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。
