Quick sort javascript

quickSort.js

/**
 * Quick sort algorithm
 * Complexity: O(n log n) to O(n) - depends on a pivot selection
 */
function quickSort(arr) {
    var pivotIndex,
        pivot,
        left = [],
        right = [],
        len = arr.length;

    if (len < 2) {
        return arr;
    } else {
        pivotIndex = Math.floor(Math.random() * len);
        pivot = arr[pivotIndex];

        // left from pivot
        for (var i = 0; i < pivotIndex; i++) {
            var elem = arr[i];
            if (elem > pivot) {
                right.push(elem);
            } else {
                left.push(elem);
            }
        }

        // right from pivot
        for (var i = pivotIndex + 1; i < len; i++) {
            var elem = arr[i];
            if (elem > pivot) {
                right.push(elem);
            } else {
                left.push(elem);
            }
        }

        return quickSort(left).concat(pivot, quickSort(right));
    }
}

// tests
console.log(quickSort([5, 4, 1, 6, 19])); // [1, 4, 5, 6, 19]
console.log(quickSort([5, 6, 1, 6, 19, 0, -15])); // [-15, 0, 1, 5, 6, 6, 19]
console.log(quickSort([5])); // [5]
console.log(quickSort([])); // []