手写Promise.all()

下面是手写Promise.all()的实现代码：

function promiseAll(promises) {
  return new Promise((resolve, reject) => {
    let result = [];
    let count = 0;
    for (let i = 0; i < promises.length; i++) {
      Promise.resolve(promises[i]).then(res => {
        result[i] = res;        count++;
        if (count === promises.length) {
          resolve(result);
        }
      }).catch(err => {
        reject(err);
      });
    }
  });
}