wp-calypso/apps/notifications/src/panel/state/index.js

import { applyMiddleware, combineReducers, compose, createStore } from 'redux'; // eslint-disable-line no-restricted-imports
import { thunk as thunkMiddleware } from 'redux-thunk';
import actionMiddleware from './action-middleware';
import { createListenerMiddleware } from './create-listener-middleware';
import notes from './notes/reducer';
import suggestions from './suggestions/reducer';
import ui from './ui/reducer';

const reducer = combineReducers( {
	notes,
	suggestions,
	ui,
} );

/** @see https://github.com/zalmoxisus/redux-devtools-extension */
const composeEnhancers = window.__REDUX_DEVTOOLS_EXTENSION_COMPOSE__ || compose;

const listenerMiddleware = createListenerMiddleware();
const withMiddleware = () =>
	composeEnhancers( applyMiddleware( thunkMiddleware, listenerMiddleware, actionMiddleware() ) )(
		createStore
	);

let store = null;
store = init();

// Note: this function has the unexpected side effect of modifying
// the `store` export. In order to maintain behaviour for consumers,
// it's being kept this way, but beware this is not a pure function.
function init( { customEnhancer } = {} ) {
	const middle = withMiddleware();
	const create = customEnhancer ? customEnhancer( middle ) : middle;

	store = create( reducer );
}

export { store, init };