尽管向 createStore() 提供了 initialState，为什么我得到“Reducer [...] 在初始化期间返回未定义”？

Question

我在我的 redux createStore 方法中设置了 InitialState，并且我对应 InitialState 作为第二个参数

我在浏览器中遇到错误：

<code>Uncaught Error: Reducer "postsBySubreddit" returned undefined during initialization. If the state passed to the reducer is undefined, you must explicitly return the initial state. The initial state may not be undefined.</code>

代码在这里：

import { createStore, applyMiddleware } from 'redux'
import thunkMiddleware from 'redux-thunk'
import createLogger from 'redux-logger'
import rootReducer from '../reducers/reducers'
import Immutable from 'immutable'
const loggerMiddleware = createLogger()
//const initialState=0
function configureStore() {
    return createStore(
    rootReducer,
     {postsBySubreddit:{},selectedSubreddit:'reactjs'},
     applyMiddleware(
     thunkMiddleware,
    loggerMiddleware
  )
 )
}
  export default configureStore

我在Root.js 中调用了configeStoremethod：

 import React, { Component } from 'react'
 import { Provider } from 'react-redux'
 import configureStore from '../store/configureStore'
 import AsyncApp from './AsyncApp'
 import Immutable from 'immutable'
 const store = configureStore()
 console.log(store.getState())
 export default class Root extends Component {
 render() {
   return (
     <Provider store={store}>
       <AsyncApp />
     </Provider>
  )
 }
}

但我猜这个initateState 有问题：

import { combineReducers } from 'redux'
import {reducerCreator} from '../utils/creator'
import Immutable from'immutable'
import {SELECT_SUBREDDIT, INVALIDATE_SUBREDDIT ,REQUEST_POSTS, RECEIVE_POSTS} from '../actions/action'
let initialState=Immutable.fromJS({isFetching: false, didInvalidate: false,items:[]})

function selectedSubreddit(state, action) {
  switch (action.type) {
  case SELECT_SUBREDDIT:
    return action.subreddit
  default:
    return state
  }
}
function postsBySubreddit(state, action) {
  switch (action.type) {
    case INVALIDATE_SUBREDDIT:
    case RECEIVE_POSTS:
    case REQUEST_POSTS:
      return Object.assign({}, state, {
        [action.subreddit]: posts(state[action.subreddit], action)
      })
    default:
      return state
  }
}
function posts(state=initialState,action) {
  switch (action.type) {
    case INVALIDATE_SUBREDDIT:
      return state.merge({
        didInvalidate: true
      })
    case REQUEST_POSTS:
      return state.merge({
        isFetching: true,
        didInvalidate: false
      })
    case RECEIVE_POSTS:
      return state.merge({
        isFetching: false,
        didInvalidate: false,
        items: action.posts,
        lastUpdated: action.receivedAt
      })
    default:
      return state 
    }
}

const rootReducer = combineReducers({
  postsBySubreddit,
 selectedSubreddit
})
export default rootReducer

但是如果我在我的每个子减速器中设置initialState，它可以正常运行。有问题？

Answer 1

createStore() 中的 initialState 参数经常让人绊倒。它从来都不是一种手动“初始化”应用程序状态的方法。它唯一有用的应用是：

• 从 JSON 状态负载启动服务器呈现的应用程序。
• “恢复”应用程序从保存到本地存储中的状态。

这意味着您永远不会手动编写initialState，并且在大多数应用程序中您甚至都不使用它。相反，reducer 必须始终指定自己的初始状态，initialState 只是在您拥有现有序列化版本时预填充该状态的一种方式。

所以this answer 是正确的：你需要在你的reducer 中定义初始状态。将其提供给 createStore() 是不够的，也不是在代码中定义初始状态的一种方式。

Answer 2

我遇到了同样的错误，但我没有包含默认情况

function generate(state={} ,action) {
  switch (action.type) {
    case randomNumber:
      return {
        ...state,
        random: action.payload
      }   
    default: // need this for default case
      return state 
   }
}

