Solution with comments

//===========    visualization    ====================
Initial state:
l1: { val: 1, next: { val: 2, next: { val: 4, next: null } } }
l2: { val: 1, next: { val: 3, next: { val: 4, next: null } } }
//===========    visualization    ====================

var mergeTwoLists = function(l1, l2) {
  // Step 1: Create a dummy node and initialize l3
  let l3 = {val: 0, next: null};
  const headNode = l3;

//===========    visualization    ====================
l3: { val: 0, next: null } (dummy node)
headNode: { val: 0, next: null } (dummy node)
//===========    visualization    ====================

  // Step 2: Enter the while loop
  while (l1 && l2) {
    // Step 3: Compare the current nodes in l1 and l2
    if (l1.val < l2.val) {
      // Set the next pointer of the current node in l3 to point to the current node in l1
      l3.next = l1;
      // Update the l1 pointer to point to the next node in the l1 linked list
      l1 = l1.next;
    } else {
      // Set the next pointer of the current node in l3 to point to the current node in l2
      l3.next = l2;
      // Update the l2 pointer to point to the next node in the l2 linked list
      l2 = l2.next;

//===========    visualization    ====================
l3: { val: 0, next: { val: 1, next: { val: 3, next: { val: 4, next: null } } } }
l2: { val: 3, next: { val: 4, next: null } }
//===========    visualization    ====================
    }

    // Step 4: Update l3 to point to the next node
    l3 = l3.next;
//===========    visualization    ====================
l3: { val: 1, next: { val: 3, next: { val: 4, next: null } } }
//===========    visualization    ====================
  }



//===========    visualization    ====================
l3: { val: 0, next: { val: 1, next: { val: 1, next: { val: 2, next: { val: 3, next: { val: 4, next: { val: 4, next: null } } } } } } }
//===========    visualization    ====================

  // Step 5: After the loop is finished, connect the remaining nodes from the non-empty input linked list to l3
  l3.next = l1 || l2;

  // Step 6: Return the merged linked list, skipping the dummy node at the beginning
  return headNode.next;
//===========    visualization    ====================
{ val: 1, next: { val: 1, next: { val: 2, next: { val: 3, next: { val: 4, next: { val: 4, next: null } } } } } }
//===========    visualization    ====================
};

Step by Step

Here is the visualization of the process using objects, with the corresponding line numbers from the mergeTwoLists function:

Initial state:

l1: { val: 1, next: { val: 2, next: { val: 4, next: null } } }
l2: { val: 1, next: { val: 3, next: { val: 4, next: null } } }

1. Create a dummy node and initialize l3 (lines 3-4):

l3: { val: 0, next: null } (dummy node)
headNode: { val: 0, next: null } (dummy node)

1. Enter the while loop (line 6), compare values (line 7), and execute the else block (lines 11-13):

    l3.next = l2; (line 12)
    l2 = l2.next; (line 13)
    l3: { val: 0, next: { val: 1, next: { val: 3, next: { val: 4, next: null } } } }
    l2: { val: 3, next: { val: 4, next: null } }
   

2. Update l3 (line 18):

    l3 = l3.next;
    l3: { val: 1, next: { val: 3, next: { val: 4, next: null } } }
   

3. Compare values (line 7), and execute the if block (lines 8-10):

    l3.next = l1; (line 9)
    l1 = l1.next; (line 10)
    l3: { val: 1, next: { val: 1, next: { val: 2, next: { val: 4, next: null } } } }
    l1: { val: 2, next: { val: 4, next: null } }
   

1. Update l3 (line 18):

    l3 = l3.next;
    l3: { val: 1, next: { val: 2, next: { val: 4, next: null } } }
   

2. Continue the loop, updating pointers and connecting nodes in l3:

    l3: { val: 0, next: { val: 1, next: { val: 1, next: { val: 2, next: { val: 3, next: { val: 4, next: { val: 4, next: null } } } } } } }
   

1. The loop ends. Connect the remaining nodes (line 16):

    l3.next = l1 || l2;
   

   In this case, both l1 and l2 are null, so nothing changes.

1. Return the merged linked list without the dummy node (line 20):

    return headNode.next;
   

The function returns the merged linked list:

{ val: 1, next: { val: 1, next: { val: 2, next: { val: 3, next: { val: 4, next: { val: 4, next: null } } } } } }

Visualize

Initial state:
l1: 1 -> 2 -> 4
l2: 1 -> 3 -> 4
l3: 0
--------------------
Current step:
l1: 2 -> 4
l2: 1 -> 3 -> 4
l3: 0 -> 1
--------------------
Current step:
l1: 2 -> 4
l2: 3 -> 4
l3: 0 -> 1 -> 1
--------------------
Current step:
l1: 4
l2: 3 -> 4
l3: 0 -> 1 -> 1 -> 2
--------------------
Current step:
l1: 4
l2: 4
l3: 0 -> 1 -> 1 -> 2 -> 3
--------------------
Final state:
l1: null
l2: 4
l3: 0 -> 1 -> 1 -> 2 -> 3 -> 4

original