How to apply merge sort on a single linked list

mergesortLinkedList.js

class Node{
  constructor(value){
    this.value = value;
    this.next = null;
  }
}

class LinkedList{
  constructor(value){
    this.head = value ? new Node(value): null;
  }
  insertLast(value){
    if(!value) return;
    let last = this.head;
    if(!last){
      this.head = new Node(value);
    }
    while(last.next !== null){
      last = last.next;
    }
    last.next = new Node(value);
  }
}

const ll = new LinkedList(1);
ll.insertLast(5);
ll.insertLast(3);
ll.insertLast(2);
ll.insertLast(6);

mergeSort(ll.head);
console.log(JSON.stringify(ll));

function getMiddle(node){
  if(!node){
    return node;
  }
  let fastPtr = node.next;
  let slowPtr = node;
  while(fastPtr !== null){
    fastPtr = fastPtr.next;
    if(fastPtr !== null){
      slowPtr = slowPtr.next;
      fastPtr = fastPtr.next;
    }
  }
  return slowPtr;
}

function mergeSort(head){
  if(!head || !head.next){
    return head;
  }
  
  const middle = getMiddle(head);
  const middleRight = middle.next;
  middle.next = null;
  
  const left = mergeSort(head);
  const right = mergeSort(middleRight);
  console.log('left',JSON.stringify(left));
  console.log('right',JSON.stringify(right));
  return sort(left, right);
}

function sort(left, right){
  let result;
  if(!left) return right;
  if(!right) return left;
  if(left.value <= right.value){
    result = left;
    result.next = sort(left.next, right);
  }else{
    result = right;
    result.next = sort(left, right.next);
  }
  return result;
}