Promise

async-pool-es6.js

function asyncPool(concurrency, iterable, iteratorFn) {
  let i = 0;
  const ret = []; // // Store all asynchronous tasks
  const executing = new Set(); // Stores executing asynchronous tasks
  const enqueue = function() {
    if (i === iterable.length) {
      return Promise.resolve();
    }
    const item = iterable[i++]; // Get the new task item
    const p = Promise.resolve().then(() => iteratorFn(item, iterable));
    ret.push(p);
    executing.add(p);
    const clean = () => executing.delete(p);
    p.then(clean).catch(clean);
    let r = Promise.resolve();
    if (executing.size >= concurrency) {
      r = Promise.race(executing);
    }
    // The new to-do task will be obtained from the array only
    // after the faster task in the executing task list is completed.
    return r.then(() => enqueue());
  };
  return enqueue().then(() => Promise.all(ret));
}