1 var addTwoNumbers = function(l1, l2) { 2     var sum = l1.val + l2.val, 3         l3 = new ListNode(sum % 10), 4         node = l3; 5  6     l1 = l1.next; 7     l2 = l2.next; 8     sum = Math.floor(sum / 10); 9 10     while (l1 || l2 || sum != 0) {11         sum += (l1 ? l1.val : 0) + (l2 ? l2.val : 0);12         node.next = new ListNode(sum % 10);13         node = node.next;14         l1 = l1 ? l1.next : null;15         l2 = l2 ? l2.next : null;16         sum = Math.floor(sum / 10);17     }18 19     return l3;20 };