index.js

import React from 'react';
import ReactDOM from 'react-dom';
import { Provider } from 'react-redux';
import App from './App';
import './index.css';

import configureStore from './store';

const store = configureStore({ test: "ok" });

store.dispatch({ type: 'TEST' });

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

render(App);


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

store.js

import { createStore } from 'redux';

const testReducer = (state = [], action) => {
    switch (action.type) {
        case 'TEST':
            return {
                ...state,
                cnt: 0
            };
        case 'ADD':
            return {
                ...state,
                cnt: state.cnt + 1
            };
        default:
            return state;
    }
};

export default function configureStore(initialState = {}) {
   const store = createStore(
       testReducer,
       initialState,
       window.__REDUX_DEVTOOLS_EXTENSION__ && window.__REDUX_DEVTOOLS_EXTENSION__()
   );

   return store;
}