flatten array

flatten.js

/**
 * flattens a nested array
 * 1. takes an array and optional destination array
 * 2. iterates through the array
 * 3. if an item in the array is an array
 * run through flatten with the second argument
 * as the destination array
 * this will add every item within the passed array
 * into the destination array
 * return the new flat array
 * @param  {Array}  arr     - input array
 * @param  {Array}? flatArr - optional output array address
 * @return {Array}  flatArr - output array
 */
function flatten (arr, flatArr) {
	flatArr = flatArr || [];
	
	var length  = arr.length|0;
	
	for (var index = 0; index < length; index = index + 1){
		var item = arr[index];
		item.constructor === Array ? flatten(item, flatArr) : flatArr[flatArr.length] = item;
	}
	
	return flatArr;
}

console.log(flatten([[1,2,[3]],4]));

// http://jsbin.com/yeyolo/edit?js,console