Redux - how to keep the reducer state during hot reload

Check the code related to store creation - createStore(). The store must be built outside app.js, otherwise, ot will be FLUSHED each time on each HMR update.

Wrong:

// app.js

import React from 'react';
import ReactDOM from 'react-dom';
import { hot } from 'react-hot-loader';
import { Provider } from 'react-redux';
import { PersistGate } from 'redux-persist/lib/integration/react';
import { AppWidget } from 'containers';
import createStore from 'store/create-store';

const { store, persistor } = createStore(); // <!--- NEW STORE ON HMR, BUG

const rootElement = window.document.getElementById('appWidget');

const render = (Component) => {
  ReactDOM.render(
    <Provider store={store}>
      <PersistGate loading={null} persistor={persistor}>
        <Component />
      </PersistGate>
    </Provider>,
    rootElement,
  );
};

render(process.env.NODE_ENV === 'development' ? hot(module)(AppWidget) : AppWidget);

Correct:

// app.js

import React from 'react';
import ReactDOM from 'react-dom';
import { Provider } from 'react-redux';
import { AppWidget } from 'containers';
import store from 'store/create-store';
import { AppContainer } from 'react-hot-loader';

const rootElement = window.document.getElementById('appWidget');

const render = (Component) => {
  ReactDOM.render(
    <AppContainer>
      <Provider store={store}>
        <Component />
      </Provider>
    </AppContainer>,
    rootElement,
  );
};

render(AppWidget);

if (module.hot) {
  module.hot.accept();
}

// create-store.js

import { applyMiddleware, compose, createStore } from 'redux';
import createSagaMiddleware from 'redux-saga';
import thunk from 'redux-thunk';
import rootReducer from './reducers';
import rootSaga from './sagas';

const doCreateStore = () => {
  const sagaMiddleware = createSagaMiddleware();
  const middleware = [
    thunk,
    sagaMiddleware,
  ];

  const store = createStore(
    rootReducer,
    compose(
      applyMiddleware(...middleware),
    ),
  );

  sagaMiddleware.run(rootSaga);

  return store;
};

export default doCreateStore(); // <!-- CREATE AND RETURN STORE, NO BUG

Assuming Babel 6, you need to do something along this:

import {createStore} from 'redux';
import rootReducer from '../reducers';

export default function configureStore(initialState) {
  const store = createStore(rootReducer, initialState);

  if(module.hot) {
    // Enable Webpack hot module replacement for reducers
    module.hot.accept('../reducers', () => {
      const nextReducer = require('../reducers/index').default;

      store.replaceReducer(nextReducer);
    });
  }

  return store;
}

You can see the approach in action at my redux demo.

Just shape your code like below where you render the app in the root element.

store.js:

export const store = createStore(rootReducer, integrateDevTools)

index.jsx:

This will do the trick.

ReactDOM.render(
  <Provider store={store}>
    <App />
  </Provider>,
  document.getElementById('root'),
)


if (module.hot) {
  module.hot.accept()
}