回答

class Promise {
  constructor(executor) {
    this.state = 'pending';
    this.value = undefined;
    this.reason = undefined;
    this.onFulfilledCallbacks = [];
    this.onRejectedCallbacks = [];

    let resolve = value => {
      if (this.state === 'pending') {
        this.state = 'fulfilled';
        this.value = value;
        this.onFulfilledCallbacks.forEach(callback => callback(value));
      }
    };

    let reject = reason => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach(callback => callback(reason));
      }
    };

    try {
      // 立即执行函数
      executor(resolve, reject);
    } catch (err) {
      reject(err);
    }
  }

  then(onFulfilled, onRejected) {
    return new Promise((resolve, reject) => {
      if (this.state === 'fulfilled') {
        try {
          let x = onFulfilled(this.value);
          this.handleThenCallback(x, resolve, reject);
        } catch (err) {
          reject(err);
        }
      }

      if (this.state === 'rejected') {
        try {
          let x = onRejected(this.reason);
          this.handleThenCallback(x, resolve, reject);
        } catch (err) {
          reject(err);
        }
      }

      if (this.state === 'pending') {
        this.onFulfilledCallbacks.push(value => {
          try {
            let x = onFulfilled(value);
            this.handleThenCallback(x, resolve, reject);
          } catch (err) {
            reject(err);
          }
        });

        this.onRejectedCallbacks.push(reason => {
          try {
            let x = onRejected(reason);
            this.handleThenCallback(x, resolve, reject);
          } catch (err) {
            reject(err);
          }
        });
      }
    });
  }

  handleThenCallback(x, resolve, reject) {
    if (x instanceof Promise) {
      // 如果 x 是一个 Promise，则等待其状态变更
      x.then(resolve, reject);
    } else {
      // 如果 x 是普通值，则直接将其作为新 Promise 的值进行 resolve
      resolve(x);
    }
  }

  // 可以添加其他方法，如 catch、finally 等
}

// 使用示例
const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    const randomNumber = Math.random();
    if (randomNumber > 0.5) {
      resolve('Success!');
    } else {
      reject('Failure!');
    }
  }, 1000);
});

promise
  .then(
    value => {
      console.log('Resolved:', value);
      return 'New Value';
    },
    reason => {
      console.log('Rejected:', reason);
      throw new Error('New Error');
    }
  )
  .then(
    value => {
      console.log('Resolved:', value);
    },
    reason => {
      console.log('Rejected:', reason.message);
    }
  );