MergeSort in JS

mergeSort.js

function mergeSort (arr, st, end) {
  var res;

  st = st || 0;
  end = end || arr.length;
  if(end - st > 1) {
    var mid = st + Math.round((end - st)/2);
    var a1 = mergeSort(arr, st, mid);
    var a2 = mergeSort(arr, mid, end);
    res = merge(a1, a2);
  } else {
    res = arr.slice(st, end);
  }
  return res;
}

function merge (arr1, arr2) {
  var res = [];
  var p1 = p2 = 0;
  while(p1 < arr1.length && p2 < arr2.length) {
    if(arr1[p1] < arr2[p2]) {
      res.push(arr1[p1++]);
    } else {
      res.push(arr2[p2++]);
    }
  }

  while(p1 < arr1.length) {
    res.push(arr1[p1++]);
  }

  while(p2 < arr2.length) {
    res.push(arr2[p2++]);
  }

  return res;
}

console.log(mergeSort([9,3,1,6,65,32,2,1,1,4,9]));