class LinkedListNode {

  constructor(val, next = null) {

    this.val = val;

    this.next = next;

  }

}

​

l1 = new LinkedListNode(1);

l2 = new LinkedListNode(2);

l1.next = l2;

​

// we start at head

let head = l1;

let newHead = null;

while (head != null) {

  // store the node to the right to reuse later

  let nextNode = head.next;

  // set the current node's next to point backwards 

  head.next = newHead;

  // store the current node, to be used as the new next later

  newHead = head;

  // the previously right-side node is now processed

  head = nextNode;

}

​

console.log(l2);