Develop Reference

State change isn't re-rendering component after axios data fetch in react-redux useSelector()

I'm trying to fetch and display data on the initial load of an application using react and redux. But the component that should display the data does not have the data by the time it is rendered. It eventually gets the data but doesn't re-render for some reason.
Here are my two components in question:
App.js
import React, {useEffect} from 'react';
import './App.css';
import RecordList from './components/RecordList';
import CreateRecord from './components/CreateRecord';
import {useDispatch} from 'react-redux';
import { initRecords } from './actions/recordActions';
function App() {
const dispatch = useDispatch();
// Gets initial record list.
useEffect(() => {
dispatch(initRecords());
}, [dispatch])
return (
<div className="App">
<CreateRecord />
<RecordList/>
</div>
);
}
export default App;
RecordList.js
import React from 'react'
import { useSelector, useDispatch } from 'react-redux'
export default function RecordList() {
const dispatch = useDispatch();
const records = useSelector(state=>state);
console.log('state: ', records)
return (
<div>
<h3>Albums</h3>
{records.map(record =>
<div key={record.id}>
{record.albumName} by {record.artist}
</div>
)}
</div>
)
}
The issue I'm having is that initial data fetch in App.js isn't returning fast enough by the time the RecordList.js component is rendered. So in RecordList.js this bit throws an error saying map is not a function or cannot map on undefined:
{records.map(record =>
<div key={record.id}>
{record.albumName} by {record.artist}
</div>
)}
The component does eventually get the data if you comment out the JSX throwing the error. Initially it logs records as undefined but after a second it logs it with correct values.
Here are my reducer and actions:
recordActions.js
import recordService from '../services/records';
export const initRecords = () => {
return async dispatch => {
const records = await recordService.getAll();
console.log('from actions: ', records);
dispatch({
type: 'INIT_RECORDS',
data: records
})
};
}
reducer
const recordReducer = (state = [], action) => {
console.log('state now: ', state)
console.log('action', action)
switch(action.type) {
case 'CREATE':
return [...state, action.data];
case 'INIT_RECORDS':
return action.data;
default: return state;
}
}
export default recordReducer
Lastly, here is where I am making the axios call:
service
const getAll = async () => {
const response = await axios.get('someapi.com/records');
return response.data;
}
I've tried to conditionally render both the entire recordsList component and the records.map but the conditions only check once on the first load and never check again.
From my understanding, useSelector() should re-render the component when there's a state change, is it possible the state is just being mutated and not changed and how can I fix this?

Figured it out! Turns out in useSelector(state=>state) I needed to change it to useSelector(state=>state.records.Items) to get to the array.

UseSelector State is Undefined

The page does not render, citing TypeError: state is undefined, tracing back to this line in SelectForm.js: const filter = useSelector(state => state.filter);.
I've spent hours trying to figure out what I'm doing wrong. I've tried createSelector but that didn't work. I've tried dispatching a "Fetch Initial State" action, and that didn't work. The component is wrapped in provider tags. I'm not sure why I don't have access to the state. At this point I'm unable to see any flaws I've been looking at it for so long.
Code Snippets
reducer.js
let initialState = {
filter: {
country: null,
state: null,
level: null,
team: null
},
isDBConnecting: false,
isDBConnected: false,
isDBError: false
}
const SelectorReducer = (state=initialState, action) => {
switch (action.type) {
case 'DB_CONNECT_INIT':
return {
...state,
isDBConnecting: true,
isDBConnected: false,
isDBError: false,
};
...
...
}
export default SelectorReducer;
actions.js
export const initializeDBConnection = () => {
return {
type: 'DB_CONNECT_INIT'
}
};
ParentComponent.js
import React from 'react';
import { createStore } from 'redux';
import { Provider } from 'react-redux'; //import provider to provide component access to the state
//Component imports
import SelectForm from './components/SelectForm'
import SelectorReducer from '.../reducer.js'
const SelectorStore = createStore(SelectorReducer);
const ParentComponent = () => {
return (
<div className="page-container">
<div id="carousel">
<div id="wrapper">
<Provider store={SelectorStore}>
<SelectForm />
</Provider>
</div>
</div>
</div>
)
}
SelectForm.js (Child Component, wrapped in Provider tags above)
//IMPORTS
import React from 'react'; //import react
import { useSelector, useDispatch } from 'react-redux';
//COMPONENT IMPORTS
import FormGroup from '../FormGroup';
import { * as actions } from '.../actions.js';
const SelectForm = (props) => {
//STATEFUL IMPORTS
//filter
const filter = useSelector(state => state.filter);

Credit to #NicholasTower for the answer in the comments. My reducer did not have a default case in which
default: return state
Putting that in solved the issue.

let filter = useSelector(state => {
console.log('State: ', state);
return state.pieChart.filter;
});
Make sure that you are using the correct object for state.
Add a console for debugging and check the state object. Many times we use multiple reducers which makes our component state nested in global object.

