相信很多人面试的时候都被问过：

你能手写一个Promise么？

肯定可以呀，手写没问题，不要求脚写就ok。

不多逼逼，show you the code。

function Promise(executor) {  this._status = "Pending";  this._subPromiseResolves = [];  this._subPromiseRejects = [];  this._value = undefined;  let resolve = (val) => {    if(this._status !=== "Pending") return    this._status = "Resolved";    this._value = val;    this._subPromiseResolves.forEach((resolve) => {      resolve(val);    })  }  let reject = (reason) => {    if(this._status !=== "Pending") return    this._status = "Rejected";    this._value = reason;    this._subPromiseRejects.forEach((reject) => {      reject(reason);    })  }  executor(resolve, reject);}Promise.resolve = (val) => {  return new Promise((resolve) => {    resolve(val);  })}Promise.reject = (val) => {  return new Promise((resolve, reject) => {    reject(val);  })}Promise.prototype.then = function(onResolved, onRejected) {  let newPromise;  switch(this._status) {    case "Resolved":       if(onResolved) {        newPromise = Promise.resolve(onResolved(this._value));      }else {        //值穿透        newPromise = Promise.resolve(this._value);      }      break;    case "Rejected":      if(onRejected) {        newPromise = Promise.resolve(onRejected(this._value));      }else {        //错误往下冒泡        newPromise = Promise.reject(this._value);      }      break;    case "Pending":      newPromise = new Promise((resolve, reject) => {        //给前一个Promise存一个能触发回调并改变状态的函数        this._subPromiseResolves.push(function(val){          resolve(onResolved(val));        });        this._subPromiseRejects.push(function(val){          reject(onRejected(val));        });      })  }  return newPromise}Promise.prototype.catch = function(onRejected){  return this.then(null, onRejected);}复制代码

ps：这段代码实现了Promise的then、catch、链式写法...等功能，还有挺多的功能没加上，不过相信看懂这段代码的大家都有能力为它添加更多的功能。

**祝大家新年快乐**