Batch promises in javascript

batch-promise.js

/**
 * Same as Promise.all(items.map(item => task(item))), but it waits for
 * the first {batchSize} promises to finish before starting the next batch.
 *
 * Usage:
 * const results = await batchPromise(STUFF_TO_DO, 5, async (ITEM_TO_DO) => {
 *   console.log('Fetching', ITEM_TO_DO.url);
 *   return await fetch(ITEM_TO_DO.url);
 * });
 *
 * @template A
 * @template B
 * @param {A[]} items Arguments to pass to the task for each call.
 * @param {int} batchSize
 * @param {function(A): B} task The task to run for each item.
 * @returns {B[]}
 */
export default async function batchPromise(items, batchSize, task) {
  let position = 0;
  let results = [];
  while (position < items.length) {
    const itemsForBatch = items.slice(position, position + batchSize);
    results = [
      ...results,
      ...await Promise.all(
        // eslint-disable-next-line no-loop-func
        itemsForBatch.map((item, index) => task(item, position + index)),
      )];
    position += batchSize;
  }
  return results;
}