code: function sort (arr, lo, hi, sortFn) {
      if (hi === lo) { return arr; }
  
      const mid = lo + Math.floor((hi - lo) / 2);
      sort(arr, lo, mid, sortFn);
      sort(arr, mid + 1, hi, sortFn);
      merge(arr, lo, mid, hi, sortFn);
  
      return arr;
  }
docs: The helper function for merge sort which would be called
  recursively for sorting the array halves.
  
  @param {Array} arr - The target array which needs to be sorted.
  @param {number} lo - The starting index of the array half.
  @param {number} hi - The ending index of the array half.
  @param {Function} sortFn - The sort function.
  @return {Array} Returns the target array itself.