SOURCE

class MyPromise {
  constructor(executor) {
    this.state = 'pending'; // 初始状态为pending
    this.value = undefined; // 成功时的值
    this.reason = undefined; // 失败时的原因
    this.onFulfilledCallbacks = []; // 成功时的回调函数队列
    this.onRejectedCallbacks = []; // 失败时的回调函数队列

    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);
    }
  }

  then(onFulfilled, onRejected) {
    if (this.state === 'fulfilled') {
      onFulfilled(this.value);
    }
    if (this.state === 'rejected') {
      onRejected(this.reason);
    }

    if (this.state === 'pending') {
      this.onFulfilledCallbacks.push(() => onFulfilled(this.value));
      this.onRejectedCallbacks.push(() => onRejected(this.reason));
    }
  }
}

// 使用示例
const promise = new MyPromise((resolve, reject) => {
  setTimeout(() => {
    resolve('Hello, World!');
    // reject('Error occurred');
  }, 1000);
});

promise.then(
  value => console.log(value), // 输出: Hello, World!
  reason => console.error(reason)
);