Solution2.js

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function(l1, l2) {
    let head = new ListNode(0)
       let cur = head
       let curry = 0
   
       while (true) {
           let sum = curry
           sum += l1 ? l1.val : 0
           sum += l2 ? l2.val : 0
           cur.val = sum % 10
           curry = parseInt(sum / 10)
           if (l1) l1 = l1.next
           if (l2) l2 = l2.next
           if (l1 != null || l2 != null) {
               cur.next = new ListNode(0)
               cur = cur.next
           } else {
               break
           }
       }
       if (curry != 0) {
           cur.next = new ListNode(0)
           cur = cur.next
           cur.val = curry
       }
       return head
   };
   
   var l1 = new ListNode(1)
   l1.next = new ListNode(8)
   
   var l2 = new ListNode(0)
   
   console.log(addTwoNumbers(l1, l2))