如何实现一个符合Promise A+规范的完整Promise库？

实现一个符合 Promise A+ 规范的 Promise 库，需定义 PENDING、FULFILLED、REJECTED 三种状态，构造函数执行 executor 并传入 resolve 和 reject 方法，通过 onFulfilledCallbacks 和 onRejectedCallbacks 存储异步回调；then 方法返回新 Promise，根据当前状态异步执行 onFulfilled 或 onRejected，并调用 resolvePromise 解析返回值 x；resolvePromise 函数处理 x 为 promise 或 thenable 的情况，防止循环引用并确保状态只改变一次；最后补充 MyPromise.resolve、reject、all、race 等静态方法以完善功能。整个实现需严格遵循规范对状态机、异步执行、错误捕获和链式调用的要求。

实现一个符合 Promise A+ 规范的 Promise 库，核心是理解并正确处理状态机、异步解析和 then 方法的链式调用。重点在于遵循规范中对 thenable 处理、状态不可逆变更、异步执行以及错误捕获的要求。

1. 定义基本状态与结构

Promise 有三种状态：pending、fulfilled、rejected，状态只能从 pending 变为 fulfilled 或 rejected，且一旦确定不可更改。

构造函数接收一个 executor 函数，立即执行，并传入 resolve 和 reject 方法：

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. 实现 then 方法

then 是 Promise A+ 的核心，必须返回一个新的 Promise，以支持链式调用。它接收两个可选参数：onFulfilled 和 onRejected。

关键逻辑在处理返回值 x，需调用 resolvePromise 函数进行“解析”：

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MyPromise.prototype.then = function(onFulfilled, onRejected) {
  onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : val => val;
  onRejected = typeof onRejected === 'function' ? onRejected : err => { throw err; };

  const promise2 = new MyPromise((resolve, reject) => {
    if (this.state === FULFILLED) {
      setTimeout(() => {
        try {
          const x = onFulfilled(this.value);
          resolvePromise(promise2, x, resolve, reject);
        } catch (e) {
          reject(e);
        }
      }, 0);
    }

    if (this.state === REJECTED) {
      setTimeout(() => {
        try {
          const x = onRejected(this.reason);
          resolvePromise(promise2, x, resolve, reject);
        } catch (e) {
          reject(e);
        }
      }, 0);
    }

    if (this.state === PENDING) {
      this.onFulfilledCallbacks.push(() => {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      });

      this.onRejectedCallbacks.push(() => {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (e) {
            reject(e);
          }
        }, 0);
      });
    }
  });

  return promise2;
};

3. 实现 resolvePromise 辅助函数

这个函数处理 then 中回调返回的值 x，判断是否为 Promise 或 thenable 对象，决定如何 resolve 新的 promise2。

function resolvePromise(promise2, x, resolve, reject) {
  if (x === promise2) {
    return reject(new TypeError('Chaining cycle detected'));
  }

  let called = false;
  if ((x !== null && typeof x === 'object') || typeof x === 'function') {
    try {
      const 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);
  }
}

4. 补充常用方法

虽然 Promise A+ 只规定了 then，但完整的库通常包含以下静态方法：

例如 all 的实现：

MyPromise.all = function(promises) {
  return new MyPromise((resolve, reject) => {
    const results = [];
    let completedCount = 0;
    const total = promises.length;

    if (total === 0) {
      resolve(results);
      return;
    }

    for (let i = 0; i < total; i++) {
      MyPromise.resolve(promises[i]).then(value => {
        results[i] = value;
        completedCount++;
        if (completedCount === total) {
          resolve(results);
        }
      }, reject);
    }
  });
};

基本上就这些。只要严格按照 Promise A+ 规范处理状态流转、回调调度和链式解析，就能实现一个合规且可用的 Promise 库。测试时建议使用官方 promises-aplus-tests 工具验证兼容性。