Promise 代表了一个异步操作的最终实现或者失败
定义状态
const PENDING = "pending";const FULFILLED = "fulfilled";const REJECTED = "rejected";构造函数定义
function MyPromise(excutor) { // 初始状态为 pending this.status = PENDING; // 胜利的值 this.value = undefined; // 失败的起因 this.reason = undefined; // then 的回调函数汇合 this.fulfilledCallback = []; this.rejectedCallback = []; const resolve = (value) => { if (this.status === PENDING) { this.status = FULFILLED; this.value = value; setTimeout(() => { // 状态变更后执行 then 的回调函数 this.fulfilledCallback.forEach((fn) => fn(value)); }); } }; const reject = (reason) => { if (this.status === PENDING) { this.status = REJECTED; this.reason = reason; setTimeout(() => { this.rejectedCallback.forEach((fn) => fn(reason)); }, 0); } }; try { excutor(resolve, reject); } catch (error) { reject(error); }}原型办法
MyPromise.prototype.then = function (onResolve, onReject) { (onResolve = onResolve === undefined ? (value) => value : onResolve), (onReject = onReject === undefined ? (error) => { throw error; } : onReject); return new MyPromise((resolve, reject) => { function handle(func, value) { try { const result = func(value); // 如果返回一个 Promise if (result instanceof MyPromise) { result.then(resolve, reject); } else { resolve(result); } } catch (err) { reject(err); } } if (this.status === FULFILLED) { setTimeout(() => { handle(onResolve, this.value); }); } if (this.status === REJECTED) { setTimeout(() => { handle(onReject, this.reason); }); } // 状态为 pending，将回调函数保留到后面定义的函数汇合中 if (this.status === PENDING) { // 保留到 fulfilledCallback this.fulfilledCallback.push((value) => { handle(onResolve, value); }); // 保留到 rejectedCallback this.rejectedCallback.push((reason) => { handle(onReject, reason); }); } });};MyPromise.prototype.catch = function (onReject) { return this.then(undefined, onReject);};MyPromise.prototype.finally = function (cb) { return this.then( (value) => MyPromise.resolve(cb()).then(() => value), (reason) => MyPromise.resolve(cb()).then(() => { throw reason; }) );};静态方法
MyPromise.resolve = function (value) { // 如果参数是MyPromise实例，间接返回这个实例 if (value instanceof MyPromise) return value; return new MyPromise((resolve) => resolve(value));};MyPromise.reject = function (value) { // 如果参数是MyPromise实例，间接返回这个实例 if (value instanceof MyPromise) return value; return new MyPromise((resolve, reject) => reject(value));};MyPromise.race = function (promises) { return new Promise((resolve, reject) => { promises.forEach((p) => { MyPromise.resolve(p).then(resolve, reject); }); });};MyPromise.all = function (promises) { let result = []; const len = promises.length; return new MyPromise((resolve, reject) => { promises.forEach((p, index) => { MyPromise.resolve(p).then( (res) => { result[index] = res; if (result.length === len) { resolve(result); } }, (error) => { reject(error); } ); }); });};MyPromise.allSettled = function (promises) { let result = []; const len = promises.length; return new MyPromise((resolve, reject) => { promises.forEach((p, index) => { MyPromise.resolve(p).then( (value) => { result[index] = { status: FULFILLED, value, }; if (result.length === len) { resolve(result); } }, (reason) => { result[index] = { status: REJECTED, reason, }; if (result.length === len) { resolve(result); } } ); }); });};