Modify contents of Linked List - Recursive approach

<script>

      // JavaScript program to modify the contents
      // of the linked list with recursion
      // Represents a Node of the linked list
      class Node {
        constructor() {
          this.data = 0;
          this.next = null;
        }
      }

      // Function to create and return a new node
      function insert(data) {
        var temp;
        temp = new Node();
        temp.data = data;
        temp.next = null;
        return temp;
      }

      // Function which returns the total number
      // of node present in the list
      function getTotalNodeCount(head) {
        var totalNodesCount = 0;

        while (head != null) {
          totalNodesCount++;
          head = head.next;
        }
        return totalNodesCount;
      }

      var temp = null;

      // Function to modify the contents
      // of the given linked lists
      function modifyContents(second_half) {
        if (second_half == null) {
          return;
        }

        modifyContents(second_half.next);

        temp.data = second_half.data - temp.data;
        temp = temp.next;

        return;
      }

      // Wrapper function which calculates the starting
      // point for second half of the list
      function modifyContentsWrapper(head) {
        // pointer to second half of list
        var ptr = null;
        temp = null;
        var diff = 0;

        // number of nodes in the list
        var length = 0;

        if (head == null) {
          return null;
        }
        length = getTotalNodeCount(head);

        // If link list has odd number of nodes,
        // then pointer for second half starts
        // from (length of link list + 1).
        // Say for an example, for the list 10.4.5.3.6
        // pointer should start from 3.
        // If list has even number of nodes,
        // 10.4.5.9.6.8, pointer starts from 9.
        diff =
          length % 2 == 0 ? 
          parseInt(length / 2) : parseInt(length / 2) + 1;

        ptr = head;

        while (diff-- > 0) ptr = ptr.next;

        temp = head;

        modifyContents(ptr);

        return head;
      }

      // Function to print the contents
      // of the linked list
      function print(nod) {
        if (nod == null) {
          return;
        }

        document.write(nod.data);

        if (nod.next != null) document.write(" -> ");

        print(nod.next);
      }

      // Driver code
      var head = insert(2);
      head.next = insert(9);
      head.next.next = insert(8);
      head.next.next.next = insert(12);
      head.next.next.next.next = insert(7);
      head.next.next.next.next.next = insert(10);

      // Modify the linked list
      head = modifyContentsWrapper(head);

      // Print the modified linked list
      print(head);
      
</script>