Because stateless functions don't have a backing instance, you can't attach a ref to a stateless function component. Normally this isn't an issue, since stateless functions do not provide an imperative API. Without an imperative API, there isn't much you could do with an instance anyway. However, if a user wants to find the DOM node of a stateless function component, they must wrap the component in a stateful component (eg. ES6 class component) and attach the ref to the stateful wrapper component.

beforeEach(() => {  
  component = TestUtils.renderIntoDocument(<div><RenderDemo /></div>);
});
it('<RenderDemo /> renders', () => {  
  expect(ReactDOM.findDOMNode(component).querySelector('h1').textContent)
.toBe('Hello World!');
});