Promise.all 简易实现

Promise.myAll = function(iterable) {
  return new Promise((resolve, reject) => {
    const arr = Array.from(iterable);
    if (arr.length === 0) return resolve([]);
    const res = new Array(arr.length);
    let count = 0;
    arr.forEach((p, i) => {
      Promise.resolve(p)
        .then(val => {
          res[i] = val;
          count++;
          if (count === arr.length) resolve(res);
        })
        .catch(reject);
    });
  });
};