map vs. flatMap

map-vs-flatmap.js

const map = (array, func) => (
  array.map(func)
);

const flatMap = (array, func) => (
  array.reduce((result, element) => (
    result.concat(func(element))
  ), [])
);

const arr = ['two birds', 'three green peas'];
const split = s => s.split(' ');

map(arr, split)
// [["two", "birds"], ["three", "green", "peas"]]

flatMap(arr, split)
// ["two", "birds", "three", "green", "peas"]