Test-Spy Implementation
What do you think about my implementation? Is is done in a basically correct way?


Yeah who is to say what is "correct"? I mostly have worked with the chaiJS spies API and it feels like a basic version of that library. 


  I'm I using the ES6-features (Classes, Rest, Spread) right?


While there isn't much to the code, I would say it looks like those features are used correctly. You could consider using more class features like getters and setters



In the constructor, there exists this line:


this.result = null;



but the result property doesn't appear to be used anywhere. What is that for?