old_contents: import {createStore, applyMiddleware} from 'redux';
  import rootReducer from '../reducers';
  import reduxImmutableStateInvariant from 'redux-immutable-state-invariant';
  import thunk from 'redux-thunk';
  
  export default function configureStore(initialState){
    return createStore(
      rootReducer,
      initialState,
      applyMiddleware(thunk, reduxImmutableStateInvariant())
    );
  }
  
new_contents: import { createStore, applyMiddleware, compose } from 'redux';
  import rootReducer from '../reducers';
  import reduxImmutableStateInvariant from 'redux-immutable-state-invariant';
  import thunk from 'redux-thunk';
  
  // In order to use the devtools (https://github.com/gaearon/redux-devtools)
  // we prepare it to enhance the store.
  const devtools = window.devToolsExtension ? window.devToolsExtension() : f => f;
  
  export default function configureStore(initialState) {
    return createStore(
      rootReducer,
      initialState,
      compose(
        applyMiddleware(thunk, reduxImmutableStateInvariant()),
        devtools
      )
    );
  }
  
subject: Enhance store with devtools (#20)