Flatten arbitrary nested array of integers.

flatten.js

var arrAllInts = [[1, 2, [3]], 4];
var arrWithOthers = [
  () => {},
  2,
  [ 4, 'five' ],
  [ Array.prototype, [
    {},
    6
  ]]
];

var oneFour = arrAllInts.reduce(flatten, []);
var twoSix = arrWithOthers.reduce(flatten, []);

console.log(arrAllInts, 'should reduce to [1, 2, 3, 4]', oneFour === [1, 2, 3, 4]);
console.log(arrWithOthers, 'should reduce to [2, 4, 6]', twoSix === [2, 4, 6]);

function flatten (prev, item) {
  if (item && Number.isInteger(item)) {
    prev.push(item);
  }
  
  if (item && Array.isArray(item)) {
    prev = item.reduce(flatten, prev);
  }
  
  return prev;
}