array sorting javascript mergesort

// Merge Sort Implentation (Recursive)

function mergeSort (unsortedArray) {
  if (unsortedArray.length <= 1) {
    return unsortedArray;
  }
  
  const middle = Math.floor(unsortedArray.length / 2);

  const left = unsortedArray.slice(0, middle);
  const right = unsortedArray.slice(middle);

  // Using recursion to combine the left and right
  return merge(
    mergeSort(left),
    mergeSort(right)
  );
}

function merge (left, right) {
  let resultArray = [], leftIndex = 0, rightIndex = 0;

  while (leftIndex < left.length && rightIndex < right.length) {
    if (left[leftIndex] < right[rightIndex]) {
      resultArray.push(left[leftIndex]);
      leftIndex++;
    } else {
      resultArray.push(right[rightIndex]);
	  rightIndex++;
    }
  }

  return resultArray
          .concat(left.slice(leftIndex))
          .concat(right.slice(rightIndex));
}

// Merge Sort (Recursive)
function mergeSort (unsortedArray) {
  // No need to sort the array if the array only has one element or empty
  if (unsortedArray.length <= 1) {
    return unsortedArray;
  }
  // In order to divide the array in half, we need to figure out the middle
  const middle = Math.floor(unsortedArray.length / 2);

  // This is where we will be dividing the array into left and right
  const left = unsortedArray.slice(0, middle);
  const right = unsortedArray.slice(middle);

  // Using recursion to combine the left and right
  return merge(
    mergeSort(left), mergeSort(right)
  );
}