promise.all,promise手写

Promise.all是一个方法，它接收一个promise对象组成的数组作为参数，当数组中所有的promise对象都变为fulfilled状态时，Promise.all返回一个新的promise对象，其状态为fulfilled，并将所有promise对象的结果组成一个数组作为该promise对象的结果。当数组中有一个promise对象变为rejected状态时，Promise.all返回一个新的promise对象，其状态为rejected，并将第一个被rejected的promise对象的结果作为该promise对象的结果。下面是一个使用Promise.all的例子：

const promise1 = Promise.resolve(1);
const promise2 = Promise.resolve(2);
const promise3 = Promise.resolve(3);

Promise.all([promise1, promise2, promise3])
  .then(values => {
    console.log(values); // [1, 2, 3]
  });

手写Promise的实现需要遵循Promise/A+规范，主要包括以下几个部分：

1. Promise构造函数：接收一个executor函数作为参数，executor函数接收两个函数参数resolve和reject，分别表示promise对象的状态变为fulfilled和rejected时的回调函数。
2. then方法：接收两个函数参数onFulfilled和onRejected，分别表示promise对象的状态变为fulfilled和rejected时的回调函数。then方法返回一个新的promise对象。
3. resolve方法：接收一个参数value，如果value是一个promise对象，则返回该promise对象；否则返回一个新的promise对象，其状态为fulfilled，并将value作为该promise对象的结果。
4. reject方法：接收一个参数reason，返回一个新的promise对象，其状态为rejected，并将reason作为该promise对象的结果。

下面是一个简单的手写Promise的实现：

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

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

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

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

    return promise2;
  }

  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;
            this.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);
    }
  }

  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 results = [];
      let count = 0;

      promises.forEach((promise, index) => {
        MyPromise.resolve(promise).then(value => {
          results[index] = value;
          count++;

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