React+Redux - Uncaught Error: Expected the reducer to be a function

const rootReducer = combineReducers({
    products,
    specialPosts
});

const store = createStore( rootReducer, applyMiddleware( thunkMiddleware ));

The initial state is then created automatically from the initial states returned by the individual reducer functions. These individual states can be accessed as state.products and state.specialPosts

The problem was due to rootReducer being imported by "require" (ES5):

var rootReducer = require('./reducers.js');

If you import it via the ES6 method, it will correctly save the rootReducer.js' default automatically into rootReducer as you expected:

import rootReducer from './reducers';

I see you are mixing ES5 (require) and ES6 (import) in that file...I was mixing in my project as well, which is why I ran into this problem. More information can be found here: https://medium.com/@kentcdodds/misunderstanding-es6-modules-upgrading-babel-tears-and-a-solution-ad2d5ab93ce0#.2x2p2dx3m

When you create store first argument must be function that means reduce(()=>[]) parameter

Error:

import {createStore,applyMiddleware} from 'redux';


export default function configureStore(initialState) {
  return createStore(
    [],
    {},
    applyMiddleware()
  );
}

Solution:

import {createStore,applyMiddleware} from 'redux';


export default function configureStore(initialState) {
  return createStore(
    ()=>[],
    {},
    applyMiddleware()
  );
}