reducer.js

const rootReducer = combineReducers({
  counter: (state = 0, action) => {
    switch (action.type) {
    case INCREMENT_COUNTER_IF_ODD:
      if (state % 2 === 1) {
        return state + 1;
      } else {
        return state;
      }
    case INCREMENT_COUNTER:
      return state + 1;
    case DECREMENT_COUNTER:
      return state - 1;
    default:
      return state;
    }
  }
});