如何在redux中合并以前的状态和新的状态？

import { combineReducers } from 'redux';

// 旧的状态
const initialState = {
  counter: 0,
  username: 'guest'
};

// reducer函数
function reducer(state = initialState, action) {
  switch (action.type) {
    case 'INCREMENT':
      return {
        ...state, // 浅拷贝原来的状态
        counter: state.counter + 1 // 更新新的状态
      };
    case 'CHANGE_USERNAME':
      return {
        ...state, // 浅拷贝原来的状态
        username: action.payload // 更新新的状态
      };
    default:
      return state;
  }
}

// 合并reducer
const rootReducer = combineReducers({
  app: reducer
});

export default rootReducer;