答案：手写符合promises/A+规范的Promise需实现状态机、then链式调用与resolvePromise递归解析；包含pending/fulfilled/rejected三种状态，通过resolve/reject函数改变状态，then方法返回新Promise并注册回调，利用queueMicrotask确保异步执行，resolvePromise处理返回值为Promise或thenable的情况，防止循环引用，同时实现catch、finally、resolve、reject等辅助方法。
要手写一个符合 Promises/A+ 规范的 Promise，关键在于理解其核心机制：状态机、异步执行、then 方法的链式调用与穿透，以及 onFulfilled 和 onRejected 的回调调度。下面从零实现一个精简但完全符合规范的 Promise 类。
Promise 核心状态与基本结构
Promise 有三种状态：pending（等待）、fulfilled（成功）、rejected（失败）。一旦状态变更，就不能再改变。
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 (error) { reject(error); } }
实现 then 方法
then 方法是 Promise 的核心，必须返回一个新的 Promise，实现链式调用。它接收两个可选参数：onFulfilled 和 onRejected。
MyPromise.prototype.then = function(onFulfilled, onRejected) { // 处理透传，支持 .then().then(value => ...) onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : value => value; onRejected = typeof onRejected === 'function' ? onRejected : reason => { throw reason; }; const promise2 = new MyPromise((resolve, reject) => { if (this.state === FULFILLED) { queueMicrotask(() => { try { const x = onFulfilled(this.value); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }); } if (this.state === REJECTED) { queueMicrotask(() => { try { const x = onRejected(this.reason); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }); } if (this.state === PENDING) { this.onFulfilledCallbacks.push(() => { queueMicrotask(() => { try { const x = onFulfilled(this.value); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }); }); this.onRejectedCallbacks.push(() => { queueMicrotask(() => { try { const x = onRejected(this.reason); resolvePromise(promise2, x, resolve, reject); } catch (e) { reject(e); } }); }); } }); return promise2; };
处理 then 返回值：resolvePromise 函数
这个函数决定如何处理 onFulfilled/onRejected 的返回值 x。根据 Promises/A+ 规范，x 可能是普通值、Promise 或带有 then 方法的对象（即“thenable”）。
function resolvePromise(promise2, x, resolve, reject) { if (promise2 === x) { return reject(new TypeError('Chaining cycle detected for promise')); } let called = false; if ((typeof x === 'object' && x !== null) || typeof x === 'function') { let then; try { then = x.then; if (typeof then === 'function') { then.call(x, y => { if (called) return; called = true; resolvePromise(promise2, y, resolve, reject); }, r => { if (called) return; called = true; reject(r); }); } else { resolve(x); } } catch (e) { if (called) return; called = true; reject(e); } } else { resolve(x); } }
补充常用方法：catch、finally、resolve、reject
这些是基于 then 的语法糖，增强使用体验。
MyPromise.prototype.catch = function(onRejected) { return this.then(null, onRejected); }; MyPromise.prototype.finally = function(callback) { return this.then( value => MyPromise.resolve(callback()).then(() => value), reason => MyPromise.resolve(callback()).then(() => { throw reason; }) ); }; MyPromise.resolve = function(value) { if (value instanceof MyPromise) return value; return new MyPromise(resolve => resolve(value)); }; MyPromise.reject = function(reason) { return new MyPromise((_, reject) => reject(reason)); };
基本上就这些。这个实现覆盖了 Promises/A+ 的主要测试用例，包括异步回调、链式调用、错误捕获、循环引用检测等。关键是理解 resolvePromise 如何递归解析 thenable，以及 queueMicrotask 确保回调异步执行，模拟原生 microtask 队列行为。