Limit promise requests.

index.js

/**
 * Created by Oleksandr Tserkovnyi on 11/3/17.
 * kemperomg@gmail.com
 */

const assert = require('assert');

const requester = strToRequest => {
  return new Promise((res, rej) => {
    const plus = Math.random() * 2000;
    setTimeout(() => {
      res(`${strToRequest} with num: ${plus}`);
    }, 1000 + plus);
  });
};

const arr = [
  () => requester('request0'),
  () => requester('request1'),
  () => requester('request2'),
  () => requester('request3'),
  () => requester('request4'),
  () => requester('request5'),
  () => requester('request6'),
  () => requester('request7'),
  () => requester('request8'),
  () => requester('request9'),
  () => requester('request10'),
  () => requester('request11'),
  () => requester('request12'),
  () => requester('request13'),
  () => requester('request14'),
  () => requester('request15'),
  () => requester('request16'),
  () => requester('request17'),
  () => requester('request18'),
  () => requester('request19'),
  () => requester('request20'),
  () => requester('request21'),
  () => requester('request22'),
  () => requester('request23'),
  () => requester('request24'),
  () => requester('request25'),
  () => requester('request26'),
  () => requester('request27'),
  () => requester('request28'),
  () => requester('request29'),
  () => requester('request30'),
  () => requester('request31'),
  () => requester('request32'),
  () => requester('request33'),
  () => requester('request34'),
  () => requester('request35'),
  () => requester('request36'),
  () => requester('request37'),
  () => requester('request38'),
  () => requester('request39'),
  () => requester('request40'),
  () => requester('request41'),
  () => requester('request42'),
  () => requester('request43'),
  () => requester('request44'),
  () => requester('request45'),
  () => requester('request46'),
  () => requester('request47'),
  () => requester('request48'),
  () => requester('request49'),
  () => requester('request50')
];

const isArraySparse = arr => [...arr].indexOf(undefined) > -1;

const scheduler = (req, index, results, currentRequests) => {
  const promise = req();
  currentRequests.push(promise);
  promise.then(data => {
    results[index] = data;
    currentRequests.splice(currentRequests.indexOf(promise), 1);
  });
  return promise;
};

const limitPromiseAll = (arr, limit) => {
  const internalArr = arr.slice(0);
  const results = [];
  const currentRequests = [];
  let lastIndex = 0;

  // [START] JUST FOR TESTING PURPOSES
  // If you want to test assertion, please uncomment
  // setTimeout(() => {
  //   limit = 4;
  // }, 3000);

  const intr = setInterval(() => {
    console.info('>>>> Current amount of requests are: ', currentRequests.length);
    assert.ok(
      currentRequests.length <= limit,
      `
        Amount of requests is bigger than expected.
        
        Current: ${currentRequests.length}
        Expected: ${limit}
        
      `
    );
    currentRequests.length === 0 && clearInterval(intr);
  }, 50);
  // [END] JUST FOR TESTING PURPOSES

  return new Promise((res, rej) => {
    internalArr.splice(0, limit).forEach(function requester (request, i) {
      lastIndex = i;
      request && scheduler(request, lastIndex, results, currentRequests)
        .then(_ => lastIndex += 1)
        .then(_ => requester(internalArr.shift(), lastIndex, results, currentRequests))
        .then(_ => results.length === arr.length && isArraySparse(results) === false && res(results))
        .catch(err => rej(err));
    });
  });
};

limitPromiseAll(arr, 7)
  .then((resCollection) => (console.log(resCollection), resCollection))
  .then((resCollection) => {
    assert.ok(resCollection[37].match(/^request37/), 'Incorrect order 37');
    assert.ok(resCollection[22].match(/^request22/), 'Incorrect order 22');
    assert.ok(resCollection[13].match(/^request13/), 'Incorrect order 13');
    assert.ok(resCollection[2].match(/^request2/), 'Incorrect order 2');
    assert.equal(resCollection.length, arr.length, 'Incorrect result length');
    assert.equal(isArraySparse(resCollection), false, 'NOT all results are with data');
  });