middle_of_the_linked_list_2.js

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} head
 * @return {ListNode}
 */

var middleNode = function(head) {
  let fast = head;
  let slow = head;

  while (fast !== null && fast.next !== null) {
    fast = fast.next.next;
    slow = slow.next.next;
  }

  return slow;
};