手写promise

好的，以下是一个简单的手写 Promise 实现：

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

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

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

    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);
            this.resolvePromise(promise2, x, resolve, reject);
          } catch (error) {
            reject(error);
          }
        }, 0);
      }

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

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

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

    return promise2;
  }

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

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

    let called = false;

    if (x instanceof MyPromise) {
      x.then(value => {
        this.resolvePromise(promise2, value, resolve, reject);
      }, reason => {
        if (called) return;
        called = true;
        reject(reason);
      });
    } else if (x !== null && (typeof x === 'object' || typeof x === 'function')) {
      try {
        const then = x.then;
        if (typeof then === 'function') {
          then.call(x, value => {
            if (called) return;
            called = true;
            this.resolvePromise(promise2, value, resolve, reject);
          }, reason => {
            if (called) return;
            called = true;
            reject(reason);
          });
        } else {
          resolve(x);
        }
      } catch (error) {
        if (called) return;
        called = true;
        reject(error);
      }
    } else {
      resolve(x);
    }
  }

  static resolve(value) {
    if (value instanceof MyPromise) {
      return value;
    }

    return new MyPromise(resolve => resolve(value));
  }

  static reject(reason) {
    return new MyPromise((resolve, reject) => reject(reason));
  }

  static all(promises) {
    return new MyPromise((resolve, reject) => {
      const values = [];
      let count = 0;

      for (let i = 0; i < promises.length; i++) {
        MyPromise.resolve(promises[i]).then(value => {
          values[i] = value;
          count++;

          if (count === promises.length) {
            resolve(values);
          }
        }, reason => {
          reject(reason);
        });
      }
    });
  }

  static race(promises) {
    return new MyPromise((resolve, reject) => {
      for (let i = 0; i < promises.length; i++) {
        MyPromise.resolve(promises[i]).then(value => {
          resolve(value);
        }, reason => {
          reject(reason);
        });
      }
    });
  }
}

这个手写 Promise 实现基本上实现了 Promise/A+ 规范，并支持 then、catch、resolve、reject、all、race 等方法。