Flat Array of Integers

FlatArray.js

/**
* Preset: ES2015
* The reduce() method applies a function against an accumulator and
* each value of the array (from left-to-right) to reduce it to a single value.
*
* Compatibility Mobile: ALL
* Compatibility Desktop: 
*  Chrome (Yes) | Firefox 3.0(1.9) | IE 9 | Opera 10.5 | Safari 4.0
*/

const flat = array => array.reduce(
  (a, b) => a.concat( Array.isArray(b) ? flat(b) : b), []
);

flat([[1,2,[3]],4]);

/**
* COMPILED CODE
*/

"use strict";

var flat = function flat(arr) {
  return arr.reduce(function (a, b) {
    return a.concat(Array.isArray(b) ? flat(b) : b);
  }, []);
};

flat([[1, 2, [3]], 4]);