手写promise
const PENDING = 'pending'const RESOLVED = 'resolved'const REJECTED = 'rejected'function MyPromise(fn) { // 保留初始化状态 var self = this // 初始化状态 this.state = PENDING // 用于保留 resolve 或者 rejected 传入的值 this.value = null // 用于保留 resolve 的回调函数 this.resolvedCallbacks = [] // 用于保留 reject 的回调函数 this.rejectedCallbacks = [] // 状态转变为 resolved 办法 function resolve(value) { // 判断传入元素是否为 Promise 值，如果是，则状态扭转必须期待前一个状态扭转后再进行扭转 if (value instanceof MyPromise) { return value.then(resolve, reject) } // 保障代码的执行程序为本轮事件循环的开端 setTimeout(() => { // 只有状态为 pending 时能力转变， if (self.state === PENDING) { // 批改状态 self.state = RESOLVED // 设置传入的值 self.value = value // 执行回调函数 self.resolvedCallbacks.forEach((callback) => { callback(value) }) } }, 0) } // 状态转变为 rejected 办法 function reject(value) { // 保障代码的执行程序为本轮事件循环的开端 setTimeout(() => { // 只有状态为 pending 时能力转变 if (self.state === PENDING) { // 批改状态 self.state = REJECTED // 设置传入的值 self.value = value // 执行回调函数 self.rejectedCallbacks.forEach((callback) => { callback(value) }) } }, 0) } // 将两个办法传入函数执行 try { fn(resolve, reject) } catch (e) { // 遇到谬误时，捕捉谬误，执行 reject 函数 reject(e) }}MyPromise.prototype.then = function (onResolved, onRejected) { // 首先判断两个参数是否为函数类型，因为这两个参数是可选参数 onResolved = typeof onResolved === 'function' ? onResolved : function (value) { return value } onRejected = typeof onRejected === 'function' ? onRejected : function (error) { throw error } // 如果是期待状态，则将函数退出对应列表中 if (this.state === PENDING) { this.resolvedCallbacks.push(onResolved) this.rejectedCallbacks.push(onRejected) } // 如果状态曾经凝固，则间接执行对应状态的函数 if (this.state === RESOLVED) { onResolved(this.value) } if (this.state === REJECTED) { onRejected(this.value) }}
手写Promise.all()
Promise.all = function(iterator) { if (!Array.isArray(iterator)) return; let count = 0; let res = []; return new Promise((resolve, reject) => { for(let item of iterator) { Promise.resolve(item) .then(data => { res[count++] = data; if (count === iterator.length) { resolve(res); } }) .catch(e => { reject(e); }) } })}// testlet p1 = Promise.resolve(3);let p2 = 4;let p3 = new Promise(resolve => { setTimeout(resolve, 100, 'lee') // setTimeout的第三个往后参数都是用来作为第一个参数也就是函数的参数,也就是其实是setTimeout(resolve('lee'), 100)});Promise.all([p1, p2, p3]).then(data => { console.log(data);})
手写Promise.race()
Promise.race = function(iterator) { return new Promise((resolve, reject) => { for(let item of iterator) { Promise.resolve(item) .then(data => { resolve(data) }) .catch(e => { reject(e) }) } })}let p1 = new Promise(resolve => { setTimeout(resolve, 105, 'p1 done')})let p2 = new Promise(resolve => { setTimeout(resolve, 100, 'p2 done')})Promise.race([p1, p2]).then(data => { console.log(data); // p2 done})