JS function that calls a set of functions that generate promises (called generators) in batches and resolves when they have all completed

queuePromises.jsx

import _ from 'underscore';

/**
 * Takes an array of generators, i.e. functions that return promises, and calls them such that there are only ever
 * 5 requests that are waiting on responses. Returns a promise that resolves to all the resulting promises only after
 * they have all been executed
 *
 * @param generators array of functions that
 * @param batchSize max number of requests to execute at a time
 * @param throttle how often the queue is checked for more requests to process
 */
export default function queuePromises(generators, { batchSize = 5, throttle = 100 } = {}) {
  if (!_.isArray(generators)) {
    return Promise.reject(new Error('Invalid argument generators: must be an array'));
  }

  if (!_.all(generators, fn => typeof fn == 'function')) {
    return Promise.reject(new Error('Invalid set of generators: one or more were not functions'));
  }

  return new Promise(resolve => {
    // this is the array we will shift items off of
    const queued = [ ...generators ],
      // this is the set of promises that have finished
      done = [];

    let inProgress = [];

    // we will throttle how often we push from queued to in progress
    const processMore = _.throttle(() => {
      if (queued.length == 0) {
        resolve(Promise.all(done.concat(inProgress)));
        return;
      }

      // remove the generators we should trigger
      const more = queued.splice(0, batchSize - inProgress.length);

      // trigger the calls and add them to in progress
      inProgress = inProgress.concat(
        more.map(fn => {
          // create the promise
          const prom = fn();
          // when it finishes, push it to the stack of done, remove it from inProgress
          prom.finally(() => {
            done.push(prom);
            inProgress = _.without(inProgress, prom);
            // add more to in progress
            processMore();
          });

          return prom;
        })
      );

    }, throttle);

    processMore();
  });
}