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：测试上述代码运行效果。
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