quicksort in Javascript

gistfile1.js

/**
 * @param {Array} array .
 * @return {Array} A sorted array.
 */
function quicksort(array) {
  if (array.length === 0) return array;

  var pivot = array.splice(0, 1)[0],
      left = [],
      right = [],
      target;

  while (array.length > 0) {
    target = array.splice(0, 1)[0];
    if (target <= pivot) {
      left.push(target);
    } else {
      right.push(target);
    }
  }
  return quicksort(left).concat(pivot, quicksort(right));
}



/**
 * @param {string} string .
 * @return {string} A sorted string.
 */
function quicksortForString(string) {
  if (string.length === 0) return string;

  var pivot = string[0],
      left = '',
      right = '',
      target;
  string = string.substr(1);

  while (string.length > 0) {
    target = string[0];
    string = string.substr(1);
    if (target <= pivot) {
      left += target;
    } else {
      right += target;
    }
  }
  return quicksortForString(left) + pivot + quicksortForString(right);
}



var assert = require('assert');
assert.deepEqual(quicksort(
    [2, 5, 7, 4, 7, 8, 3, 1, 3, 8, 9, 6, 7, 8, 6, 7, 2, 999, 1, 2, 3, 5]),
    [1, 1, 2, 2, 2, 3, 3, 3, 4, 5, 5, 6, 6, 7, 7, 7, 7, 8, 8, 8, 9, 999]);
assert.deepEqual(quicksort([]), []);
assert.strictEqual(quicksortForString(
    'The quick brown fox jumps over the lazy dog'),
    '        Tabcdeeefghhijklmnoooopqrrstuuvwxyz',
    'Fail at String.');
assert.strictEqual(quicksort(''), '');