I have a quick question about how this linked list merging code works

Im confused on how head.next returns the entire list instead of a next value such l1,l2,dummy .next does in the code below. particularly I’m wondering how head.next returns an entire sorted array and skips the -1 value that was input on the second line.

let mergeTwoLists = function (l1, l2) {
  let dummy = new ListNode(-1);
  let head = dummy;

  while (l1 !== null && l2 !== null) {
    if (l1.val <= l2.val) {
      dummy.next = l1;
      l1 = l1.next;
    } else {
      dummy.next = l2;
      l2 = l2.next;
    }
    dummy = dummy.next;
  }

  if (l1 !== null) {
    dummy.next = l1;
  } else {
    dummy.next = l2;
  }

  return head.next;
};

class ListNode {
  constructor(val = null, next = null) {
    this.val = val;
    this.next = next;
  }
}

Answer

Maybe it helps when visualising how the list is built:

Let the input be a list with values [3, 9] and another with just [4]:

 l1
 ↓
 3 → 9 → null

 l2
 ↓
 4 → null

Before the loop starts a new node is created:

head
 ↓
-1
 ↑
dummy

The loop will make its first iteration, and the if condition is true. First dummmy.next is adapted, which leads to this situation:

head l1
 ↓   ↓
-1 → 3 → 9 → null
 ↑
dummy    

 l2
 ↓
 4 → null

… and then l1 is reassigned a new reference:

head     l1
 ↓       ↓
-1 → 3 → 9 → null
 ↑
dummy    

 l2
 ↓
 4 → null

The last statement in the loop assigns a new reference to dummy:

head     l1
 ↓       ↓
-1 → 3 → 9 → null
     ↑
    dummy    

 l2
 ↓
 4 → null

The loop iterates a second time, and the if condition is now false, so we get in the else block. First dummmy.next is adapted (this breaks the link it had with l1, and so I move the visualisation of l1 and l2):

head     l2
 ↓       ↓
-1 → 3 → 4 → null
     ↑
    dummy    

 l1
 ↓
 9 → null

… and then l1 is reassigned a new reference, in this case it becomes null:

head          l2
 ↓            ↓
-1 → 3 → 4 → null
     ↑
    dummy    

 l1
 ↓
 9 → null

The last statement in the loop assigns a new reference to dummy:

head          l2
 ↓            ↓
-1 → 3 → 4 → null
         ↑
        dummy    

 l1
 ↓
 9 → null

At this stage the loop condition is no longer true (l2 is null), and so the if block that follows the loop is executed. This links dummy.next with the remaining (not null) reference. Again, for sake of the visualisation, I swap the position of l1 and l2:

head         l1
 ↓           ↓
-1 → 3 → 4 → 9 → null
         ↑
        dummy    

 l2
 ↓
null

Now we get to the final statement: return head.next. Notice how head did not ever move away from the new node that was created at the start.

So the returned reference is:

head         l1
 ↓           ↓
-1 → 3 → 4 → 9 → null
     ↑
    returned    

 l2
 ↓
null

Note how head keeps pointing to the node with -1 during the whole execution of this function. The temporary node with value -1 will be garbage collected, as there is no variable referencing it any more once the function has returned (head is a local variable).