Reducerのオーソドックスな書き方

configureStore.js

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

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

counter.js

import * as types from '../constants/ActionTypes';

const initialState = {
  value: 0,
};

export default function counter(state = initialState, action) {
  switch (action.type) {
    case types.INCREMENT:
      return { value: state.value + 1 };
    case types.DECREMENT:
      return { value: state.value - 1 };
    default:
      return state;
  }
}

index.js

import { combineReducers } from 'redux';
import counter from './counter';

const rootReducer = combineReducers({
  counter,
});

export default rootReducer;