How to find the kth to last element of a singly linked list

function findKthToLast(head, k) {
  let p1 = head;
  let p2 = head;
  
  for (let i = 0; i < k; i++) {
    if (p1 === null) return null; // Out of bounds
    p1 = p1.next;
  }
  
  while (p1 !== null) {
    p1 = p1.next;
    p2 = p2.next;
  }
  
  return p2;
}
function Node(data) {
  this.next = null;
  this.data = data;
}
Node.prototype.addToTail = function(data) {
  const end = new Node(data);
  let current = this;
  while (current.next !== null) {
    current = current.next;
  }
  current.next = end;
  return end;
};