function quickSort(arr) {
  if (arr.length <= 1) {
    return arr;
  }

  const less = [];
  const greater = [];
  const pivot = arr[arr.length - 1];
  for (let i = 0; i < arr.length - 1; ++i) {
    const num = arr[i];
    if (num < pivot) {
      less.push(num);
    } else {
      greater.push(num);
    }
  }

  return [...quickSort(less), pivot, ...quickSort(greater)];
}

const arr = [9, 4, 1, 6, 7, 3, 8, 2, 5];
quickSort(arr);