Solution: Sum Lists problem for LinkedList (in Typescript)

This post just posts the code for the solution for the sum lists problem on Linkedlist. You can see that question here,

https://leetcode.com/problems/add-two-numbers/

Now onto the code

class LinkedListNode {
    next: LinkedListNode = null;
    data: number;

    constructor(data?: number, next?: LinkedListNode) {
        this.data = data;
        this.next = next;
    }
    appendToTail(d: number) {
        let end = new LinkedListNode(d);
        let n: LinkedListNode = this;
        while (n.next != null) {
        n = n.next;
        }
        n.next = end;
    }
    length(): number {
        var len = 1;
        var current = this.next;
        while (current != null) {
        len += 1;
        current = current.next;
        }
        return len;
    }
}
class LinkedlistService {
    sumLists(l1: LinkedListNode, l2: LinkedListNode, carry: number): LinkedListNode {
        if (l1 == null && l2 == null && carry == 0) {
            return null;
        }
        var result: LinkedListNode = new LinkedListNode();
        var value = carry;
        if (l1 != null) {
            value += l1.data;
        }
        if (l2 != null) {
            value += l2.data;
        }
        result.data = value % 10;

        //recurse
        if (l1 != null || 12 != null) {
            var more: LinkedListNode = this.sumLists(l1 == null ? null : l1.next,
            l2 == null ? null : l2.next,
            value >= 10 ? 1 : 0);
            result.next = more;
        }
        return result;
    }
}