Promise

这个实现会包含Promise的基本功能,包括状态管理、then方法、resolve和reject方法。以下是一个基本的Promise实现:

class MyPromise {
  constructor(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(callback => callback(value));
      }
    };

    const reject = (reason) => {
      if (this.state === 'pending') {
        this.state = 'rejected';
        this.reason = reason;
        this.onRejectedCallbacks.forEach(callback => callback(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.state === 'fulfilled') {
        setTimeout(() => {
          try {
            const x = onFulfilled(this.value);
            resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      }

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

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

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

    return promise2;
  }
}

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

  if (x && (typeof x === 'object' || typeof x === 'function')) {
    let used = false;
    try {
      const then = x.then;
      if (typeof then === 'function') {
        then.call(
          x,
          y => {
            if (used) return;
            used = true;
            resolvePromise(promise2, y, resolve, reject);
          },
          r => {
            if (used) return;
            used = true;
            reject(r);
          }
        );
      } else {
        resolve(x);
      }
    } catch (error) {
      if (used) return;
      used = true;
      reject(error);
    }
  } else {
    resolve(x);
  }
}

这个实现包括以下主要部分：

构造函数：初始化Promise的状态和相关属性。

resolve和reject方法：用于改变Promise的状态和设置结果值。

then方法：返回一个新的Promise，并处理回调函数。

resolvePromise函数：处理Promise的解析过程，确保正确处理各种情况。

这个实现涵盖了Promise的基本功能，但还不包括catch、finally等方法。你可以根据需要进一步扩展这个实现。

要使用这个MyPromise，你可以这样做：

const promise = new MyPromise((resolve, reject) => {
  setTimeout(() => {
    resolve('Success!');
  }, 1000);
});

promise.then(
  value => console.log(value),
  reason => console.error(reason)
);

这个实现应该能够处理大多数基本的Promise用例。