I am having trouble coming up an alternate solution to my mergesort. I think using slice is an expensive operation. I have two implementations, recursive and iterative.

function mergeSortRecursive(arr) { // base case if (arr.length <= 1) {  return arr; }  var leftArr =  arr.slice(0, arr.length/2); var rightArr = arr.slice(arr.length/2); var leftSorted = mergeSortRecursive(leftArr); var rightSorted = mergeSortRecursive(rightArr); // merge subarrays return merge(leftSorted, rightSorted); }  mergeSortRecursive([1,4,5,245,7, 3,3,6,31]);  function merge(left, right) { var result = []; var indexLeft = 0; var indexRight = 0; // while result is not fully populated while(result.length < (left.length + right.length)) {   // if all elements in left have been added, then add remaining  right elements   if (indexLeft === left.length) {     result = result.concat(right.slice(indexRight));   }   // if all elements in right have been added, then add remaining left elements   else if (indexRight === right.length) {    result = result.concat(left.slice(indexLeft));   }   // compare elements in subarrays and add lower of the two to result   else if (left[indexLeft] <= right[indexRight]) {   result.push(left[indexLeft++]);   } else {     result.push(right[indexRight++]);   }  } return result; }    function mergeSortIterative(arr) { // create an array of subarrays with each element var splitArr = arr.map(function(element) {   return [element]; }); // while there is more than one subarray while (splitArr.length > 1) {   var result = [];   // merge adjacent;   for (var i = 0; i < splitArr.length; i+=2) {     // for pairs merge     if (splitArr[i + 1]) result.push(merge(splitArr[i],   splitArr[i+1]));     // for last odd element, just add to results     else result.push(splitArr[i]);   }   // overwrite old splitArr   splitArr = result;   }   return splitArr[0]; }