Answer 3

另外，请确保您在 reducer 中最后返回 默认状态。有时您可能会忘记确保这是您的 switch 语句的默认值（在重构和移动代码时）。

...
 default:
  return state

Answer 4

当你的 reducer 第一次被调用时，状态是未定义的。然后您必须返回初始状态（这就是错误消息告诉您的内容）。 定义初始状态值的常用方法是为状态参数设置一个默认值：

function postsBySubreddit(state = {}, action) {}

posts 函数中有一个初始状态，但在初始化期间没有调用它。

Answer 5

我刚刚遇到了同样的问题，因为我不小心在减速器中重新定义了 state：

const initialState = {
  previous: true,
  later: true
}

const useTypeReducer = (state = initialState, action) => {
  switch (action.type) {
    case 'TOGGLE_USE_TYPES':
      let state = Object.assign({}, state);   // DON'T REDEFINE STATE LIKE THIS!
      state[action.use] = !state[action.use];
      return state;

    default:
      return state;
  }
}

export default useTypeReducer;

为了解决这个问题，我需要避免重新定义状态：

const initialState = {
  previous: true,
  later: true
}

const useTypeReducer = (state = initialState, action) => {
  switch (action.type) {
    case 'TOGGLE_USE_TYPES':
      let _state = Object.assign({}, state);   // BETTER
      _state[action.use] = !state[action.use];
      return _state;

    default:
      return state;
  }
}

export default useTypeReducer;

Answer 6

根据redux的文档：https://redux.js.org/basics/reducers

你需要在 switch 函数之外返回状态 像这样：

 function posts(state=initialState,action) {
  switch (action.type) {
    case INVALIDATE_SUBREDDIT:
      return state.merge({
        didInvalidate: true
      })
    case REQUEST_POSTS:
      return state.merge({
        isFetching: true,
        didInvalidate: false
      })
    case RECEIVE_POSTS:
      return state.merge({
        isFetching: false,
        didInvalidate: false,
        items: action.posts,
        lastUpdated: action.receivedAt
      })
    default:
      return state 
    }

//here you should put the return state
return state
}

Answer 7

确保你在最后有一个默认状态

...
default:
  return state;

Answer 8

tl;博士

确保：

• 您实际上是通过动作/动作创建器将数据传递给减速器！
• reducer 不返回 undefined。

示例

const reducer = (state = initialState, action) => {
  switch (action.type) {
    case 'SET_DATA':
      debugger // Place a breakpoint for investigation
      const result = update(state, {$set: action.data});
      // `result` is `undefined` if `action.data` is too
      return result;
    default:
      return state;
  }
}

在这种情况下，如果您不小心通过动作创建器将undefined 传递给action.data，那么react-addons-update 将返回来自该变量的undefined。由于这会覆盖整个 reducer 状态，因此它将返回将触发错误的 undefined。

调试

你可以通过检查这些地方来调试它：

• reducer 创建新状态的位置。在该部分放置一个断点，以确保您不会在那里收到或返回undefined。
• 数据传递给操作创建者的位置。

Answer 9

对我来说，我有我的减速器

import * as actions from '../actions/budgetActions';

export const initialState = {
    budget: {},
    loading: false,
    hasErrors: false,
}

export default function budgetReducer(state = initialState, action) {

    switch (action.type) {
        case actions.GET_BUDGET:
            return { ...state, loading: true };
        case actions.GET_BUDGET_SUCCESS:
            return { budget: action.payload, loading: false, hasErrors: false };
        case actions.GET_BUDGET_FAILURE:
            return { ...state, loading: false, hasErrors: true };
    }
}

而不是（我错过了默认情况）

   import * as actions from '../actions/budgetActions';
    
    export const initialState = {
        budget: {},
        loading: false,
        hasErrors: false,
    }
    
    export default function budgetReducer(state = initialState, action) {
    
        switch (action.type) {
            case actions.GET_BUDGET:
                return { ...state, loading: true };
            case actions.GET_BUDGET_SUCCESS:
                return { budget: action.payload, loading: false, hasErrors: false };
            case actions.GET_BUDGET_FAILURE:
                return { ...state, loading: false, hasErrors: true };
            default:
                return state;
        }
    }