We should import useSelector form react-redux and then select user details form store in the following way
import { useSelector } from 'react-redux'
const User = () => {
const userInfo= useSelector(state => state.user)
return <div>{userInfo.name}</div>
}

Inside ParentComponent.js you are saying <Provider store={TeamSelectorStore}>. I think you meant to say <Provider store={SelectorStore}>

In my case I was storing the data coming from Redux state at didMount stage whereas the Redux states weren't fully uploaded yet.
const currentUser = useAuth();
useEffect(() => {
form.setFieldValue('email', currentUser.email);
}, []);
Adding currentUser to useEffect dependency array, get it resolved for me:
useEffect(() => {
form.setFieldValue('email', currentUser.email);
}, [currentUser]);

Sometimes having a break in the switch block of the reducer can cause the prop not defined in the state.

This error can also happen because of an uppercase / lowercase mistake.
let horns = useSelector(state => state.largeRhinohorns);
let horns = useSelector(state => state.largeRhinoHorns);

How to update React component after changing state through redux?

I was learning React and Redux and while doing that I decided to make webpage with a button which on clicking would change the state. Below the button I wanted to display the current state in a different component. Though the button on clicking changes the state, but it is not getting reflected in the component. Here is my code:
App.js
import React from 'react'
import Name from './Name'
import {changeName} from './Action';
export default function App () {
return (
<div>
<button onClick={changeName}>Click me</button>
<Name />
</div>
)
}
Name.js
import React from 'react'
import {store} from './Store'
function Name(props) {
return (
<div>
My name is: {store.getState()}
</div>
)
}
export default Name
Store.js
import { createStore } from 'redux';
import {reducer} from './Reducer';
export const store = createStore(reducer, 'Tarun');
Action.js
import {store} from './Store';
export const changeName = () => {
if (store.getState() === "Tarun"){
store.dispatch({ type: 'name', payload: 'Subhash' });
}
else{
store.dispatch({ type: 'name', payload: 'Tarun' });
}
}
Reducer.js
export const reducer = function(state, action) {
if (action.type === 'name') {
return action.payload;
}
return state;
};
When I click the button, The text inside the Name component does not change. What is the issue?

You need to set up your reducer and initial store properly following the Redux documentation.
You're missing a Provider, which will provide your store to your application.
const store = createStore(reducer, applyMiddleware(thunk));
const rootElement = document.getElementById("root");
ReactDOM.render(
<Provider store={store}>
<App />
</Provider>,
rootElement
);
Now, your store is available to your components.
Your reducer needs an initial state too and you're always supposed to return an updated copy of your state. That said, don't change the state directly, but make a copy, change it, then return that copy.
const initialState = {
name: ""
};
const reducer = function(state = initialState, action) {
if (action.type === "name") {
return { ...state, name: action.payload };
} else {
return state;
}
};
export default reducer;
You might have noticed that I added a middleware to your store, and that's because it's usually the way to go when accessing your current reducer's state in your actions. That said, I installed redux-thunk for that, so in your action, you can have something like this:
export const changeName = () => {
return (dispatch, getState) => {
if (getState().name === "Tarun") {
dispatch({ type: "name", payload: "Subhash" });
} else {
dispatch({ type: "name", payload: "Tarun" });
}
};
};
Now, with your store being provided to your app, your reducer being done and your actions being ready to go, you can connect different components to your reducer.
You use the high order component in react-redux called connect for that. For example, in your Name component, we can connect the name to be displayed to your reducer by mapping your state to the component's props:
function Name(props) {
return <div>My name is: {props.name}</div>;
}
const mapStateToProps = state => {
return {
name: state.name
};
};
export default connect(mapStateToProps)(Name);
The nice thing here is that you can also leave the first parameter in the connect high order component empty and just pass the second, which would be the dispatch functions. Well, that's what you would do in your App component, you would connect it to the changeName action.
function App(props) {
return (
<div>
<button onClick={props.changeName}>Click me</button>
<Name />
</div>
);
}
const mapDispatchToProps = dispatch => {
return {
changeName: () => dispatch(changeName())
};
};
export default connect(
null,
mapDispatchToProps
)(App);
Now, when App dispatches a changeName action, your reducer state will be updated and the other components that are connected to the reducer's state will re-render.
Summary: Try to think of your store as an empty jar of candies. Your jar starts empty, but different actions could change what's inside the jar. On top of that, different people in the house that know where the jar is can go get some candy. Translating to your problem, your app begins with an empty name and you have an action that sets up a name. The components that know where to find that name by being connected to your reducer will know when that name changes and will get the updated name.
The final code can be found here:

