Comprehensive analysis of this in JavaScript

JavaScript Of course it is indispensable to have objects in it, the this keyword! With the this keyword JavaScript, the code will be reduced. Today we will analyze this in JavaScript!

Implicit binding

Regarding this, generally speaking, whoever calls a method will point to whom this method points to, such as:

function foo(){
    console.log(this.a)
}
var a = 3;
var obj = {
    a: 2,
    foo: foo
};
obj.foo(); // 输出2,因为是obj调用的foo，所以foo的this指向了obj，而obj.a = 2

If there are multiple calls, the objectPropertiesOnly the upper or last layer of the reference chain works in the calling location, such as:

function foo() {
    console.log( this.a )
}
var obj2 = { 
    a: 42,
    foo: foo
}
var obj1 = {
    a: 2,
    obj2: obj2
}
obj1.obj2.foo(); // 42

Implicit loss

One of the most common this binding problems is that implicitly bound functions will lose the binding object. That is to say, it should apply the default binding, thereby binding this to the global object or undefined, depending on whether it is in strict mode.

function foo() {
    console.log( this.a )
}
var obj1 = {
    a: 2,
    foo: foo
}
var bar = obj1.foo; // 函数别名！
var a = "oops, global"; // a是全局对象的属性
bar(); // "oops, global"

Although bar is a reference to obj.foo, in fact, it refers to the foo function itself, so bar() at this time is actually a function call without any modification, so it is applied Default binding

A subtler, more common and more unexpected situation occurs when passing in the callback function:

function foo() {
    console.log( this.a )
}
function doFoo( fn ){
    // fn 其实引用的是 foo
    fn(); // <-- 调用位置！
}
var obj = {
    a: 2,
    foo: foo
}
var a = "oops, global"; // a是全局对象的属性
doFoo( obj.foo ); // "oops, global"

Parameter passing is actually an implicit formula assignment, so when we pass in the function, it will also be implicitly assigned, so the result is the same as the previous example. If the function is passed into the language's built-in function instead of passing in the self-declared function (such as setTimeout, etc.), the result will be the same.

Explicit binding

Simply put, it means specifying this, such as: call, apply, bind, new binding, etc.

Hard binding

function foo( something ) {
    console.log( this.a, something)
    return this.a + something
}
var obj = {
    a: 2
}
var bar = function() {
    return foo.apply( obj, arguments)
}
var b = bar(3); // 2 3
console.log(b); // 5

Here is a brief explanation: In the bar function, foo uses the apply function to bind obj, which means that this in foo will point to obj, at the same time, uses arguments (no limit on the number of parameters passed in) as parameters and passes them into the foo function; so when running bar(3), first output obj.a, which is 2 and the passed in 3, and then foo returns the sum of the two, so the value of b is 5

Similarly, this example can also use bind:

function foo( something ) {
    console.log( this.a, something)
    return this.a + something
}
var obj = {
    a: 2
}
var bar = foo.bind(obj)
var b = bar(3); // 2 3
console.log(b); // 5

new binding

In traditional class-oriented languages, when using new to initialize a class, the constructor in the class will be called, but new in JS The mechanism is actually completely different from class-oriented and language-oriented.

Use new to call a function, or when a constructor call occurs, the following operations will be automatically performed:

Create (or construct) a brand new object

This The new object will be executed [[Prototype]] connection

This new object will be bound to this

of the function call If the function does not return other objects, then newExpression## The function in # will automatically return this new object such as:

function foo(a){
    this.a = a
}
var bar = new foo(2);
console.log(bar.a); // 2

When using new to call foo(...), we will construct a new object and bind it to this in the call to foo(...) . new is the last method that can affect the this binding behavior when a function is called. We call it new binding.

#this's priority

There is no doubt that the priority of the default binding is the lowest among the four rules , so we can ignore it first.

Which one has higher priority, implicit binding or explicit binding? Let’s test it:

function foo(a){
    console.log(this.a)
}
var obj1 = {
    a: 2,
    foo: foo
}
var obj2 = {
    a: 3,
    foo: foo
}
obj1.foo(); // 2
obj2.foo(); // 3
obj1.foo.call(obj2); // 3
obj2.foo.call(obj1); // 2

As you can see, explicit binding has a higher priority, which means that whether explicit binding can exist should be considered first when making a judgment.

Now we need to figure out who has a higher priority and which has a lower priority between new binding and implicit binding:

function foo(something){
    this.a = something
}
var obj1 = {
    foo: foo
}
var obj2 = {}
obj1.foo(2); 
console.log(obj1.a); // 2
obj1.foo.call(obj2,3);
console.log(obj2.a); // 3
var bar = new obj1.foo(4)
console.log(obj1.a); // 2
console.log(bar.a); // 4

You can see that new binding has a higher priority than implicit binding. But which one has higher priority, new binding or explicit binding?

function foo(something){
    this.a = something
}
var obj1 = {}
var bar = foo.bind(obj1);
bar(2);
console.log(obj1.a); // 2
var baz = new bar(3);
console.log(obj1.a); // 2
console.log(baz.a); // 3

You can see that new binding modifies this in hard binding, so new binding has a higher priority than explicit binding.

The main purpose of using hard-bound functions in new is to pre-set some parameters of the function, so that only the rest can be passed in when initializing with new. parameters. One of the functions of bind(...) is that it can pass all parameters except the first parameter (the first parameter is used to bind this) to the underlying function (this technique is called "partial application" and is A type of "currying"). For example:

function foo(p1,p2){
    this.val = p1 + p2;
}
// 之所以使用null是因为在本例中我们并不关心硬绑定的this是什么
// 反正使用new时this会被修改
var bar = foo.bind(null,'p1');
var baz = new bar('p2');
baz.val; // p1p2
}

Currying: Intuitively, currying states that "if you fix some parameters, you will get a function that accepts the remaining parameters." So for the function yx with two variables, if y = 2 is fixed, we get the function 2x with one variable

##This application in arrow functions The arrow function does not use the four standard rules of this, but determines this based on the outer (function or global) scope.

我们来看一下箭头函数的词法作用域：

function foo() {
    // 返回一个箭头函数
    return (a) => {
        // this继承自foo()
        console.log(this.a)
    };
}
var obj1 = {
    a: 2
};
var obj2 = {
    a: 3
};
var bar = foo.call(obj1);
bar.call(obj2); // 2, 不是3！

foo()内部创建的箭头函数会捕获调用时foo()的this。由于foo()的this绑定到obj1，bar（引用箭头函数）的this也会绑定到obj1，箭头函数的绑定无法被修改。（new也不行!）

总结

如果要判断一个运行中的函数的this绑定，就需要找到这个函数的直接调用位置。找到之后就可以顺序应用下面这四条规则来判断this的绑定对象。

由new调用？绑定到新创建的对象。

由call或者apply（或者bind）调用？绑定到指定的对象。

由上下文对象调用？绑定到那个上下文对象。

默认：在严格模式下绑定到undefined，否则绑定到全局对象。

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