根据标准，我们知道，对于如下代码，promise2的值取决于then里面函数的返回值：
promise2 = promise1.then(function(value) { return 4 }, function(reason) { throw new Error('sth went wrong') })如果promise1被resolve了，promise2的将被`4` resolve，如果promise1被reject了，promise2将被`new Error('sth went wrong')` reject，更多复杂的情况不再详述。
3、完整的promise
try { module.exports = Promise } catch (e) {} function Promise(executor) { var self = this self.status = 'pending' self.onResolvedCallback = [] self.onRejectedCallback = [] function resolve(value) { if (value instanceof Promise) { return value.then(resolve, reject) } setTimeout(function() { // 异步执行所有的回调函数 if (self.status === 'pending') { self.status = 'resolved' self.data = value for (var i = 0; i < self.onResolvedCallback.length; i++) { self.onResolvedCallback[i](value) } } }) } function reject(reason) { setTimeout(function() { // 异步执行所有的回调函数 if (self.status === 'pending') { self.status = 'rejected' self.data = reason for (var i = 0; i < self.onRejectedCallback.length; i++) { self.onRejectedCallback[i](reason) } } }) } try { executor(resolve, reject) } catch (reason) { reject(reason) } } function resolvePromise(promise2, x, resolve, reject) { var then var thenCalledOrThrow = false if (promise2 === x) { return reject(new TypeError('Chaining cycle detected for promise!')) } if (x instanceof Promise) { if (x.status === 'pending') { //because x could resolved by a Promise Object x.then(function(v) { resolvePromise(promise2, v, resolve, reject) }, reject) } else { //but if it is resolved, it will never resolved by a Promise Object but a static value; x.then(resolve, reject) } return } if ((x !== null) && ((typeof x === 'object') || (typeof x === 'function'))) { try { then = x.then //because x.then could be a getter if (typeof then === 'function') { then.call(x, function rs(y) { if (thenCalledOrThrow) return thenCalledOrThrow = true return resolvePromise(promise2, y, resolve, reject) }, function rj(r) { if (thenCalledOrThrow) return thenCalledOrThrow = true return reject(r) }) } else { resolve(x) } } catch (e) { if (thenCalledOrThrow) return thenCalledOrThrow = true return reject(e) } } else { resolve(x) } } Promise.prototype.then = function(onResolved, onRejected) { var self = this var promise2 onResolved = typeof onResolved === 'function' ? onResolved : function(v) { return v } onRejected = typeof onRejected === 'function' ? onRejected : function(r) { throw r } if (self.status === 'resolved') { return promise2 = new Promise(function(resolve, reject) { setTimeout(function() { // 异步执行onResolved try { var x = onResolved(self.data) resolvePromise(promise2, x, resolve, reject) } catch (reason) { reject(reason) } }) }) } if (self.status === 'rejected') { return promise2 = new Promise(function(resolve, reject) { setTimeout(function() { // 异步执行onRejected try { var x = onRejected(self.data) resolvePromise(promise2, x, resolve, reject) } catch (reason) { reject(reason) } }) }) } if (self.status === 'pending') { // 这里之所以没有异步执行，是因为这些函数必然会被resolve或reject调用，而resolve或reject函数里的内容已是异步执行，构造函数里的定义 return promise2 = new Promise(function(resolve, reject) { self.onResolvedCallback.push(function(value) { try { var x = onResolved(value) resolvePromise(promise2, x, resolve, reject) } catch (r) { reject(r) } }) self.onRejectedCallback.push(function(reason) { try { var x = onRejected(reason) resolvePromise(promise2, x, resolve, reject) } catch (r) { reject(r) } }) }) } } Promise.prototype.catch = function(onRejected) { return this.then(null, onRejected) } Promise.deferred = Promise.defer = function() { var dfd = {} dfd.promise = new Promise(function(resolve, reject) { dfd.resolve = resolve dfd.reject = reject }) return dfd }
四、promise的常用方法是如何实现
1、Promise.resolve / Promise.reject 实现
// 原生的Promise.resolve使用 Promise.resolve('hello swr').then((data)=>{ // 直接把成功的值传递给下一个then console.log(data) // hello swr }) // 那么Promise.resolve内部是怎么实现的呢？ Promise.resolve = function(value){ return new Promise((resolve,reject)=>{ // 在内部new一个Promise对象 resolve(value) }) } // 同理，Promise.reject内部也是类似实现的 Promise.reject = function(reason){ return new Promise((resolve,reject)=>{ reject(reason) }) }