HMR create-react-app

index.js

/*
 * index.js
 */

import React from 'react';
import ReactDOM from 'react-dom';
import App from 'App';
import { Provider } from 'react-redux';
import configureStore from 'state/store'; // see store.js in the gist below
import { BrowserRouter } from 'react-router-dom';
import registerServiceWorker from 'registerServiceWorker';

const store = configureStore();

registerServiceWorker();

const render = Component => {
  return ReactDOM.render(
    <Provider store={store}>
      <BrowserRouter>
        <Component />
      </BrowserRouter>
    </Provider>,
    document.getElementById('root')
  );
};

render(App);

if (module.hot) {
  module.hot.accept('./App', () => {
    const NextApp = require('./App').default;
    render(NextApp);
  });
}

store.js

/*
 * store.js
 */

import { createStore } from 'redux';
import { reducer } from 'state/nav/reducers'; // aka this is your rootReducer

const reduxDevTools =
  window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__();

const configureStore = () => {
  const store = createStore(reducer, reduxDevTools);

  if (process.env.NODE_ENV !== 'production') {
    if (module.hot) {
      module.hot.accept('./nav/reducers', () => {
        store.replaceReducer(reducer);
      });
    }
  }

  return store;
};

export default configureStore;