app.js

import {hot} from 'react-hot-loader';

export default hot(module)(
    <Provider store={store}>
      <Router>
        <div>This is a test.</div>
      </Router>
    </Provider>
);

index.js

import {render} from 'react-dom';
import App from './app.js';

render(
  <App/>,
  document.getElementById('root')
);