promise封装

众所周知promise是一个解决异步编程，也就是回调地狱的解决方案，他有三种状态：1.resolve（已成功），2.reject（已失败），3.pendding（进行中）。
那他是怎么用的呢？

    let p = new Promise((resolve,reject) => {
       setTimeout(() => {
            resolve(123);
            reject(456);
        },1000)
    })
    p.then(res => {
        console.log(res);
    }).catch(err => {
        console.log(err);
    })

然后咱们就用原生方法去实现一个promise

    class MyPromise{
        constructor(callback){
            this.status = 'pendding';
            this.result = null;
            this.err = null;
            this.resolveFn = null;
            this.rejectFn = null;
            this.resolve = function(val){
                if(this.status === 'pendding'){
                    this.status = 'resolve';
                    this.result = val;
                    this.resolveFn && this.resolveFn(this.result);
                }
            }
            this.reject = function(err){
                if(this.status === 'pendding'){
                    this.status = 'reject';
                    this.err = err;
                    this.rejectFn && this.rejectFn(this.err);
                }
            }
            callback(this.resolve.bind(this),this.reject.bind(this));
          }
        then(callback){
            if(this.status === 'resolve' ){
                callback(this.result);
            }else{
                this.resolveFn = callback;
            }
            return this;
        }
        catch(callback){
            if(this.status === 'reject' ){
                callback(this.err);
            }else{
                this.rejectFn = callback;
            }
        }  
    }

现在去用一下：

      let p = new MyPromise((resolve,reject) => {
        setTimeout(() => {
            resolve(123);
            reject(456);
          },1000)
      })
    p.then(res => {
        console.log(res);
    }).catch(err => {
        console.log(err);
    })

结果请自行打印。此方法我已试验，暂无问题，欢迎来怼！