I really struggled at first because of the lack of sample tests. I ended up just guessing based on comments that you could new Node() and passed that into loop_size in a test. Then console.logged it to see what I was actually dealing with. That gave me a look at the Node object. From there, the light bulbs started to go on, and I created this really ugly test. My first CW test! :-P

describe("Ugly Test", function(){
  it("LinkedList of 7 nodes connected at 3rd should have a loop link of 5", function(){
    var first = new Node()
    var second = new Node()
    first.setNext(second);    
    var third = new Node()
    second.setNext(third); 
    var fourth = new Node();
    third.setNext(fourth);
    var fifth = new Node();
    fourth.setNext(fifth);
    var sixth = new Node();
    fifth.setNext(sixth);
    var seventh = new Node();
    sixth.setNext(seventh);
    seventh.setNext(third);
    
    var size = loop_size(first);
    Test.assertEquals(size, 5, "The size should be 5");
  });
});