index.js

module.exports = (function () {
  'use strict';

  /**
   * Sort an array using the merge sort algorithm.
   *
   * @param {function} comparatorFn The comparator function.
   * @param {array} arr The array to sort.
   * @returns {array} The sorted array.
   */
  function mergeSort(comparatorFn, arr) {
    var len = arr.length, firstHalf, secondHalf;
    if (len >= 2) {
      firstHalf = arr.slice(0, len / 2);
      secondHalf = arr.slice(len / 2, len);
      return merge(comparatorFn, mergeSort(comparatorFn, firstHalf), mergeSort(comparatorFn, secondHalf));
    } else {
      return arr.slice();
    }
  }

  /**
   * The merge part of the merge sort algorithm.
   *
   * @param {function} comparatorFn The comparator function.
   * @param {array} arr1 The first sorted array.
   * @param {array} arr2 The second sorted array.
   * @returns {array} The merged and sorted array.
   */
  function merge(comparatorFn, arr1, arr2) {
    var result = [], left1 = arr1.length, left2 = arr2.length;
    while (left1 > 0 && left2 > 0) {
      if (comparatorFn(arr1[0], arr2[0]) <= 0) {
        result.push(arr1.shift());
        left1--;
      } else {
        result.push(arr2.shift());
        left2--;
      }
    }
    if (left1 > 0) {
      result.push.apply(result, arr1);
    } else {
      result.push.apply(result, arr2);
    }
    return result;
  }

  return mergeSort;
})();