yandex-load-queue.js

const urls = [
  'https://ya.ru',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'https://vk.com',
  'http://twosphere.ru',
  'https://vc.ru',
  'https://javascript-future.io',
  'https://google.com',
  'https://yandex.ru',
  'https://ya.ru',
  'https://ya.ru',
  'https://ya.ru',
  'https://ya.ru',
];

function loadAll (urls, limit, callback) {
  let finished = 0;
  let processing = 0;

  const processed = Object.create( null );
  const processingOps = Object.create( null );
  const queue = [];

  function load (url) {
    if (processing >= limit) {
      return void queue.push( url );
    }
    processing++;

    const onResultBound = onResult.bind( null, url );

    if (url in processed) {
      onResultBound( processed[ url ] );
    } else if (url in processingOps) {
      const promise = processingOps[ url ];
      promise.then( onResultBound );
    } else {
      processingOps[ url ] = fetch(url).then( onResultBound ).catch( onResultBound );
    }
  }

  function onResult (url, result) {
    processed[ url ] = result;
    processingOps[ url ] = null;
    delete processingOps[ url ];
    processing--;
    finished++;

    if (finished >= urls.length) {
      callback( getProcessed() );
    } else {
      next();
    }

    return result;
  }

  function next () {
    if (!queue.length) {
      return;
    }
    load( queue.shift() );
  }

  function getProcessed () {
    return urls.map(url => {
      return processed[ url ];
    });
  }

  for (let i = 0; i < urls.length; ++i) {
    load( urls[i] );
  }
}

loadAll(urls, 3, function (results) {
  console.log( results );
});