Partitioned iterator

iterator.js

const Promise = require('bluebird');

function iterator(entities = [], mapFn, concurrent = 2) {
  const partitionSize = entities.length / concurrent;
  const partitions = [];
  for (let i = 0; i < concurrent; i++) {
    const start = i * partitionSize;
    const entitiesInPartition = entities.slice(start, start + partitionSize);
    partitions.push(Promise.mapSeries(entitiesInPartition, mapFn));
  }

  return Promise.all(partitions);
}

const entities = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14];
const fn = (x) => x * x;
const concurrent = 2;

iterator(entities, fn, concurrent).then(console.log);
/*
  [
    [ 1, 4, 9, 16, 25, 36, 49 ],
    [ 64, 81, 100, 121, 144, 169, 196 ]
  ]
*/