The meaning of javascript callback function

Simply put, callback function: that is, the function to be executed.

(1) Definition of callback function

##

function add(x,y,fn) {
        /*
        * this.x  this.y：属于add类的全局变量的定义
        * */
        this.x=x||1;
        this.y=y||1;

        if(fn){     /*判断是否有回调函数，有的话执行传入的函数（传入参数）*/
            fn(this.x+this.y);
        }
    }

2) The callback function is usually an anonymous function with a return value

##

  add(1,2,function (v) {//回调函数有返回值
        if(v>0){
            alert("result>0")
        }else{
            alert("result<0")
        }
    })

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