日期: 2020-06-22 17:46:14
本文实例讲述了JS手写一个自定义Promise操作。分享给大家供大家参考,具体如下:
经常在面试题中会看到,让你实现一个Promsie,或者问你实现Promise的原理,所以今天就尝试利用class类的形式来实现一个Promise
为了不与原生的Promise命名冲突,这里就简单命名为MyPromise.
class MyPromise { constructor(executor) { let _this = this this.state = 'pending' // 当前状态 this.value = undefined // 存储成功的值 this.reason = undefined // 存储失败的值 // 利用发布订阅模式,让Promise支持异步 this.onFulfilledFunc = [] // 存储成功的回调 this.onRejectedFunc = [] // 存储失败的回调 function resolve (value) { // Promise对象已经由pending状态改变为了成功态(resolved)或是失败态(rejected)就不能再次更改状态了。因此我们在更新状态时要判断,如果当前状态是pending(等待态)才可更新 if (_this.state === 'pending') { _this.value = value //依次执行成功回调 _this.onFulfilledFunc.forEach(fn => fn(value)) _this.state = 'resolved' } } function reject (reason) { // Promise对象已经由pending状态改变为了成功态(resolved)或是失败态(rejected)就不能再次更改状态了。因此我们在更新状态时要判断,如果当前状态是pending(等待态)才可更新 if (_this.state === 'pending') { _this.reason = reason //依次执行失败回调 _this.onRejectedFunc.forEach(fn => fn(reason)) _this.state = 'rejected' } } try { // 当实例化Promise时,构造函数中就要马上调用传入的executor函数执行 executor(resolve, reject) } catch (error) { reject(error) } } _resolvePromise (promise2, x, resolve, reject) { // 如果返回了自己的Promise对象,状态永远为等待态(pending),再也无法成为resolved或是rejected,程序会死掉,因此首先要处理它 if (promise2 === x) { reject(new TypeError('Promise存在循环引用')) } if (x !== null && (typeof x === 'object' || typeof x === 'function')) { // x可能是一个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 { //否则是个普通值 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 } }
运行测试:
var promise = new MyPromise((resolve, reject) => { console.log(1) setTimeout(() => { resolve(2) }, 1000); console.log(3) }).then(value => console.log(value))
结果真香:
感兴趣的朋友可以使用在线HTML/CSS/JavaScript代码运行工具:http://tools.jb51.net/code/HtmlJsRun测试上述代码运行效果。
更多关于JavaScript相关内容感兴趣的读者可查看本站专题:《javascript面向对象入门教程》、《JavaScript错误与调试技巧总结》、《JavaScript数据结构与算法技巧总结》、《JavaScript遍历算法与技巧总结》及《JavaScript数学运算用法总结》
希望本文所述对大家JavaScript程序设计有所帮助。