text: import {
    ADD_NUMBER,
    SUB_NUMBER,
    INCREMENT,
    DECREMENT
  } from './constants.js';
  
  // 拆分counterReducer
  const initialCounterState = {
    counter: 0
  };
  function counterReducer(state = initialCounterState, action) {
    switch (action.type) {
      case ADD_NUMBER:
        return { ...state, counter: state.counter + action.num };
      case SUB_NUMBER:
        return { ...state, counter: state.counter - action.num };
      case INCREMENT:
        return { ...state, counter: state.counter + 1 };
      case DECREMENT:
        return { ...state, counter: state.counter - 1 };
      default:
        return state;
    }
  }
  
  export default counterReducer;
  