如何控制异步请求并发数

function batchRequest(urls, max = 6) {
  // 请求并发数
  const results = [];
  let together = new Array(max).fill(null);
  let index = 0;
  // 生成promise任务 使用定时器模拟
  const tasks = urls.map((item, index) => {
    return function () {
      return new Promise(function (resolve, reject) {
        // 使用定时器 模拟请求成功和失败的结果
        setTimeout(() => {
          Math.random() > 0.5
            ? resolve({ status: "fulfilled", value: "成功", index: index + 1 })
            : reject({ status: "rejected", reason: "失败", index: index + 1 });
        }, Math.random() * 1000);
      });
    };
  });
  together = together.map(() => {
    return new Promise((resolve, reject) => {
      const run = function run() {
        // 如果索引大于等于任务数量表示 全部请求已执行完成
        if (index >= tasks.length) {
          resolve();
          return;
        }
        const old_index = index; // 0
        // 获取当前执行任务
        console.log("请求" + old_index);
        const task = tasks[index++];
        // 如果当前任务执行完成则执行下一个任务
        task()
          .then((result) => {
            console.log("result", result);
            results[old_index] = result;
          })
          .catch((reason) => {
            console.log("result", reason);
            results[old_index] = reason;
          })
          .finally(() => {
            run();
          });
      };
      run();
    });
  });
  return Promise.all(together).then(() => results);
}

// 测试
 
const urls = Array.from({ length: 10 }).map((v, i) => `url_${i + 1}`);
 
batchRequest(urls).then((res) => {
 
  console.log(res); // 响应结果数组
 
});