使用class手写一个promise

Promise是一种异步编程的解决方案，可以避免回调地狱的问题。下面是使用class手写一个Promise的示例代码：

class MyPromise {
  constructor(executor) {
    this.status = 'pending'; // Promise的状态
    this.value = undefined; // Promise的结果
    this.reason = undefined; // Promise的错误信息

    const resolve = (value) => {
      if (this.status === 'pending') {
        this.status = 'fulfilled';
        this.value = value;
      }
    };

    const reject = (reason) => {
      if (this.status === 'pending') {
        this.status = 'rejected';
        this.reason = reason;
      }
    };

    try {
      executor(resolve, reject);
    } catch (error) {
      reject(error);
    }
  }

  then(onFulfilled, onRejected) {
    onFulfilled = typeof onFulfilled === 'function' ? onFulfilled : (value) => value;
    onRejected = typeof onRejected === 'function' ? onRejected : (reason) => { throw reason };

    const promise2 = new MyPromise((resolve, reject) => {
      if (this.status === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, );
      } else if (this.status === 'rejected') {
        setTimeout(() => {
          try {
            const x = onRejected(this.reason);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, );
      } else {
        this.onFulfilledCallbacks.push(() => {
          setTimeout(() => {
            try {
              const x = onFulfilled(this.value);
              resolvePromise(promise2, x, resolve, reject);
            } catch (error) {
              reject(error);
            }
          }, );
        });

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

    return promise2;
  }

  catch(onRejected) {
    return this.then(null, onRejected);
  }
}

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

  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 (error) {
      if (called) return;
      called = true;
      reject(error);
    }
  } else {
    resolve(x);
  }
}

以上是一个简单的Promise实现，其中包含了Promise的基本功能，如状态管理、then方法、catch方法等。需要注意的是，Promise的实现并不是一件简单的事情，需要考虑到各种边界情况，才能保证其正确性和稳定性。