面试手写 Promise：链式 + 静态方法全实现

用 ES5 语法 实现一个 符合 Promise/A+ 规范 的完整 Promise，附 Promise.all/race/finally 等静态方法，面试手写无压力。

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一、手写核心 Promise（符合 A+）

1. 结构

const PENDING = 'pending';
const FULFILLED = 'fulfilled';
const REJECTED = 'rejected';function MyPromise(executor) {this.state = PENDING;this.value = undefined;this.reason = undefined;this.onFulfilledCallbacks = [];this.onRejectedCallbacks = [];const resolve = value => {if (this.state === PENDING) {this.state = FULFILLED;this.value = value;this.onFulfilledCallbacks.forEach(fn => fn());}};const reject = reason => {if (this.state === PENDING) {this.state = REJECTED;this.reason = reason;this.onRejectedCallbacks.forEach(fn => fn());}};try {executor(resolve, reject);} catch (err) {reject(err);}
}

2. then 方法（链式核心）

MyPromise.prototype.then = function (onFulfilled, onRejected) {onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : val => val;onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err; };return new MyPromise((resolve, reject) => {const handle = () => {try {const result = this.state === FULFILLED? onFulfilled(this.value): onRejected(this.reason);resolvePromise(result, resolve, reject);} catch (err) {reject(err);}};if (this.state === PENDING) {this.onFulfilledCallbacks.push(handle);this.onRejectedCallbacks.push(handle);} else {setTimeout(handle, 0);}});
};function resolvePromise(result, resolve, reject) {if (result instanceof MyPromise) {result.then(resolve, reject);} else {resolve(result);}
}

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二、静态方法全家桶

1. Promise.resolve / reject

MyPromise.resolve = value => new MyPromise(resolve => resolve(value));
MyPromise.reject = reason => new MyPromise((_, reject) => reject(reason));

2. Promise.all（并发等待）

MyPromise.all = function (promises) {return new MyPromise((resolve, reject) => {let count = 0;const results = [];promises.forEach((p, i) => {MyPromise.resolve(p).then(val => {results[i] = val;if (++count === promises.length) resolve(results);},reject);});});
};

3. Promise.race（谁先完成用谁）

MyPromise.race = function (promises) {return new MyPromise((resolve, reject) => {promises.forEach(p => MyPromise.resolve(p).then(resolve, reject));});
};

4. Promise.finally（无论成功失败都执行）

MyPromise.prototype.finally = function (callback) {return this.then(val => MyPromise.resolve(callback()).then(() => val),err => MyPromise.resolve(callback()).then(() => { throw err; }));
};

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三、完整测试用例

// 1. 基本链式
new MyPromise(resolve => resolve(1)).then(val => val + 1).then(val => console.log(val)); // 2// 2. 并发
MyPromise.all([new MyPromise(resolve => setTimeout(() => resolve('a'), 100)),new MyPromise(resolve => setTimeout(() => resolve('b'), 200))
]).then(console.log); // ['a', 'b']