Understanding this in Javascript

Why write this article?

I have been asked this question before during interviews at Alibaba and other companies, so I want to share it with everyone. If you can clearly understand the difference between the following seven situations, you can follow the interview The officer's clear explanation is undoubtedly a big plus. After understanding this article, the interviewer will no longer be afraid of asking you this.

Talking about this in Javascript, it actually makes people feel headache. Unlike Java, this in C++ points to the object that calls this.

The value of this in a function is determined when the function is actually called and executed, and cannot be determined when the function is defined.

Because the value of this is part of the execution context, every time a function is called, a new execution context will be generated. When you use this in your code, the value of this is obtained directly from the execution context without searching in the scope chain.

Regarding the value of this, it can be roughly divided into the following seven situations:

Case 1: Global & calling ordinary functions

In the global environment, this always points to window.

console.log(this === window);     //true

When a normal function is called (note that it is not a constructor, no new is added in front), this also points to the window.

var x = 10;
function foo(){
    console.log(this);     //Window
    console.log(this.x);   //10
}
foo();

Case 2: Constructor

The so-called constructor is an object derived from a function new. Generally, the first letter of the function name of the constructor is capitalized, such as Object, Function, and Array. Belongs to the constructor function.

function Foo(){
    this.x = 10;
    console.log(this);    //Foo {x:10}
}
var foo = new Foo();
console.log(foo.x);      //10

In the above code, if the function is used as a constructor, then this represents the object it is about to new.

But if you call the Foo function directly instead of new Foo(), it becomes case 1, and Foo() becomes a normal function.

function Foo(){
    this.x = 10;
    console.log(this);    //Window
}
var foo = Foo();
console.log(foo.x);      //undefined

Case 3: Object method

If the function is used as a method of the object, this in the method points to the object.

var obj = {
    x: 10,
    foo: function () {
        console.log(this);        //Object
        console.log(this.x);      //10
    }
};
obj.foo();

Note: If the function is defined in the object method, the situation is different.

var obj = {
    x: 10,
    foo: function () {
        function f(){
            console.log(this);      //Window
            console.log(this.x);    //undefined
        }
        f();
    }
}
obj.foo();

It can be understood this way: although function f is defined inside obj.foo, it still belongs to an ordinary function, and this still points to window. (This is a pitfall, please keep it in mind)

Here, if you want to call the variable obj.x in the upper scope, you can use self to cache the external this variable.

var obj = {
    x: 10,
    foo: function () {
        var self = this;
        function f(){
            console.log(self);      //{x: 10}
            console.log(self.x);    //10
        }
        f();
    }
}
obj.foo();

If the foo function is not called as an object method:

var obj = {
    x: 10,
    foo: function () {
        console.log(this);       //Window
        console.log(this.x);     //undefined
    }
};
var fn = obj.foo;
fn();

obj.foo is assigned to a global variable and is not called as a property of obj, then the value of this at this time It's a window.

Case 4: Constructor prototype property

function Foo(){
    this.x = 10;
}
Foo.prototype.getX = function () {
    console.log(this);        //Foo {x: 10, getX: function}
    console.log(this.x);      //10
}
var foo = new Foo();
foo.getX();

In the Foo.prototype.getX function, this points to the foo object. Not only that, even in the entire prototype chain, this represents the value of the current object.

Case 5: The function is called using call, apply or bind.

var obj = {
    x: 10
}
function foo(){
    console.log(this);     //{x: 10}
    console.log(this.x);   //10
}
foo.call(obj);
foo.apply(obj);
foo.bind(obj)();

When a function is called by call, apply or bind, the value of this takes the value of the passed in object.

Case 6: DOM event this

In an HTML DOM event handler, this always points to the HTML DOM node bound to this handler:

function Listener(){   
    document.getElementById('foo').addEventListener('click', this.handleClick);     //这里的 this 指向 Listener 这个对象。不是强调的是这里的 this
}
Listener.prototype.handleClick = function (event) {
    console.log(this);    //<p id="foo"></p>
}
var listener = new Listener();
document.getElementById(&#39;foo&#39;).click();

This is very It's easy to understand. It's equivalent to passing parameters to the function, which changes the handleClick runtime context. It's equivalent to the following code:

var obj = {
    x: 10,
    fn: function() {
        console.log(this);         //Window
        console.log(this.x);       //undefined
    }
};
function foo(fn) {
    fn();
} 
foo(obj.fn);

You can also use bind to switch context:

function  Listener(){
    document.getElementById('foo').addEventListener('click',this.handleClick.bind(this));      
}
Listener.prototype.handleClick = function (event) {
    console.log(this);    //Listener {}
}
var listener = new Listener();
document.getElementById('foo').click();

before The six situations can be summarized in one sentence: this points to the object that calls the method.

Case 7: this in the arrow function

When using the arrow function, the situation is different: this inside the arrow function is the lexical scope, determined by the context.

var obj = {
    x: 10,
    foo: function() {
        var fn = () => {
            return () => {
                return () => {
                    console.log(this);      //Object {x: 10}
                    console.log(this.x);    //10
                }
            }
        }
        fn()()();
    }
}
obj.foo();

Now, the arrow function completely fixes the pointing of this. This always points to the lexical scope, which is the outer caller obj.

If you use arrow functions, the previous hack writing method:

var self = this;

is no longer needed.

var obj = {
    x: 10,
    foo: function() {
        var fn = () => {
            return () => {
                return () => {
                    console.log(this);    // Object {x: 10}
                    console.log(this.x);  //10
                }
            }
        }
        fn.bind({x: 14})()()();
        fn.call({x: 14})()();
    }
}
obj.foo();

Since this has been bound according to the lexical scope in the arrow function, when calling the arrow function with call() or apply(), this cannot be bound, that is, the first passed in parameters are ignored.

Supplementary Note:

• this is a reserved word, you cannot rewrite this.

function test(){
    var this = {};     //Uncaught SyntaxError: Unexpected token this
}

• Host object:

1. When a language is running, it needs a The environment is called the host environment.

2. For JavaScript, the most common host environment is a web browser. The browser provides an environment for JavaScript to run. In this environment, some interfaces need to be provided so that the JavaScript engine can interact with Host environment docking.

3. The JavaScript engine is where JavaScript code is actually executed. Common engines include V8 (currently the fastest JavaScript engine, produced by Google) and JavaScript core.

4. The browser or server (nodejs) has its own JS engine. In the browser, the global object is window, and in nodejs, the global object is global.

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