[Handwritten question] Implement a batch request function multiRequest(urls, maxNum)

Subject requirements

1. The maximum number of concurrent requests maxNum
2. Whenever a request is returned, a space is left and a new request can be added
3. After all the requests are completed, the results will be printed in the order in the urls

answer

// 模拟请求
function request(url) {
  return new Promise((resolve) => {
    const time = Math.random() * 1000;
    setTimeout(() => resolve(url), time);
  });
}
async function multiRequest(urls, maxNum) {
  let data = urls.map((url, index) => ({ index, url })) // 因为最终要按照顺序输出，所以给每个url追加一个index属性，用于记录在数组中的位置，确保按序输出
  let result = [] // 存放结果的数组
  // 巧用Array.from, length是开辟的数组长度，这个可以控制最大的并发数量。后面回调方法用于存放异步请求的函数
  let promises = Array.from({ length: Math.min(maxNum,data.length) }, () => getChain(data, result))
  // 利用Promise.all并发执行异步函数
  await Promise.all(promises)
  // 通过函数参数接收最终的一个结果
  return result
}

async function getChain(data, res = []) {
  // 利用队列的思想，一个个pop出来执行，只要urls还有，就继续执行
  while (data.length) {
    let one = data.pop()
    try {
      let urlRes = await request(one.url)
      // 结果按照索引顺序存储
      res[one.index] = urlRes
    }
    catch (e) {
      res[one.index] = e
    }
  }
}
// 调用
const urls = ['www.example1.com', 'www.example2.com', 'www.example3.com', 'www.example4.com', 'www.example5.com']
multiRequest(urls, 5).then(finalRes => {
  console.log('done', finalRes)
})