Merge Two Sorted Lists

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

function mergeTwoLists(list1: ListNode | null, list2: ListNode | null): ListNode | null {
  let head = new ListNode(0);
  let tail = head;

  while (list1 !== null || list2 !== null) {
    if (list1 === null) {
      tail.next = new ListNode(list2.val);
      tail = tail.next;
      list2 = list2.next !== null ? list2.next : null;
    } else if (list2 === null) {
      tail.next = new ListNode(list1.val);
      tail = tail.next;
      list1 = list1.next !== null ? list1.next : null;
    } else {
      if (list1.val <= list2.val) {
        tail.next = new ListNode(list1.val);
        tail = tail.next;
        list1 = list1.next !== null ? list1.next : null;
      } else if (list2.val < list1.val) {
        tail.next = new ListNode(list2.val);
        tail = tail.next;
        list2 = list2.next !== null ? list2.next : null;
      }
    }
  }
  const result = head.next;
  head.next = null;
  return result;
}