本文實(shí)例講述了JS手寫一個(gè)自定義Promise操作。分享給大家供大家參考，具體如下：

經(jīng)常在面試題中會(huì)看到，讓你實(shí)現(xiàn)一個(gè)Promsie,或者問(wèn)你實(shí)現(xiàn)Promise的原理，所以今天就嘗試?yán)胏lass類的形式來(lái)實(shí)現(xiàn)一個(gè)Promise

為了不與原生的Promise命名沖突，這里就簡(jiǎn)單命名為MyPromise.

class MyPromise { constructor(executor) { let _this = this this.state = ’pending’ // 當(dāng)前狀態(tài) this.value = undefined // 存儲(chǔ)成功的值 this.reason = undefined // 存儲(chǔ)失敗的值 // 利用發(fā)布訂閱模式，讓Promise支持異步 this.onFulfilledFunc = [] // 存儲(chǔ)成功的回調(diào) this.onRejectedFunc = [] // 存儲(chǔ)失敗的回調(diào) function resolve (value) { // Promise對(duì)象已經(jīng)由pending狀態(tài)改變?yōu)榱顺晒B(tài)（resolved）或是失敗態(tài)（rejected）就不能再次更改狀態(tài)了。因此我們?cè)诟聽(tīng)顟B(tài)時(shí)要判斷，如果當(dāng)前狀態(tài)是pending（等待態(tài)）才可更新 if (_this.state === ’pending’) { _this.value = value //依次執(zhí)行成功回調(diào) _this.onFulfilledFunc.forEach(fn => fn(value)) _this.state = ’resolved’ } } function reject (reason) { // Promise對(duì)象已經(jīng)由pending狀態(tài)改變?yōu)榱顺晒B(tài)（resolved）或是失敗態(tài)（rejected）就不能再次更改狀態(tài)了。因此我們?cè)诟聽(tīng)顟B(tài)時(shí)要判斷，如果當(dāng)前狀態(tài)是pending（等待態(tài)）才可更新 if (_this.state === ’pending’) { _this.reason = reason //依次執(zhí)行失敗回調(diào) _this.onRejectedFunc.forEach(fn => fn(reason)) _this.state = ’rejected’ } } try { // 當(dāng)實(shí)例化Promise時(shí)，構(gòu)造函數(shù)中就要馬上調(diào)用傳入的executor函數(shù)執(zhí)行 executor(resolve, reject) } catch (error) { reject(error) } } _resolvePromise (promise2, x, resolve, reject) { // 如果返回了自己的Promise對(duì)象，狀態(tài)永遠(yuǎn)為等待態(tài)（pending），再也無(wú)法成為resolved或是rejected，程序會(huì)死掉，因此首先要處理它 if (promise2 === x) { reject(new TypeError(’Promise存在循環(huán)引用’)) } if (x !== null && (typeof x === ’object’ || typeof x === ’function’)) { // x可能是一個(gè)promise try { let then = x.then if (typeof then === ’function’) { then.call(x, (y) => { _resolvePromise(promise2, y, resolve, reject) }) } else { resolve(x) } } catch (err) { reject(err) } } else { //否則是個(gè)普通值 resolve(x) } } then (onFulfilled, onRejected) { let promise2 onFulfilled = typeof onFulfilled === ’function’ ? onFulfilled : function (val) { return val } onRejected = typeof onRejected === ’function’ ? onRejected : function (reason) { throw reason } if (this.state === ’resolved’) { promise2 = new MyPromise((resolve, reject) => { setTimeout(() => { try { let x = onFulfilled(this.value) this._resolvePromise(promise2, x, resolve, reject) } catch (error) { reject(error) } }, 0); }) } if (this.state === ’rejected’) { promise2 = new MyPromise((resolve, reject) => { setTimeout(() => { try { let x = onRejected(this.reason) this._resolvePromise(promise2, x, resolve, reject) } catch (error) { reject(error) } }, 0); }) } if (this.state === ’pending’) { promise2 = new MyPromise((resolve, reject) => { this.onFulfilledFunc.push(() => { setTimeout(() => { try { let x = onFulfilled(this.value) this._resolvePromise(promise2, x, resolve, reject) } catch (error) { reject(error) } }, 0); }) this.onRejectedFunc.push(() => { setTimeout(() => { try { let x = onRejected(this.reason) this._resolvePromise(promise2, x, resolve, reject) } catch (error) { reject(error) } }, 0); }) }) } return promise2 }}

運(yùn)行測(cè)試：

var promise = new MyPromise((resolve, reject) => { console.log(1) setTimeout(() => { resolve(2) }, 1000); console.log(3)}).then(value => console.log(value))

結(jié)果真香：

感興趣的朋友可以使用在線HTML/CSS/JavaScript代碼運(yùn)行工具：http://tools.jb51.net/code/HtmlJsRun測(cè)試上述代碼運(yùn)行效果。

更多關(guān)于JavaScript相關(guān)內(nèi)容感興趣的讀者可查看本站專題：《javascript面向?qū)ο笕腴T教程》、《JavaScript錯(cuò)誤與調(diào)試技巧總結(jié)》、《JavaScript數(shù)據(jù)結(jié)構(gòu)與算法技巧總結(jié)》、《JavaScript遍歷算法與技巧總結(jié)》及《JavaScript數(shù)學(xué)運(yùn)算用法總結(jié)》

希望本文所述對(duì)大家JavaScript程序設(shè)計(jì)有所幫助。