实现一个最大个数的并发请求函数

function multiRequest(urls = [], max) {
  const len = urls.length;
  const result = Array(len).fill(null);
  let count = 0;
  return new Promise((resolve, reject) => {
    while (count < max) {
      next();
    }
    function next() {
      let current = count++;
      if (current >= len) {
        !result.includes(null) && resolve(result);
        return;
      }
      let url = urls[current];
      fetch(url)
        .then(res => {
          result[current] = res;
        })
        .catch(err => {
          result[current] = err;
        })
        .finally(() => {
          if (current < len) {
            next();
          }
        });
    }
  });
}