Merge Two Sorted Lists solution using TypeScript

Below is my TypeScript solution to the LeetCode "Merge Two Sorted Lists" question using a recursive approach.

Time Complexity: Because each node in each of the linked lists will be visited one time, the time complexity is O(m + n), where m represents the number of nodes in the first linked list and n represents the number of nodes in the second linked list.

Space Complexity: This approach will place m + n function calls on the stack, making the space complexity O(m + n).

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     val: number
 *     next: ListNode | null
 *     constructor(val?: number, next?: ListNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.next = (next===undefined ? null : next)
 *     }
 * }
 */

/**
 * Merge two sorted linked lists and return it as a new sorted list. 
 *
 *   Time Complexity: O(m + n)
 *   Space Complexity: O(m + n)
 * 
 *   Input: 1->2->4, 1->3->4
 *   Output: 1->1->2->3->4->4
 */
function mergeTwoLists(l1: ListNode | null, l2: ListNode | null): ListNode | null {
    if (l1 === null) {
        return l2;
    } 
    else if (l2 === null) {
        return l1;
    }
    else if (l1.val < l2.val) {
        l1.next = mergeTwoLists(l1.next, l2);
        return l1;
    } else {
        l2.next = mergeTwoLists(l1, l2.next);
        return l2
    }
}