Run promises in parallel, applying the concurrency limit

parallel-promises.js

**
 * Run Promises in parallel with a concurrency limit
 * @param args - List of arguments to be passed to 'fn'
 * @param fn - A function to apply to each argument. MUST return a Promise
 * @param concurrency - Allowed amount of Promises to be run in parallel
 * @returns {Promise} - A Promise, which eventually resolved with a list of results.
 * Order corresponds to the list of arguments.
 */
function mapPromises(args, fn, concurrency = 1) {
  if (!args.length) return Promise.resolve([]);
  const argsCopy = [].concat(args.map((val, ind) => ({ val, ind })));
  const result = new Array(args.length);
  const promises = new Array(concurrency).fill(Promise.resolve());

  function chainNext(p) {
    if (argsCopy.length) {
      const arg = argsCopy.shift();
      return p.then(() => {
        const operationPromise = fn(arg.val).then(r => { result[arg.ind] = r; });
        return chainNext(operationPromise);
      });
    }
    return p;
  }

  return Promise.all(promises.map(chainNext)).then(() => result);
}