class LinkedListNode {

  constructor(val, next = null) {

    this.val = val;

    this.next = next;

  }

}

​

l1 = new LinkedListNode(8);

l2 = new LinkedListNode(4);

l1.next = l2;

​

// start at head, 8

let head = l1;

// example: 8 -> 4

let newHead = null;

while (head) {

  /* FIRST PASS */

  // store the node to the right

  let nextNode = head.next;

  // nextNode = 4, still 8 -> 4

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

  head.next = newHead;

  // 8 -> null

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

  newHead = head;

  // newHead = 8

  // the previously right-side node is now processed

  head = nextNode;

  // head = 4