Merge Two Sorted Lists

Problem statement

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]Output: [1,1,2,3,4,4]

Example 2:

Input: list1 = [], list2 = []Output: []

Example 3:

Input: list1 = [], list2 = [0]Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

My solution

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
var mergeTwoLists = function(list1, list2) {
    let sorted = curr = new ListNode();
    
    while (list1 || list2) {
        const l1Val = list1 && list1.val;
        const l2Val = list2 && list2.val;
        
        if (l1Val === l2Val) {
            curr.next = new ListNode(l1Val);
            curr = curr.next;
            curr.next = new ListNode(l2Val);
            curr = curr.next
            
            if (list1) {
                list1 = list1.next;
            }
            
            if (list2) {
                list2 = list2.next
            }
            
        } else if ((l1Val < l2Val && l1Val !== null) || l2Val === null ) {
            curr.next = new ListNode(l1Val);
            curr = curr.next;
            
            if (list1) {
                list1 = list1.next;
            }
        } else if ((l2Val < l1Val && l2Val !== null) || l1Val === null) {
            curr.next = new ListNode(l2Val);
            curr = curr.next;
            
            if (list2) {
                list2 = list2.next
            }
        }
    }
    
    return sorted.next;
};