parallelFetch.ts

function mockupFetch(cache: object, url: stirng) {
  return new Promise((resolve) => {
    if (cache.hasOwnProperty(url)) {
      return resolve(cache[url]);
    }
    
    setTimeout(() => {
      const date = new Date();
      const min = date.getMinutes();
      const sec = date.getSeconds();
      
      resolve(`${min}:${sec}-${url}`);
    }, 1000);
  });
}

function parallelFetch(urls: string[], limit: number, cb: Function) {
  const cache: object = {};
  
  let workIndex= 0;
  let total = 0;
  
  if (urls.length < limit) {
    limit = urls.length;
  }
  
  function request(index: number) {
    const url = urls[index];

    workIndex++;
    total++;

    mockupFetch(cache, url)
      .then((res) => {
        cache[url] = res;
      
        total--;
      
        if (total < limit && workIndex < urls.length) {
          request(workIndex);
        } else if (total === 0) {
          cb(urls.map((url) => cache[url]));
        }
      });
  }
  
  while (workIndex < limit) {
    request(total);
  }
}

parallelFetch(
  ['a', 'b', 'c', 'a', 'd'],
  1,
  console.log
)