Ice Wilder Ice Wilder - 7 months ago 63
Javascript Question

Why do I get “Reducer [...] returned undefined during initialization” despite providing initialState to createStore()?

I had set InitialState in my redux createStore method ,and I corresponding InitialState as second arguments

I got a error in browser:

<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>


code is here:

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


and I invoked
configeStore
method in
Root.js
:

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>
)
}
}


but I guess this
initateState
has something wrong:

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


but if I set
initialState
in my every sub reducer it can did word normally. Something wrong?

Answer

When your reducers are called for the first time, state is undefined. You must then return the initial state (that's what the error message is telling you). The usual way of defining the initial state value is to set a default value for the state parameter:

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

You have an initial state in the posts function but it is not called during initialization.

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