map();
+
+  for(let word of arr) {
+    let sorted = word.toLowerCase().split("").sort().join("");
+    map.set(sorted, word);
+  }
+
+  return Array.from(map.values());
+}

1-js/4-data-structures/9-map-set-weakmap-weakset/1-filter-anagrams/_js.view/test.js

@@ -0,0 +1,24 @@
+function intersection(arr1, arr2) {
+  return arr1.filter(item => arr2.includes(item));
+}
+
+describe("aclean", function() {
+
+  it("returns exactly 1 word from each anagram set", function() {
+    let arr = ["nap", "teachers", "cheaters", "PAN", "ear", "era", "hectares"];
+
+    let result = aclean(arr);
+    assert.equal(result.length, 3);
+
+    assert.equal(intersection(result, ["nap", "PAN"]).length, 1);
+    assert.equal(intersection(result, ["teachers", "cheaters", "hectares"]).length, 1);
+    assert.equal(intersection(result, ["ear", "era"]).length, 1);
+
+  });
+
+  it("is case-insensitive", function() {
+    let arr = ["era", "EAR"];
+    assert.equal(aclean(arr).length, 1);
+  });
+
+});

1-js/4-data-structures/9-map-set-weakmap-weakset/1-filter-anagrams/solution.md

@@ -0,0 +1,81 @@
+To find all anagrams, let's split every word to letters and sort them. When letter-sorted, all anagrams are same.
+
+For instance:
+
+```
+nap, pan -> anp
+ear, era, are -> aer
+cheaters, hectares, teachers -> aceehrst
+...
+```
+
+We'll use the letter-sorted variants as map keys to store only one value per each key:
+
+```js run
+function aclean(arr) {
+  let map = new Map();
+
+  for(let word of arr) {
+    // split the word by letters, sort them and join back
+*!*
+    let sorted = word.toLowerCase().split('').sort().join(''); // (*)
+*/!*
+    map.set(sorted, word); 
+  }
+
+  return Array.from(map.values());
+}
+
+let arr = ["nap", "teachers", "cheaters", "PAN", "ear", "era", "hectares"];
+
+alert( aclean(arr) );
+```
+
+Letter-sorting is done by the chain of calls in the line `(*)`.
+
+For convenience let's split it into multiple lines:
+
+```js
+var sorted = arr[i] // PAN
+  .toLowerCase() // pan
+  .split('') // ['p','a','n']
+  .sort() // ['a','n','p']
+  .join(''); // anp
+```
+
+Two different words `'PAN'` and `'nap'` receive the same letter-sorted form `'anp'`.
+
+The next line put the word into the map:
+
+```js
+map.set(sorted, word);
+```
+
+If we ever meet a word the same letter-sorted form again, then it would overwrite the previous value with the same key in the map. So we'll always have at maximum one word per letter-form.
+
+At the end `Array.from(map.values())` takes an iterable over map values (we don't need keys in the result) and returns an array of them.
+
+Here we could also use a plain object instead of the `Map`, because keys are strings.
+
+That's how the solution can look:
+
+```js run
+function aclean(arr) {
+  var obj = {};
+
+  for (var i = 0; i < arr.length; i++) {
+    var sorted = arr[i].toLowerCase().split("").sort().join("");
+    obj[sorted] = arr[i];
+  }
+
+  return Array.from(Object.values(obj));
+}
+
+let arr = ["nap", "teachers", "cheaters", "PAN", "ear", "era", "hectares"];
+
+alert( aclean(arr) );
+```
+
+
+
+

1-js/4-data-structures/9-map-set-weakmap-weakset/1-filter-anagrams/task.md

@@ -0,0 +1,28 @@
+importance: 4
+
+---
+
+# Filter anagrams
+
+[Anagrams](https://en.wikipedia.org/wiki/Anagram) are words that have the same number of same letters, but in different order.
+
+For instance:
+
+```
+nap - pan
+ear - are - era
+cheaters - hectares - teachers
+```
+
+Write a function `aclean(arr)` that returns an array cleaned from anagrams.
+
+For instance:
+
+```js
+let arr = ["nap", "teachers", "cheaters", "PAN", "ear", "era", "hectares"];
+
+alert( aclean(arr) ); // "nap,teachers,ear" or "PAN,cheaters,era"
+```
+
+From every anagram group should remain only one word, no matter which one.
+

1-js/4-data-structures/9-map-set-weakmap-weakset/article.md

@@ -134,9 +134,9 @@ Here, `Object.entries` returns the array of key/value pairs: `[ ["name","John"],
 
 For looping over a `map`, there are 3 methods:
 
-- `map.keys()` -- returns an array-like object for keys,
-- `map.values()` -- returns an array-like object for values,
-- `map.entries()` -- returns an array-like object for entries `[key, value]`, it's used by default in `for..of`.
+- `map.keys()` -- returns an iterable object for keys,
+- `map.values()` -- returns an iterable object for values,
+- `map.entries()` -- returns an iterable object for entries `[key, value]`, it's used by default in `for..of`.
 
