在reducers中设置redux的初始状态

// 初始状态
const initialState = {
  counter: 0,
  loggedIn: false,
  user: null
};

// reducer函数
const rootReducer = (state = initialState, action) => {
  switch (action.type) {
    case 'INCREMENT':
      return {
        ...state,
        counter: state.counter + 1
      };
    case 'LOGIN':
      return {
        ...state,
        loggedIn: true,
        user: action.payload
      };
    case 'LOGOUT':
      return {
        ...state,
        loggedIn: false,
        user: null
      };
    default:
      return state;
  }
};

export default rootReducer;