rootreducer.js

/* src/store/rootReducer.js */
import { combineReducers } from "redux";

const exampleReducer = (state = { counter: 0 }, action) => {
  switch (action.type) {
    case TEST_INCREMENT_ACTION: {
      return {
        ...state,
        counter: state.counter + action.payload
      };
    }
    default:
      return state;
  }
};

export const rootReducer = combineReducers({
  exampleReducer
});