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this 指向

var number = 2
var obj = {
  number: 4,
  fn1: (function() {
    // 立即执行函数中的this指向window，因为立即执行函数是window调用的
    // var number = undefined;
    this.number *= 2 // this.number = window.number 、2 *= 2 = 4
    number = number * 3 // undefined *= 3 NaN
    var number = 3 //number = 3
    return function() {
      this.number *= 2
      number *= 3
      console.log(number)
    }
  })(),
  db2: function() {
    this.number *= 2
  },
}
var fn1 = obj.fn1
console.log(number)
fn1()
/**
function fn1(){
    var number = 3
    this.number *= 2 ; this.number = window.number = 4*2 = 8;
    number *= 3  ; 3*3 = 9
    console.log(9) ;
}
*/

总结:this几种情况

1.在 DOM 事件下调用 this = DOM;
2.在对象后面跟个. (obj.fn this = obj;) (obj.hello.fn this = obj.hello)
3.全局作用下 this = window
4.函数作用下 this = 当前作用域
5.构造函数中 this. = 构造函数返回的实例
6.自执行函数/定时器 this = window

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