为什么我得到“Reducer [...]在初始化过程中返回未定义“尽管向createStore()提供了initialState？
为什么我得到“Reducer [...]在初始化过程中返回未定义“尽管向createStore()提供了initialState？
提问于 2016-04-14 17:33:58
我在redux createStore方法中设置了InitialState，并将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>代码如下：
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configeStore方法
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出了什么问题：
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但如果我在我的每个子减速器中设置initialState，它就可以正常工作。有什么问题吗？
回答 5
Stack Overflow用户
发布于 2016-04-14 18:29:39
当您的缩减程序第一次被调用时，状态是未定义的。然后必须返回初始状态(这就是错误消息告诉您的)。定义初始状态值的常用方法是为state参数设置默认值：
function postsBySubreddit(state = {}, action) {}您在posts函数中有一个初始状态，但在初始化期间不会调用它。
Stack Overflow用户
发布于 2018-01-03 00:41:18
我遇到了同样的问题，因为我意外地在reducer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;要解决我需要避免重新定义状态的问题：
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;Stack Overflow用户
发布于 2020-08-02 00:22:37
根据redux的文档：https://redux.js.org/basics/reducers
您需要在switch函数的外部返回状态，如下所示：
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
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/36619093
复制相关文章
相似问题
腾讯云开发者
