solution.js

/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
class ListNode {
  constructor(val, next) {
    this.val = (val === undefined ? 0 : val);
    this.next = (next === undefined ? null : next);
  }
}

/**
 * @param {ListNode} list1
 * @param {ListNode} list2
 * @return {ListNode}
 */
var mergeTwoLists = function (list1, list2) {
  if (null === list1) return list2;
  if (null === list2) return list1;

  let dummy = new ListNode(), head = dummy;

  while (list1 && list2) {
    if (list1.val <= list2.val) {
      dummy.next = list1;
      list1 = list1.next;
    } else {
      dummy.next = list2;
      list2 = list2.next;
    }

    dummy = dummy.next;

    dummy.next = (null === list1) ? list2 : list1;
  }

  return head.next;
};