prottle.js

const EventEmitter = require('events');

module.exports = (limit, arr) => {
  limit = parseInt(limit);

  if (isNaN(limit) || limit <= 0) {
    return Promise.reject(Error('Limit must be at least 1'));
  }

  if (!Array.isArray(arr)) {
    return Promise.reject(Error('Array of promises required'));
  }

  const promises = [].concat(arr);
  const results = [];
  const emitter = new EventEmitter();

  const run = (batch, id) => {
    Promise.all(batch.map(
      (fn) => Promise.resolve(fn())
    ))
      .then((res) => {
        results.push.apply(results, res);
        emitter.emit(`end:${id}`);
      })
      .catch((err) => {
        emitter.emit('error', err);
      });
  };

  let add = (batch, id) => {
    if (id === 0) {
      return run(batch, id);
    }

    emitter.once(`end:${id-1}`, () => run(batch, id));
  };

  let idx = -1;
  while (promises.length) {
    add(
      promises.splice(0, limit),
      ++idx
    );
  }

  return new Promise((resolve, reject) => {
    emitter
      .once(`end:${idx}`, () => {
        resolve(results);
      })
      .once('error', err => {
        emitter.removeAllListeners();
        reject(err);
      });
  });
};