var addTwoNumbers = function(l1, l2) {
    let result = new ListNode(0);
    let carryOver = 0;
    let current = result;

    while (l1 !== null || l2 !== null || carryOver > 0) {
        let sum = carryOver;

        if (l1 !== null) {
            sum += l1.val;
            l1 = l1.next;
        };

        if (l2 !== null) {
            sum += l2.val;
            l2 = l2.next;
        }

        carryOver = sum >= 10 ? 1 : 0;
        current.next = new ListNode(sum % 10);
        current = current.next;
    }

    return result.next;
};