实现一个promiseAll函数

const myPromiseAll = (promiseArr) => {
  let flag = 0;
  let resArr = [];
  return new Promise((resolve, reject) => {
    promiseArr.forEach(p => {
      Promise.resolve(p).then(
        res => {
          resArr.push(res);
          flag++;
          if(flag === promiseArr.length) resolve(resArr);
        },
        err => {
          reject(err);
        }
      );
    });
  });
}