 For instance:
 
@@ -182,7 +182,7 @@ recipeMap.forEach( (value, key, map) => {
 
 The main methods are:
 
-- `new Set([values])` -- creates the set, optionally with an array of values (any iterable will do). 
+- `new Set(iterable)` -- creates the set, optionally from an array of values (any iterable will do). 
 - `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`.
@@ -236,6 +236,12 @@ Note the funny thing. The `forEach` function in the `Set` has 3 arguments: a val
 
 That's made for compatibility with `Map` where `forEach` has three arguments.
 
+The same methods as `Map` has for iterators are also supported:
+
+- `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`.
+
 ## WeakMap and WeakSet
 
 `WeakSet` is a special kind of `Set` that does not prevent JavaScript from memory cleaning. `WeakMap` is the same thing for `Map`.

1-js/5-deeper/1-recursion/article.md

@@ -22,7 +22,7 @@ There are two ways to solve it.
 
 1. Iterative thinking -- the `for` loop:
 
-    ```js  run
+    ```js run
     function pow(x, n) {
       let result = 1;
 
@@ -132,7 +132,8 @@ function pow(x, n) {
 alert( pow(2, 3) );
 ```
 
-The line changes, so the context is now:
+
+The variables are same, but the line changes, so the context is now:
 
 <ul class="function-execution-context-list">
   <li>

1-js/5-deeper/1-recursion/head.html

@@ -8,16 +8,16 @@
 .function-execution-context {
   border: 1px solid black;
   font-family: "Consolas", monospace;
-  padding: 5px 8px;
+  padding: 4px 6px;
+  margin: 0 4px;
 }
 
 .function-execution-context-call {
   color: gray;
-  padding-left: 20px;
 }
 
 .function-execution-context-call::before {
-  content: '@';
+  content: ' call: ';
 }
 
 .function-execution-context-list li:first-child {

1-js/5-deeper/2-closure/article.md

@@ -373,7 +373,7 @@ alert( counter() ); // 10
 
 Sometimes such possibility can be a plus, but usually we want more control over `count`, and the other way is prefered.
 
-## Code blocks and loops
+## Code blocks and loops, IIFE
 
 A code block has it's own Lexical Environment and hence local variables.
 
@@ -400,7 +400,16 @@ The new Lexical Environment gets the enclosing one as the outer reference, so `p
 
 After `if` finishes, its Lexical Environment is normally destroyed (unless there's a living nested function). That's why the `alert` below won't see the `user`.
 
-**We also can use a "bare" code block to isolate variables.**
+For a loop, every run has a separate Lexical Environment. The loop variable is its part:
+
+```js run
+for(let i = 0; i < 10; i++) {
+  // Each loop has its own Lexical Environment
+  // {i: value}
+}
+```
+
+We also can use a "bare" code block to isolate variables.
 
 For instance, in-browser all scripts share the same global area. So if we create a global variable in one script, it becomes available to others. That may be a source of conflicts if two scripts use the same variable name and overwrite each other.
 
@@ -418,15 +427,61 @@ If we don't want that, we can use a code block to isolate the whole script or an
 alert(message); // Error: message is not defined
 ```
 
-For a loop, every run has a separate Lexical Environment. The loop variable is its part:
+In old scripts, you can find immediately-invoked function expressions (abbreviated as IIFE) used for this purpose.
+
+They look like this:
 
 ```js run
-for(let i = 0; i < 10; i++) {
-  // Each loop has its own Lexical Environment
-  // {i: value}
-}
+(function() {
+  
+  let message = "Hello";
+
+  alert(message); // Hello
+
+})();
 ```
 
+Here a Function Expression is created and immediately called. So the code executes right now and has its own private variables.
+
+The Function Expression is wrapped with brackets `(function {...})`, because otherwise Javascript would try to read it as Function Declaration:
+
+```js run
+// Error: Unexpected token (
+function() { // <-- JavaScript assumes it is a Function Declarations, but no name
+  
+  let message = "Hello";
+
+  alert(message); // Hello
+
+}();
+```
+
+...And we can't actually use Function Declaration here, because Javascript does not allow them to be called immediately:f
+
+```js run
+// syntax error
+function go() {
+  
+}();
+```
+
+So the brackets are needed to show Javascript that we make a function in the context of another expression. Other means to do that:
+
+```js run
+!function() {
+  alert("Bitwise NOT operator starts the expression");
+}();
+
++function() {
+  alert("Unary plus starts the expression");
+}();
+```
+
+
+
+
+
+
 ## The old "var"
 
 In the very first chapter about [variables](info:variables), we mentioned three ways of variable declaration:

1-js/plan2.txt

@@ -63,6 +63,9 @@ closures
   counter object?
 
 new function
+
+
+
 bind, currying
 decorators
 constructors