“TypeError: Cannot set properties of undefined (setting ‘next’)”

Input: Given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

Input & Output Illustration (open for understanding)

var addTwoNumbers = function(l1, l2) {
let carry = 0,
    sum = 0;
let runningNode = new ListNode(0, null);
let headNode = runningNode;

while (l1 !== null || l2 !== null) {

    sum = l1 != null ? l1.val : 0 + l2 != null ? l2.val : 0 + carry;
    carry = 0;
    // Error is in below line it states "TypeError: Cannot set properties of undefined (setting 'next')" 
    runningNode.next = ListNode(sum % 10, null)
    runningNode = runningNode.next;
    //How to fix it?
    if (l1) {
        l1.next;
    }
    if (l2) {
        l2.next;
    }
}

if (carry) {
    runningNode.next = ListNode(carry);
}

return headNode;

};