js quick sort

/*
 *  Quicksort
 *
 *  Prompt:    Given an unsorted array of integers, return the array
 *             sorted using quicksort.
 *
 *  Input:     input {Array}
 *  Output:    {Array}
 *
 *  Example:   [3,9,1,4,7] --> [1,3,4,7,9]
 */


// Worse Time Complexity: O(n^2)
// Worse Auxiliary Space Complexity: O(n)
// Average Time Complexity: O(nlogn)
// Average Auxiliary Space Complexity: O(logn)
//}}}
function quicksort(input) {

  function divide(start, end) {
    if (start >= end) { return; }

    let pivotIndex = end;
    let pivot = input[pivotIndex];
    let big = start;

    for (let i = start; i < end; i++) {
      if (input[i] < pivot) {
        swap(i, big, input);
        big++;
      }
    }
    swap(big, pivotIndex, input);

    divide(start, big - 1);
    divide(big + 1, end);

  }

  divide(0, input.length - 1);
  return input;
}

function swap(i, j, arr) {
  return [arr[i], arr[j]] = [arr[j], arr[i]];
}