Promise

回调地狱

        var base = 0;
        var img = new Image();
        img.src = "./images/3-.jpg";
        img.onload = function(){
            base += img.width;
            var img1 = new Image();
            img1.src = "./images/4-.jpg";
            img1.onload = function(){
                base += img1.width;
                var img2 = new Image();
                img2.src = "./images/5.jpg";
                img2.onload = function(){
                    base += img2.width;
                    console.log(base);
                }
            }
        }

使用函数式编程解决回调地狱

        var base = 0;
        init();
        function init(){
            var img = new Image();
            img.onload =  loadHandler;
            img.src = "./images/3-.jpg";
        }

        function loadHandler(e){
            base += this.width;
            if(this.src.indexOf("3-.jpg")>-1){
                this.src = "./images/4-.jpg";
            }else if(this.src.indexOf("4-.jpg")>-1){
                this.src = "./images/5-.jpg";
                console.log(base);
            }
        }

非函数式编程只能使用Promise解决回调地狱

　　

 1         // resolve是成功时要执行的回调函数
 2         // reject是失败时要执行的回调函数
 3         /*  var p=new Promise(function(resolve,reject){
 4              var img=new Image();
 5              img.src="./img/3-.jpg";
 6              img.onload=function(){
 7                  // resolve函数可以带入一个参数
 8                  resolve(img);
 9              }
10              img.onerror=function(){
11                  // reject函数也可以带入一个参数
12                  reject(img.src);
13              }
14          }) */
15 
16 
17         p.then(function(img){
18             // reslove调用
19              console.log(img);
20          },function(src){
21             // reject调用
22              console.log(src);
23          }) 

简单封装下Promise

        class Promise{
            status = "pending";
            constructor(fn){
                console.log(this);
                fn(this.resolve,this.reject.bind(this));
            }
            resolve(result){
                if(this.status !== "pending") return;
                var ids = setTimeout((function(){
                    this.setVal("resolve",result);
                    clearTimeout(ids);
                }).bind(this),0);
            }
            reject(error){
                if(this.status !== "pending") return;
                var ids = setTimeout((function(){
                    this.setVal("reject",error);
                    clearTimeout(ids);
                }).bind(this),0);
            }
            then(_resolve,_reject){
                this._resolve = _resolve;
                this._reject = _reject;
            }
            catch(reject){
                this._reject = _reject;
            }
            setVal(_status,_arg){
                this.status = _status;
                if(_status === "resolve" && this._resolve){
                    this._resolve(_arg);
                }else if(_status === "reject" && this._reject){
                    this._reject(_arg);
                }
            }
        }

        
        function loadImage(){
            return new Promise(function(resolve,reject){
                var img = new Image();
                img.src = "./images/3-.jpg";
                img.onload = function(){
                    resolve(img);
                }
                img.onerror = function(){
                    reject(img.src);
                }
            });
        }
        loadImage().then(function(img){
            console.log(img);
        },function(msg){
            console.log(msg);
        });