The `concurRequest` function allows concurrent HTTP requests to multiple URLs, returning the results in the original order. It improves efficiency by fetching data from multiple endpoints simultaneously.

concur-request

/**
 * concur request and return result sortby.
 *
 * @param {string[]} urlList - The url which be will requested.
 * @param {number} capacity - The max number of concurRequest.
 * @returns {Promise<any[]>}.
 */
function concurRequest(urlList, capacity) {
  return new Promise((resolve) => {
    if (urlList.length === 0) {
      resolve([]);
      return;
    }

    const results = [];
    let index = 0; // next request
    let count = 0; // current request finished count
    // send request
    async function request() {
      if (index === urlList.length) {
        return;
      }
      const currentIdx = index;
      const url = urlList[index];
      index++;
      console.log(`url: ${url} start fetch`);
      try {
        const resp = await _fetch(url);
        // resp add into results
        results[currentIdx] = resp;
      } catch (error) {
        // err add into results
        results[currentIdx] = `err:${error}`;
      } finally {
        // condition about
        count++;
        if (count === urlList.length) {
          console.log("finish");
          resolve(results);
        }
        request();
      }
    }

    const times = Math.min(capacity, urlList.length);
    for (let index = 0; index < times; index++) {
      request();
    }
  });
}

function getRandomNumber(min, max) {
  return Math.floor(Math.random() * (max - min + 1)) + min;
}

function _fetch(str) {
  return new Promise((resolve, reject) => {
    const needResolve = Math.floor(Math.random() * 100) % 2 === 0;
    const result = str.repeat(2);
    setTimeout(() => {
      if (needResolve) {
        resolve(result);
      } else {
        reject(str);
      }
    }, getRandomNumber(1000, 3000));
  });
}

concurRequest(["1", "2", "3", "4", "5", "6", "7", "8"], 3).then((data) => {
  console.log(data);
});