function detectCycle(head) {

  let fast = (slow = head);

  while (fast && fast.next) {

    fast = fast.next.next;

    slow = slow.next;

    if (fast === slow) {

      return true;

    }

  }

  return false;

}

​

function Node(val) {

  this.data = val;

  this.next = null;

}

​

const parent = new Node(1);

const child = new Node(2);

parent.next = child;

child.next = parent;

console.log(detectCycle(parent));