setTimeout

var x = 1
var obj = {
    x:2,
    y:function(){
        console.log(this.x)
    }
}
setTimeout(obj.y,1000)

谁调用它，this就指向谁。上面例子是 setTimeout 函数调用了 obj.y 方法，而setTimeout 属于 window。

var num = 0
function Obj(){
    this.num = 1
    this.getNum = function(){
        console.log(this.num)
    }
    this.getNumLater = function(){
        setTimeout(function(){
            console.log(this.num)
        },1000)
    }
}
var obj = new Obj
obj.getNum()      //obj.num    1
obj.getNumLater() //window.num 0

会出现什么问题

为什么要用 var that = this

文章链接

https://www.jianshu.com/p/3e482748369d?from=groupmessage

https://set.sh/post/180206-keyword-this