Perform async map in batches

batched-map.js

const batchedMap = batchSize => data => async (fn) => {
  const batches = data.length / batchSize;
  let result = [];
  for (let i = 0; i < batches; i++) {
    const res = await Promise.all(data
      .slice(i * batchSize, (i + 1) * batchSize)
      .map((d, j) => fn(d, (i * batchSize) + j)),
    );
    result = result.concat(res);
  }
  return result;
};

// usage
await batchedMap(batchSize)(data)((item, index) => somethingAsync(item, index));