The only way your name component will rerender is its props or state change, or if a parent component rerenders. Making a change in redux will not automatically do this. In order to see changes to the state, you'd need to subscribe to those changes. You could do this yourself, but a far better solution is to use react-redux, which is designed for connecting react components to redux stores.
For example, you'd add a provider to your app:
import { Provider } from 'react-redux';
import { store } from './Store'
export default function App () {
return (
<Provider store={store}>
<div>
<button onClick={changeName}>Click me</button>
<Name />
</div>
</Provider>
)
}
And then you'd use connect with your Name component:
import { connect } from 'react-redux';
function Name(props) {
return (
<div>
My name is: {props.name}
</div>
)
}
const mapStateToProps = (state) => {
return { name: state };
}
export default connect(mapStateToProps)(Name)

React context with hooks prevent re render

I use React context with hooks as a state manager for my React app. Every time the value changes in the store, all the components re-render.
Is there any way to prevent React component to re-render?
Store config:
import React, { useReducer } from "react";
import rootReducer from "./reducers/rootReducer";
export const ApiContext = React.createContext();
export const Provider = ({ children }) => {
const [state, dispatch] = useReducer(rootReducer, {});
return (
<ApiContext.Provider value={{ ...state, dispatch }}>
{children}
</ApiContext.Provider>
);
};
An example of a reducer:
import * as types from "./../actionTypes";
const initialState = {
fetchedBooks: null
};
const bookReducer = (state = initialState, action) => {
switch (action.type) {
case types.GET_BOOKS:
return { ...state, fetchedBooks: action.payload };
default:
return state;
}
};
export default bookReducer;
Root reducer, that can combine as many reducers, as possible:
import userReducer from "./userReducer";
import bookReducer from "./bookReducer";
const rootReducer = ({ users, books }, action) => ({
users: userReducer(users, action),
books: bookReducer(books, action)
});
An example of an action:
import * as types from "../actionTypes";
export const getBooks = async dispatch => {
const response = await fetch("https://jsonplaceholder.typicode.com/todos/1", {
method: "GET"
});
const payload = await response.json();
dispatch({
type: types.GET_BOOKS,
payload
});
};
export default rootReducer;
And here's the book component:
import React, { useContext, useEffect } from "react";
import { ApiContext } from "../../store/StoreProvider";
import { getBooks } from "../../store/actions/bookActions";
const Books = () => {
const { dispatch, books } = useContext(ApiContext);
const contextValue = useContext(ApiContext);
useEffect(() => {
setTimeout(() => {
getBooks(dispatch);
}, 1000);
}, [dispatch]);
console.log(contextValue);
return (
<ApiContext.Consumer>
{value =>
value.books ? (
<div>
{value.books &&
value.books.fetchedBooks &&
value.books.fetchedBooks.title}
</div>
) : (
<div>Loading...</div>
)
}
</ApiContext.Consumer>
);
};
export default Books;
When the value changes in Books component, another my component Users re-renders:
import React, { useContext, useEffect } from "react";
import { ApiContext } from "../../store/StoreProvider";
import { getUsers } from "../../store/actions/userActions";
const Users = () => {
const { dispatch, users } = useContext(ApiContext);
const contextValue = useContext(ApiContext);
useEffect(() => {
getUsers(true, dispatch);
}, [dispatch]);
console.log(contextValue, "Value from store");
return <div>Users</div>;
};
export default Users;
What's the best way to optimize context re-renders? Thanks in advance!

