An article that understands this point and catches up with 70% of front-end people-headlines-php.cn

Colleagues got stuck because of the problem pointed by thisbug, vue2’s this pointed problem, using the arrow function, resulting in the inability to get the corresponding props . He didn't know it when I introduced it to him, and then I deliberately looked at the front-end communication group. So far, at least 70% of front-end programmers still don't understand it. I'll share it with you today thisPoint, if you haven’t learned anything, please give me a big mouth.

1. Calling location

The scope is related to where it is defined and has nothing to do with where it is executed
thisThe pointer has nothing to do with where it is defined, it has to do with how it is called and in what form
this(this) how this function is called (convenient to remember)
In order to facilitate understanding, strict mode is not enabled by default

2. Binding rules

We introduced above,this Pointing mainly depends on the form of calling. Next, I will introduce the calling rules to you. Without rules, nothing is complete. Just keep these calling rules in mind. There is nothing difficult.

You must find the calling location, and then determine which of the following four binding rules is
Secondly, you must also know the priority of these four binding rules.
You know these two points and it will be easy for you to know where this points.

2.1 Default binding

Functions are the most commonly used The calling method, the type of calling function: independent function call

function bar() {
  console.log(this) // window
}

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bar is a direct call without any modifier, so the default binding is window
In strict mode, this here is undefined

##2.2 Implicit binding

Use the most In layman's terms, it means: the object has a certain method, and the method is accessed through this object and called directly (note: the arrow function is special, which will be explained below)

const info = {
  fullName: 'ice',
  getName: function() {
    console.log(this.fullName)
  }
}

info.getName() // 'ice'

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info The call is initiated and implicit binding is performed, so the current this is info, and the value accessed through this.fullName is undoubtedly ice

Implicit loss normal

In some cases, implicit loss will be performed, and the implicitly bound function will lose the bound object. , that is to say, it becomes the default binding. The

this value of the default binding is window or undefined, depending on the environment you are currently in. Whether it is strict mode.

const info = {
  fullName: 'ice',
  getName: function() {
    console.log(this.fullName)
  }
}

const fn = info.getName

fn() //undefined

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In this case, implicit loss is performed and the bound object is lost. Why does such a problem occur? If you are familiar with memory, it will be easy to understand.

getName is found through info and assigned to the variable fn
fn
window, from windowTake out the fullName attribute, which must be undefined

Implicit loss advancedWhat do you need to understand first? Is the callback function. In fact, it can be understood this way, that is, I don't call it now, but pass it to other places in the form of parameters, and call it somewhere else.

//申明变量关键字必须为var
var fullName = 'panpan'

const info = {
  fullName: 'ice',
  getName: function() {
    console.log(this.fullName)
  }
}

function bar(fn) {
  //fn = info.getName
  fn() // panpan
}

bar(info.getName)

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fn in bar is a callback function
fn = info.getName parameter passing It is an implicit assignment. In fact, it has the same meaning as the implicit loss above. They all point to fn = info.getName references, which is their memory addresses
this is lost, that is, the function is called independently, the default binding rule is, this is the global window object
var?
- var will be added to the global window object
- let\const is used, then No, I will introduce the two keywords for declaring variables in detail

2.3 Display binding

However, in some scenarios, the changes in

this are unexpected. In fact, we There is no control over how the callback function is executed, so there is no way to control that the calling location has the expected binding that this points to.

The next display binding can be used to solve this implicit loss problem.

2.3.1 call/apply/bind

“All” functions in js have some useful features. This is related to its prototype chain. Follow-up I will introduce in the prototype, the method of implementing inheritance in disguise through the prototype chain js, among which the three methods

call/apply/bind are the methods on the function prototype chain, and they can be called in the function.

2.3.2 call

call() 方法使用一个指定的 this 值和单独给出的一个或多个参数来调用一个函数。
- 第一个参数为固定绑定的this对象
- 第二个参数以及二以后的参数，都是作为参数进行传递给所调用的函数
备注
- 该方法的语法和作用与 apply() 方法类似，只有一个区别，就是 call() 方法接受的是一个参数列表，而 apply() 方法接受的是一个包含多个参数的数组。

var fullName = 'panpan'

const info = {
  fullName: 'ice',
  getName: function(age, height) {
    console.log(this.fullName, age, height)
  }
}

function bar(fn) {
  fn.call(info, 20, 1.88) //ice 20 1.88
}

bar(info.getName)

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2.3.3 apply

与call的方法类似，只是参数列表有所不同
- 参数
  - call 参数为单个传递
  - apply 参数为数组传递

var fullName = 'panpan'

const info = {
  fullName: 'ice',
  getName: function(age, height) {
    console.log(this.fullName, age, height)
  }
}

function bar(fn) {
  fn.apply(info, [20, 1.88]) //ice 20 1.88
}

bar(info.getName)

