Hello my question will be about data structures in javascript. I try to solve a problem on hackerrank with javascript. merge-two-sorted-list-problem

I can solve that by writing this code

```
function mergeLists(head1, head2) {
let headNode = {
data: 0,
next: null
}
let tail = headNode;
let node1 = head1;
let node2 = head2;
while (true) {
if (!node1) { tail.next = node2; break; }
if (!node2) { tail.next = node1; break; }
if (node1.data <= node2.data) {
let temp = node1;
if (!temp) return;
node1 = node1.next;
temp.next = tail.next;
tail.next = temp;
} else {
let temp = node2;
if (!temp) return;
node2 = node2.next;
temp.next = tail.next;
tail.next = temp;
}
tail = tail.next;
}
return headNode.next;
}
// src -> source node reference
// dest -> tail reference
function moveNode(node, tail) {
let temp = node; // temp taşınacak node
if (!temp) return;
node = node.next;
temp.next = tail.next;
tail.next = temp;
}
```

But when I trying to structure my code better, I try to write function named **addNode** I am getting unexpected behaviour in my code. When I write this code:

```
const SinglyLinkedListNode = class {
constructor(nodeData) {
this.data = nodeData;
this.next = null;
}
};
function mergeLists(head1, head2) {
let headNode = new SinglyLinkedListNode(0);
let tail = headNode;
let node1 = head1;
let node2 = head2;
while (true) {
if (!node1) { tail.next = node2; break; }
if (!node2) { tail.next = node1; break; }
if (node1.data <= node2.data) {
addNode(node1, tail);
} else {
addNode(node2, tail);
}
tail = tail.next;
}
return headNode.next;
}
// node -> source node reference
// tail -> tail reference
function addNode(node, tail) {
let temp = node; // the moving node
if (!temp) return;
node = node.next;
temp.next = tail.next;
tail.next = temp;
}
```

As you can see in the if blocks I simply call the function and add to reference node to that function. When I debug my code node variable refers to this:

After the function finished I expect that node1 variable = node variable inside that function.

I looked at pass by reference and pass by value topics but I do not understan how javascript do this behind the since. And I am really want to imporve myself at data structures with javascript.

All help is really appreciated.

Thank you.

**Furkan**