https://leetcode.com/problems/middle-of-the-linked-list/

middleOfALinkedList.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, slow] = [head, head];
    
    while(fast && fast.next){
        slow = slow.next;
        fast = fast.next.next;
    }
    return slow;
};