How to test arrow function in React ES6 class

I used arrow function inside of my React component to avoid binding this context, for example my component look like this;

class Comp extends Component {
   _fn1 = () => {}
   _fn2 = () => {}
   render() {
      return (<div></div>);
   }
}

How do I test _fn1 and _fn2 function in my test cases? Because these kind of function did not associated with React component itself, so when I do

 fnStub = sandbox.stub(Comp.prototype, "_fn1");

it is not going work, since _fn did not bind with Comp.prototype. Thus, how can I test those functions in React if I want to create function with arrow syntax? Thanks!