长处：1.反对链式调用，能够解决回调天堂问题
export default function Promise(executor) { this.PromiseState = 'pending' this.PromiseResult = null this.callbacks = [] const self = this function resolve(data) { if (self.PromiseState !== 'pending') return//不可反复更改状态 self.PromiseState = 'fulfilled' //批改状态 self.PromiseResult = data//后果赋值 //异步函数执行回调函数 setTimeout(()=>{ self.callbacks.forEach(item => { item.onResolved(data) }) }) } function reject(data) { if (self.PromiseState !== 'pending') return self.PromiseState = 'rejected' self.PromiseResult = data setTimeout(()=>{ self.callbacks.forEach(item => { item.onRejected(data) }) }) } try { executor(resolve, reject) } catch (error) { console.log('err' + error) reject(error) } return executor(resolve, reject)}Promise.prototype.then = function (onResolved, onRejected) { const self = this if(typeof onRejected !=='function'){ onRejected = reason =>{ throw reason } } if(typeof onResolved!=='function'){ onResolved = value =>value } return new Promise((resolve, reject) => { function callback(type) { try { let result = type(self.PromiseResult) if (result instanceof Promise) { result.then(v => { resolve(v) }, r => { resolve(r) }) } else { resolve(result) } } catch (e) { reject(e) } } if (this.PromiseState === 'fulfilled') { callback(onResolved) } else if (this.PromiseState === 'rejected') { callback(onRejected) } else if (this.PromiseState === 'pending') { //异步调用时，在扭转状态后执行函数 this.callbacks.push({ onResolved: function () { setTimeout(()=>{ callback(onResolved) }) }, onRejected: function () { setTimeout(()=>{ callback(onRejected) }) } }) } })}Promise.prototype.catch = function(onRejected){ return this.then(undefined,onRejected)}