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Detailed explanation of this pointing in javascript

零下一度
零下一度Original
2017-07-24 16:00:411386browse

The pointing of this in JavaScript has always been a worry for front-end colleagues, and it is also the first choice for interview questions. Now we will summarize the pointing of this in js. First, you need to understand a few concepts:

1: Global variables are mounted by default under the window object

2: Under normal circumstances, this points to its call Author

3: In the arrow function of es6, this points to the creator, not the caller

4: You can change the point of this through call, apply, and bind

The following Let’s analyze it in detail

1: When calling a function

　(non-strict mode)

const func = function () {
    console.log(this);
    const func2 = function () {
      console.log(this);
    };
    func2(); //Window
  };
  func(); //Window

　(strict mode) 

'use strict'
  const func = function () {
    console.log(this);
    const func2 = function () {
      console.log(this);
    };
    func2(); //undefined
  };
  func(); //undefined

Combining the fourth and first two rules: the func function is global and is mounted under the window object by default. This points to its caller, window, so the window object is output, but in strict mode Under this, this is not allowed to point to the global variable window, so the output is undefined (func2 points to the global window by default when the function is called directly. In fact, this is a flaw in javascript design. The correct design method is internal function this should be bound to the object corresponding to its outer function. In order to circumvent this design flaw, smart JavaScript programmers came up with the method of variable substitution. By convention, the variable is generally named that. This method will be used next. will be discussed).

2: As an object method

const user = {

    userName: '小张',
    age: 18,
    selfIntroduction: function () {
      const str = '我的名字是:' + this.userName + ",年龄是:" + this.age;
      console.log(str);

      const loop = function () {
        console.log('我的名字是:' + this.userName + ",年龄是:" + this.age);
      };

      loop();   //我的名字是:undefined,年龄是:undefined

    }
  };

  user.selfIntroduction();  //我的名字是:小张,年龄是:18

According to our first rule, this points to his caller, the selfIntroduction() method The caller is user, so inside the selfIntroduction() method this points to its parent object, user, and the reason why the loop method outputs undefined is the JavaScript design flaw I mentioned above. In this case, We usually choose to cache this in the selfIntroduction() method. 

const user = {
    userName: '小张',
    age: 18,
    selfIntroduction: function () {
      const str = '我的名字是:' + this.userName + ",年龄是:" + this.age;
      console.log(str);

      const that=this;

      const loop = function () {
        console.log('我的名字是:' + that.userName + ",年龄是:" + that.age);
      };

      loop();   //我的名字是:小张,年龄是:18

    }
  };

  user.selfIntroduction();  //我的名字是:小张,年龄是:18

At this time, the this point of the loop is ideal. 

const user={

    userName:'小张',
    age:18,
    selfIntroduction:function(){
      const str='我的名字是:'+this.userName+",年龄是:"+this.age;
      console.log(str); 
    }
  };

  const other =user.selfIntroduction;
  other(); //我的名字是:undefined,年龄是:undefined

  const data={
    userName:'小李',
    age:19,
  };
  data.selfIntroduction=user.selfIntroduction;
  data.selfIntroduction(); //我的名字是:小李,年龄是:19

When looking at this code, selfIntroduction() is assigned to the global variable other, and the other() method is called. other is mounted under the global functionwindow object, and there is no window object. These two properties are userName and age, so the output is undefined. The second piece of code declares the data object, which contains the username and age attributes. Remember our second rule. Generally, this points to its caller. Everyone will understand that data is the caller of the selfIntroduction() function. , so the userName and age of data are output.

3: Triggered as an event in html

<body>
  <p id="btn">点击我</p>
</body>
     const btn=document.getElementById(&#39;btn&#39;);
    btn.addEventListener(&#39;click&#39;,function () {
      console.log(this); //<p id="btn">点击我</p>
    })

In this case, the second rule is actually followed. Generally, this points to it. The caller, this points to the event source of the event, that is, event.

4:new keyword (constructor)


const fun=function(userName){
    this.userName=userName;
  }
  const user=new fun(&#39;郭德纲&#39;);  
  console.log(user.userName); //郭德纲

This Without going into details, the new keyword constructs an object instance and assigns it to user, so userName becomes an attribute of the user object.

5:es6 (arrow function) 

const func1=()=>{
    console.log(this); 
  };
  func1(); //Window
const data={
    userName:&#39;校长&#39;,
    selfIntroduction:function(){
      console.log(this); //Object {userName: "校长", selfIntroduction: function}
      const func2=()=>{
        console.log(this); //Object {userName: "校长", selfIntroduction: function}
      }
      func2();
    }
  }
  data.selfIntroduction();

Everyone is looking at the third rule I mentioned at the beginning: the arrow function of es6 , this points to the creator, not the caller. fun1 is created under the global function, so this points to the global window, and fun2 is created under the object data, and this points to the data object, so inside the func2 function, this points to the data object. Personally, I think es6 The this point of the arrow function is an improvement on the JavaScript design flaws I mentioned above (personal knowledge).

#6: Change the point of this

The three functions call, apply, and bind can artificially change the point of this of the function. I won’t go into detail about the differences between the three here. I will explain the differences between the three in detail in future blogs. Now let’s take one as an example 

const func=function(){
   console.log(this);
 }; 
 func(); //window
 func.apply({userName:"郭德纲"}); //Object {userName: "郭德纲"}

These three methods can artificially change the pointer of this. The difference is that call and apply will execute the method immediately after binding this, while the bind method will return An executable function.

To sum up a lot of this, it is the 4 points I mentioned at the beginning

1: Global variables are mounted under the window object by default

2: Generally, this points to its caller

3: In the arrow function of es6, this points to the creator, not the caller

4: Through call, apply, bind You can change the pointer of this

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