Flatten

flatten.js

var _ = require('lodash');

var concat = function (arr) {
	var args = arguments,
	len = args.length,
	ArrayTypeOf = "[object Array]",
	toString = Object.prototype.toString,
	push = Array.prototype.push;
 
	for (var i = 1; i < len; i++) {
		if (toString.call(args[i]) === ArrayTypeOf) {
			push.apply(arr, args[i]);
		} else {
			arr.push(args[i]);
		}
	}
	return arr;
};

Array.prototype.concat = function(arr) {
	return concat(this, arr);
}

function stupidFln(arr) {
	let result = [];
	_.forEach(arr, (a) => result = result.concat(_.isArray(a) ? stupidFln(a) : a));
	return result;
}

Array.prototype.stupidFln = function() {
	return stupidFln(this);
}

const oneLinerFln = (arr) => arr.reduce((a, b) => a.concat(Array.isArray(b) ? oneLinerFln(b) : b), [])

Array.prototype.oneLinerFln = function() {
	return oneLinerFln(this);
}

const A = [[1,2,[[3,[4]]],5]];
const expected = [1,2,3,4,5];

console.assert(_.isEqual(expected, A.stupidFln()) && _.isEqual(expected, A.oneLinerFln()), "DON'T TOUCH TESTS, FIX YOUR CODE");