```
var addTwoNumbers = function(l1, l2, overflow) {
overflow = overflow ? overflow : 0;
if (l1 === null && l2 === null && !overflow)
return null;
let val = ((l1 ? l1.val : 0) + (l2 ? l2.val : 0) + overflow);
let newNode = new ListNode(val%10);
newNode.next = addTwoNumbers(l1 ? l1.next : null, l2 ? l2.next : null, Math.floor(val/10))
return newNode;
};
```