Merge sort in javascript

mergesort.js

/**

Imagine we have [1, 2] and [3,4]
So using merge_union we will concatenate these arrays and sort them right

merge_union([2,1], [3,4]) ---> [1,2,3,4]
merge_union([2,3,4,5], [-5,-3,100]) ---> [-5, -3, 2, 3, 4, 5, 100]

**/
function merge_union(arr1, arr2) {
  let result = [];
  // i is for arr1 and j is for arr2
  let i = 0; let j = 0;
  /**
    if we are done with one of the arrays, another array could be not empty
    so we use stopped variable to fill result with this "another not empty" array
    And countOut variable is how much elements we've left, so we fill result array with this amount of elements
  **/
  let stopped, countOut;
  /**
    set_vars is for assigning a value to variables above
  **/
  function set_vars(stop) {
    if (stop === 0) {
      countOut = arr2.length - j;
      j = arr2.length;
      stopped = 0;
    } else {
      countOut = arr1.length - i;
      i = arr2.length;
      stopped = 1;
    }
  }


  for (i; i < arr1.length;) {
    for (j; j < arr2.length;) {
      if (arr1[i] < arr2[j]) { // if you want another order, you need to change ">" to "<" and visa versa
        result.push(arr1[i]);
        i += 1;
        // we might be done with arr1 array but arr2 could be not empty
        if (i == arr1.length) set_vars(0);
      } else if (arr1[i] == arr2[j]) {
        result.push(arr1[i], arr1[i]);
        i += 1, j += 1;
        if (i == arr1.length && j == arr2.length) break;
        // we might be done with arr1 (arr2) array but arr2 (arr1) could be not empty
        if (i == arr1.length) set_vars(0);
        else if (j == arr2.length) set_vars(1);
      } else {
        result.push(arr2[j]);
        j += 1;
        // we might be done with arr2 array but arr1 could be not empty
        if (j == arr2.length) set_vars(1);
      }
    }
  }

  if (stopped === undefined) return result; // in case we took every element from both arrays, so everything s ok
  else if (stopped === 0) { // in case we have stopped at the arr1
    result = result.concat(arr2.slice(-countOut));
    return result;
  } else { // in case we have stopped at the arr2
    result = result.concat(arr1.slice(-countOut));
    return result;
  }
}


function merge_sort(arr) {
  if (arr.length <= 1) return arr;
  return merge_union(merge_sort(arr.slice(0, Math.floor(arr.length/2))),merge_sort(arr.slice(-Math.ceil(arr.length/2))));
}