控制并发请求

实现方式：

function concurrentPoll(promiseList, limit) {
  const restPromiseList = Array.prototype.slice.call(promiseList);
  const length = promiseList.length;
  const result = [];

  let handling = 0; // handling count
  let resolved = 0; // resolved count

  return new Promise((resolve, reject) => {
    // Core method
    // trigger request function
    const run = () => {
      if (restPromiseList.length === 0) return;

      const index = length - restPromiseList.length;
      const fn = restPromiseList.shift();

      handling++;
      Promise.resolve(fn())
        .then((res) => {
          result[index] = res; // save response data
        })
        .catch((err) => {
          console.log(`error:`, err);
        })
        .finally(() => {
          handling--;
          resolved++;
          if (resolved === length) {
            resolve(result); // all settled, return result
          } else {
            run(); // recursion
          }
        });
    };

    for (let i = 0; i < limit; i++) {
      run();
    }
  });
}

测试代码：

function mockHttpRequestList(delayList) {
  return delayList.map((delay, index) => () =>
    new Promise((resolve, reject) => {
      console.log('run: ', index, delay);
      setTimeout(() => {
        console.log('resolved:', index, delay);
        resolve(index);
      }, delay);
    })
  );
}

const httpRequestList = mockHttpRequestList([200, 20, 100, 300, 600, 30]);
// test
concurrentPoll(httpRequestList, 3);