用Promise实现前端并发请求

/**
* 构造假请求
*/
async function request(url) {
  return new Promise((resolve) => {
    setTimeout(
      () => {
        resolve(url);
      },
      // Math.random() * 500 + 800,
      1000,
    );
  });
}

请求一次，查看耗时，预计应该是1s：

async function requestOnce() {
  const t1 = Date.now();
  const res = await request(1);
  console.log('一次请求耗时=', Date.now() - t1, 'ms,请求结果=', res);
}
requestOnce();

结果：在这里插入图片描述

12个数据发起并行请求，预计耗时1s：

async function requestAsync() {
  const t2 = Date.now();
  const requests = new Array(12).fill(2).map(request);
  const res2 = await Promise.all(requests);
  console.log('批量请求耗时=', Date.now() - t2, 'ms,请求结果=', res2, ',预计是 1000 ms');
}

requestAsync();

结果：
在这里插入图片描述

12个请求并发进行，但限制最多3个请求可并发，预计耗时4s：

async function concurrencyRequest(urls, maxNum) {
  const buffer = [];//并发请求池，最多有maxNum个
  const t3 = Date.now();
  const res = [];
  for (let i = 0; i < urls.length && buffer.length < maxNum; ++i) {
    // console.log('next=', i);
    const promise = new Promise((resolve) => request(urls[i]).then(resolve));
    buffer.push(promise);
    promise
      .then((response) => (res[i] = response))
      .finally(() => {//每个请求完成后都从buffer中删除并保存响应
        const index = buffer.findIndex((item) => item === promise);
        buffer.splice(index, 1);
        // console.log('删除一个promise', buffer.length);
      });
    if (buffer.length === maxNum) {//必须等待buffer size小于maxNum才能继续请求
      await Promise.race(buffer);
    }
  }
  await Promise.all(buffer);//需要等待最后剩下的几个请求完成才算完成
  console.log('并发请求耗时=', Date.now() - t3, '请求结果=', res, '预计时间是 4000 ms');
}

concurrencyRequest(
  new Array(12).fill(1).map((_, i) => i + 1),
  3,
);