I wrote a helper to limit Q promise concurrency.
If I have a promise-returning function promiseSomething, writing

promiseSomething = PromiseScheduler.limitConcurrency(promiseSomething, 5);

ensures no more than 5 promises are pending at the same time.

I'm coming to JS from .NET world, and I welcome critique of this code.
It seems to work fine, but I wonder if it can be made shorter / more concise / plain better.

One of the thing worth remembering is that Q always schedules continuations on the next tick.

'use strict';

var q = require('q');

 * Constructs a function that proxies to promiseFactory
 * limiting the count of promises that can run simultaneously.
 * @param promiseFactory function that returns promises.
 * @param limit how many promises are allowed to be running at the same time.
 * @returns function that returns a promise that eventually proxies to promiseFactory.
function limitConcurrency(promiseFactory, limit) {
  var running = 0,

  function scheduleNextJob() {
    if (running < limit) {
      return q();

    if (!semaphore) {
      semaphore = q.defer();

    return semaphore.promise

  function processScheduledJobs() {

    if (semaphore && running < limit) {
      semaphore = null;

  return function () {
    var _this = this,
        args = arguments;

    function runJob() {
      return promiseFactory.apply(_this, args);

    return scheduleNextJob()

module.exports = {
  limitConcurrency: limitConcurrency

1 Answer 1


Your code looks good to me. Promises/A are a great pattern - however it can be hard to grasp in the beginning, I personally still got problems in understanding the Q library and Promises/A spec to wrap my head around

Your implementation looks familiar to control flow patterns presented here: http://book.mixu.net/node/ch7.html

Here is another nice presentation on Promises and control flow issues: http://trevorburnham.com/presentations/flow-control-with-promises/#/

and here is a list of other Promise/A spec implementations:




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