How to determine the start of a circular linked list

function findCycleStart(head) {
  if (!head) return null;
  let turtle = head.next;
  let hare = turtle && turtle.next;
  
  // Phase 1 - cycle detection
  while (hare) {
    if (hare === turtle) break;
    turtle = turtle.next;
    hare = hare.next;
    hare = hare && hare.next;
  }
  
  if (hare === null) return null;
  
  // Phase 2 - identify start of cycle
  turtle = head;
  while (true) {
    if (turtle === hare) return turtle;
    turtle = turtle.next;
    hare = hare.next;
  }
}
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;
};