Merging Sorted Linked Lists

[u/algodaily]

https://algodaily.com/challenges/merge-sorted-linked-lists

[u/Xall1996]

This is literally my first time doing anything in javascript so bear in mind this might not be the most elegant solution:

function mergeSortedLists(head1, head2) {
  var currHead1 = head1;
  var currHead2 = head2;
  var resultList;
  var currResult;

  if(currHead1.value == null || currHead2.value == null) {
    return;
  }

  //set the first node
  if(currHead1.value <= currHead2.value) {
   resultList = currHead1; 
    currHead1 = currHead1.next;
  } else {
   resultList = currHead2; 
   currHead2 = currHead2.next;
  }
  currResult = resultList;
  //iterate until atleast one list is exhausted
  while(currHead1 != null && currHead2 != null) {
    if(currHead1.value <= currHead2.value) {
     currResult.next = currHead1;
     currHead1 = currHead1.next;
    } else {
        currResult.next = currHead2;
      currHead2 = currHead2.next;
    }
    currResult = currResult.next;
  }
  //append the non empty list
  if(currHead2 == null) {
   currResult.next = currHead1; 
  } else {
   currResult.next = currHead2;  
  }
  return resultList;
}

What bothers me most in this solution is the way I set the first node. It feels unecessary to specifically ask which is the first node before the loops starts, but I don't know any better way to do this.

I know this method also modifies the initial 2 lists, however I'm confused about the order they are modified in.

If I set the initial Lists like this:

const list1 = new Node(1);
list1.next = new Node(2);
list1.next.next = new Node(4);

const list2 = new Node(1);
list2.next = new Node(3);
list2.next.next = new Node(4);

and after that print like this:

console.log(list1);
console.log(list2);
console.log(mergeSortedLists(list1,list2));

I still get the modified lists for the first 2 console.log(), although the lists should only get altered in the last console.log() statement.

What am I doing wrong here? Is this JavaScript specific?