Process promise-based tasks in batches

batched.js

const splitArrayIntoBatches = (arr, limit) => arr.reduce((memo, item) => {
  if (memo.length && memo[memo.length - 1].length < limit) {
    memo[memo.length - 1].push(item);
    return memo;
  }
  return [...memo, [item]];
}, []);


const batched = (tasks, concurrency) => {
  const processBatches = (batches, prevResults = []) => (
    (!batches.length)
      ? Promise.resolve(prevResults)
      : Promise.all(batches[0].map(fn => fn()))
        .then(batchResults => processBatches(batches.slice(1), [
          ...prevResults,
          ...batchResults,
        ]))
  );

  return processBatches(splitArrayIntoBatches(tasks, concurrency));
};