this

this is the object that the function is a property of

1. 单独的this

In this case,the window object is our global object.
this指的就是 global object，即this = window

 console.log(this)
 //this means window

 function a(){
     console.log(this)
 }
 a()
 
 /*
 this === window
 this is the object that the function is a property of
 That means we're calling window.a(),so,the function A is a property of the window object.
 */

2. 在严格模式中单独的this

"use strict"
console.log(this)

// this means global object,so this = window

 function a(){
     "use strict"
     console.log(this)
 }
 
 // this is undefined

3. 对象方法中的this

(1).gives methods access to their object(properties or methods)

 const obj = {
     name: 'Billy',
     sing(){
         return 'lalala' + this.name
     },
     singAgain(){
         return this.sing() + '!'
     }
 }
 obj.sing()  
 // this means obj's object

(2).execute same code for multiple objects

 function importantPerson(){
     console.log(this.name)
 }
 const name = "Sunny";
 const obj1 = {
     name: 'Cassy',
     importantPerson: importantPerson
 }
 const obj2 = {
     name: 'Jacob',
     importantPerson: importantPerson
 }
 
 importantPerson()
 // Sunny,Sunny is a global variable, this means window,so this.name = Sunny
 
 obj1.importantPerson()
 // Cassy, this means obj1's object,so this.name = Cassy
 obj2.importantPerson()
 // Jacob

Exercise1

 const a = function (){
    console.log('a', this)
    const b = function () {
        console.log('b', this)
        const c = {
            hi: function(){
                console.log('c', this)
            }
        }
        c.hi()
    }
    b()
 }

 a()
 
 /* a.this = window  because window.a()
    b.this = window  because window.a(b())
    c.this = c's object  because c called hi()
    */

Exercise2

 const obj = {
    name: 'Billy',
    sing(){
        console.log('a',this);
        var anotherFunc = function(){
            console.log('b',this)
        }
        anotherFunc()
    }
 }
 obj.sing()
 
 /*
  a.this is obj's object
  b.this is window
  注：It matters how the function was called
 */

解决嵌套函数this指向问题 (1).Arrow functions

 const obj = {
    name: 'Billy',
    sing(){
        console.log('a',this);
        var anotherFunc = () => {
            console.log('b',this)
        }
        anotherFunc()
    }
 }
 obj.sing()
 
 /* a.this is obj's object
    b.this is also obj's object */

(2).bind(this)

const obj = {
  name: 'Billy',
  sing(){
      console.log('a',this);
      var anotherFunc = function() {
          console.log('b',this)
      }
      return anotherFunc.bind(this)
  }
}
obj.sing()()

/* a.this is obj's object
  b.this is also obj's object */

(3).outsie of the function self,create a reference

const obj = {
   name: 'Billy',
   sing(){
       console.log('a',this);
       var self = this;
       var anotherFunc = () => {
           console.log('b',self)
       }
       anotherFunc()
   }
}
obj.sing()

/* a.this is obj's object
   b.this is also obj's object */