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2.3.4 bind

bind与apply/call之间有所不同，bind传入this，则是返回一个this绑定后的函数，调用返回后的函数，就可以拿到期望的this。
参数传递则是
- 调用bind时，可以传入参数
- 调用bind返回的参数也可以进行传参

var fullName = 'panpan'

const info = {
  fullName: 'ice',
  getName: function(age, height) {
    console.log(this.fullName, age, height) //ice 20 1.88
  }
}

function bar(fn) {
  let newFn = fn.bind(info, 20)
  newFn(1.88)
}

bar(info.getName)

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2.4 new绑定

谈到new关键字，就不得不谈构造函数，也就是JS中的 "类"，后续原型篇章在跟大家继续探讨这个new关键字，首先要明白以下几点，new Fn()的时候发生了什么，有利于我们理解this的指向。

创建了一个空对象
将this指向所创建出来的对象
把这个对象的[[prototype]] 指向了构造函数的prototype属性
执行代码块代码
如果没有明确返回一个非空对象，那么返回的对象就是这个创建出来的对象

function Person(name, age) {
  this.name = name
  this.age = age

}

const p1 = new Person('ice', 20)

console.log(p1) // {name:'ice', age:20}

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当我调用new Person()的时候，那个this所指向的其实就是p1对象

3. 绑定优先级

3.1 隐式绑定 > 默认绑定

function bar() {
  console.log(this) //info
}

const info = {
  bar: bar
}

info.bar()

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虽然这边比较有些勉强，有些开发者会认为这是默认绑定的规则不能直接的显示谁的优先级高
但是从另外一个角度来看，隐式绑定，的this丢失以后this才会指向widonw或者undefined，变相的可以认为隐式绑定 > 默认绑定

3.2 显示绑定 > 隐式绑定

var fullName = 'global ice'
const info = {
  fullName: 'ice',
  getName: function() {
    console.log(this.fullName) 
  }
}

info.getName.call(this) //global ice
info.getName.apply(this) //global ice
info.getName.bind(this)() //global ice

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通过隐式绑定和显示绑定的一起使用很明显显示绑定 > 隐式绑定

3.3 bind(硬绑定) > apply/call

function bar() {
  console.log(this) //123
}

const newFn = bar.bind(123)
newFn.call(456)

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3.4 new绑定 > bind绑定

首先我们来说一下，为什么是和bind比较，而不能对call和apply比较，思考下面代码

const info = {
  height: 1.88
}

function Person(name, age) {
  this.name = name
  this.age = age
}

const p1 = new Person.call('ice', 20)

//报错: Uncaught TypeError: Person.call is not a constructor

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new绑定和bind绑定比较

const info = {
  height: 1.88
}

function Person(name, age) {
  this.name = name
  this.age = age
}

const hasBindPerson = Person.bind(info)

const p1 = new hasBindPerson('ice', 20)

console.log(info) //{height: 1.88}

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我们通过bind对Person进行了一次劫持，硬绑定了this为info对象
new 返回的固定this的函数
但是我们发现并不能干预this的指向

3.5 总结

new关键字 > bind > apply/call > 隐式绑定 > 默认绑定

4. 箭头函数 (arrow function)

首先箭头函数是ES6新增的语法

const foo = () => {}

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4.1 箭头函数this

var fullName = 'global ice'

const info = {
  fullName: 'ice',
  getName: () => {
    console.log(this.fullName)
  }
}

info.getName() //global ice

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你会神奇的发现？为什么不是默认绑定，打印结果为ice
其实这是ES6的新特性，箭头函数不绑定this，它的this是上一层作用域，上一层作用域为window
所以打印的结果是 global ice

4.2 箭头函数的应用场景进阶

需求: 在getObjName通过this拿到info中的fullName (值为ice的fullName)

const info = {
  fullName: 'ice',
  getName: function() {
    let _this = this
    return {
      fullName: 'panpan',
      getObjName: function() {
        console.log(this) // obj
        console.log(_this.fullName)
      }
    }
  }
}

const obj = info.getName()
obj.getObjName()

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当我调用 info.getName() 返回了一个新对象
当我调用返回对象的getObjName方法时，我想拿到最外层的fullName，我通过，getObjName的this访问，拿到的this却是obj，不是我想要的结果
我需要在调用info.getName() 把this保存下来，info.getName() 是通过隐式调用，所以它内部的this就是info对象
getObjName是obj对象，因为也是隐式绑定，this必定是obj对象，绕了一大圈我只是想拿到上层作用域的this而已，恰好箭头函数解决了这一问题

const info = {
  fullName: 'ice',
  getName: function() {
    return {
      fullName: 'panpan',
      getObjName: () => {
        console.log(this.fullName)
      }
    }
  }
}

const obj = info.getName()
obj.getObjName()

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