detectLoop.js

class Node {
    constructor(value) {
        this.value = value;
        this.next = null;
    }
}

class LinkedList {
    constructor() {
        this.head = null;
        this.tail = null;
        this.length = 0;
    }

    push(value) {
        const newNode = new Node(value);
        if(!this.head) {
            this.head = newNode;
            this.tail = newNode;
        } else {
            this.tail.next = newNode;
            this.tail = newNode;
        }
        this.length++;
        return this;
    }

    hasLoopInLinkedList() {
        let slow = this.head;
        let fast = this.head;
        while(fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow === fast) {
                return true
            }
        }
        return false;
    }
}

const myLinkedList = new LinkedList();
myLinkedList.push(1);
myLinkedList.push(2);
myLinkedList.push(3);
myLinkedList.push(4);
myLinkedList.push(5);
console.log(myLinkedList.hasLoopInLinkedList());

// Create a loop
myLinkedList.tail.next = myLinkedList.head;
console.log(myLinkedList.hasLoopInLinkedList());