手写 Promise

当然可以！以下是一个简单的手写Promise的示例：

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

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

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

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

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

    const promise2 = new Promise((resolve, reject) => {
      const fulfillHandlerWrapper = () => {
        setTimeout(() => {
          try {
            const result = fulfilledHandler(this.value);
            resolvePromise(promise2, result, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      };

      const rejectHandlerWrapper = () => {
        setTimeout(() => {
          try {
            const result = rejectedHandler(this.reason);
            resolvePromise(promise2, result, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      };

      if (this.state === 'fulfilled') {
        fulfillHandlerWrapper();
      } else if (this.state === 'rejected') {
        rejectHandlerWrapper();
      } else {
        this.onResolveCallbacks.push(fulfillHandlerWrapper);
        this.onRejectCallbacks.push(rejectHandlerWrapper);
      }
    });

    return promise2;
  }

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

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

  let thenCalledOrThrow = false;

  if (result instanceof Promise) {
    result.then(
      value => resolvePromise(promise, value, resolve, reject),
      reason => reject(reason)
    );
  } else if (result !== null && (typeof result === 'object' || typeof result === 'function')) {
    try {
      const then = result.then;

      if (typeof then === 'function') {
        then.call(
          result,
          value => {
            if (thenCalledOrThrow) return;
            thenCalledOrThrow = true;
            resolvePromise(promise, value, resolve, reject);
          },
          reason => {
            if (thenCalledOrThrow) return;
            thenCalledOrThrow = true;
            reject(reason);
          }
        );
      } else {
        resolve(result);
      }
    } catch (error) {
      if (thenCalledOrThrow) return;
      thenCalledOrThrow = true;
      reject(error);
    }
  } else {
    resolve(result);
  }
}

这只是一个基本的实现，还有很多细节和功能可以改进。希望对你有所帮助！