Flattens an arbitrary nested arrays of integers

gistfile1.js

/**
 * Flattens an arbitrary nested arrays of integers
 * e.g. [[1,2,[3]],4] -> [1,2,3,4]
 *
 * @param arr The input array
 * @param responseArr The output array we will be filling via recursion
 * @returns {Array} The flattened array
*/
function flattenArray(arr, responseArr) {

	for(var i in arr) {
		if(Array.isArray(arr[i])) {
			flattenArray(arr[i], responseArr);
		} else {
			responseArr.push(arr[i]);
		}
	}
	
	return responseArr;
}

console.log(flattenArray([[1,2,[3]],4], []));
console.log(flattenArray([[[1,2,[3]],4]], []));
console.log(flattenArray([[1],[2,3],[4,5,6]], []));