this指向兩句話口訣：

1.this永遠指向函式執行的環境，而不是宣告的環境

2.箭頭函式的this指向父級

一、正常函式的this

function fn1(){
  console.log(this)  
}

fn1() //this===window

fn1.call({x:100}) //this==={x:100}

const fn2 = fn1.bind({x:200})

fn2() //this==={x:200}

二、物件的this和箭頭函式的this

const zhangsna = {
  name:'張三',
  sayHi(){
    console.log(this)  //this是當前物件  
 

  }  
  wait(){
    settimeOut(function(){
      console.log(this)  //this是window
    })  
  }
}

const zhangsna = {
  name:'張三',
  sayHi(){
    console.log(this)  //this是當前物件  
  }  
  wait(){
    settimeOut(()={
      console.log(this)  //this是當前物件
    })  
  }
}

三、建構函式的this

 1 class People {
 2   constructor(name){
 
 3      this.name = name
 4      this.age = age  
 5   }  
 6   sayHi(){
 7      console.log(this)  
 8   }
 9 }
10 
11 let person = new People('張三')
12 person.sayHi()  //person物件