Flattens an array of arbitrarily nested arrays of integers into a flat array of integers

flatten.js

'use strict';

Array.prototype.flatten = function() {
	var flattened = [];

	this.map(function(element) {
		if (Array.isArray(element)) {
			// If the array element is an array,
			// flatten it, then push each element
			// onto the master flattened array.
			element.flatten().map(function(e) {
				flattened.push(e);
			});
		} else {
			flattened.push(element);
		}
	});

	return flattened;
};

var input = [[1,2,[3]],4],
    expected = [1,2,3,4],
    actual = input.flatten();

// Array comparison in Javascript does not behave as expected
// since arrays are really objects
if (!(actual < expected || actual > expected)) {
	console.log('PASS', actual);
} else {
	console.log('FAIL, expected', expected, 'but got', actual);
}