Functional batching

functional_batching.js

// batch([1,2,3,4,5,6,7,8,9,10], 5) -> [[1,2,3,4,5], [6,7,8,9,10]]
const batch = (collection, batchSize) => {
  return collection.reduce((accumulator, value, index) => {
    const newAccumulator = index % batchSize === 0 ?
      [...accumulator, []] :
      accumulator;
    const init = newAccumulator.slice(0, -1);
    const last = newAccumulator.slice(-1)[0];
    
    return [...init, [...last, value]];
  }, []);
}