Books and Users currently re-render on every cycle - not only in case of store value changes.
1. Prop and state changes
React re-renders the whole sub component tree starting with the component as root, where a change in props or state has happened. You change parent state by getUsers, so Books and Users re-render.
const App = () => {
const [state, dispatch] = React.useReducer(
state => ({
count: state.count + 1
}),
{ count: 0 }
);
return (
<div>
<Child />
<button onClick={dispatch}>Increment</button>
<p>
Click the button! Child will be re-rendered on every state change, while
not receiving any props (see console.log).
</p>
</div>
);
}
const Child = () => {
console.log("render Child");
return "Hello Child ";
};
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.0/umd/react.production.min.js" integrity="sha256-32Gmw5rBDXyMjg/73FgpukoTZdMrxuYW7tj8adbN8z4=" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.0/umd/react-dom.production.min.js" integrity="sha256-bjQ42ac3EN0GqK40pC9gGi/YixvKyZ24qMP/9HiGW7w=" crossorigin="anonymous"></script>
<div id="root"></div>
Optimization technique
Use React.memo to prevent a re-render of a comp, if its own props haven't actually changed.
// prevents Child re-render, when the button in above snippet is clicked
const Child = React.memo(() => {
return "Hello Child ";
});
// equivalent to `PureComponent` or custom `shouldComponentUpdate` of class comps
Important: React.memo only checks prop changes (useContext value changes trigger re-render)!
2. Context changes
All context consumers (useContext) are automatically re-rendered, when the context value changes.
// here object reference is always a new object literal = re-render every cycle
<ApiContext.Provider value={{ ...state, dispatch }}>
{children}
</ApiContext.Provider>
Optimization technique
Make sure to have stable object references for the context value, e.g. by useMemo Hook.
const [state, dispatch] = useReducer(rootReducer, {});
const store = React.useMemo(() => ({ state, dispatch }), [state])
<ApiContext.Provider value={store}>
{children}
</ApiContext.Provider>
Other
Not sure, why you put all these constructs together in Books, just use one useContext:
const { dispatch, books } = useContext(ApiContext);
// drop these
const contextValue = useContext(ApiContext);
<ApiContext.Consumer> /* ... */ </ApiContext.Consumer>;
You also can have a look at this code example using both React.memo and useContext.

I believe what is happening here is expected behavior. The reason it renders twice is because you are automatically grabbing a new book/user when you visit the book or user page respectively.
This happens because the page loads, then useEffect kicks off and grabs a book or user, then the page needs to re-render in order to put the newly grabbed book or user into the DOM.
I have modified your CodePen in order to show that this is the case.. If you disable 'autoload' on the book or user page (I added a button for this), then browse off that page, then browse back to that page, you will see it only renders once.
I have also added a button which allows you to grab a new book or user on demand... this is to show how only the page which you are on gets re-rendered.
All in all, this is expected behavior, to my knowledge.

I tried to explain with different example hope that will help.
Because context uses reference identity to determine when to re-render, that could trigger unintentional renders in consumers when a provider’s parent re-renders.
for example: code below will re-render all consumers every time the Provider re-renders because a new object is always created for value
class App extends React.Component {
render() {
return (
<Provider value={{something: 'something'}}>
<Toolbar />
</Provider>
);
}
}
To get around this, lift the value into the parent’s state
class App extends React.Component {
constructor(props) {
super(props);
this.state = {
value: {something: 'something'},
};
}
render() {
return (
<Provider value={this.state.value}>
<Toolbar />
</Provider>
);
}
}

This solution is used to prevent a component from rendering in React is called shouldComponentUpdate. It is a lifecycle method which is available on React class components. Instead of having Square as a functional stateless component as before:
const Square = ({ number }) => <Item>{number * number}</Item>;
You can use a class component with a componentShouldUpdate method:
class Square extends Component {
shouldComponentUpdate(nextProps, nextState) {
...
}
render() {
return <Item>{this.props.number * this.props.number}</Item>;
}
}
As you can see, the shouldComponentUpdate class method has access to the next props and state before running the re-rendering of a component. That’s where you can decide to prevent the re-render by returning false from this method. If you return true, the component re-renders.
class Square extends Component {
shouldComponentUpdate(nextProps, nextState) {
if (this.props.number === nextProps.number) {
return false;
} else {
return true;
}
}
render() {
return <Item>{this.props.number * this.props.number}</Item>;
}
}
In this case, if the incoming number prop didn’t change, the component should not update. Try it yourself by adding console logs again to your components. The Square component shouldn’t rerender when the perspective changes. That’s a huge performance boost for your React application because all your child components don’t rerender with every rerender of their parent component. Finally, it’s up to you to prevent a rerender of a component.
Understanding this componentShouldUpdate method will surely help you out!

React Hooks: Dispatch an action on componentDidMount

I have three pages, PageA, PageB and PageC, that contain a form element formField.
State in globalReducer.js
import { fromJS } from 'immutable';
const initialState = fromJS({
userInteractionBegun: false,
pageActive: '',
refreshData: true,
})
I want to dispatch an action that sets pageActive to corresponding page value(One of A, B or C) when the component(page) mounts and refreshes formField to blank if userInteractionBegun === false.
For every page component, to get pageActive state in props from globalReducer, I do,
function PageA(props) {
//.....
}
// globalState is defined in conigureStore, I am using immutable.js. Link provided below this code.
const mapStateToProps = state => ({
pageActive: state.getIn(['globalState', 'pageActive']),
})
export default connect(mapStateToProps, null)(PageA);
Link to immutable.js getIn()
store.js
import globalReducer from 'path/to/globalReducer';
const store = createStore(
combineReducers({
globalState: globalReducer,
//...other reducers
})
)
I want to abstract the logic to update pageActive every time a component(page) mounts.
I know how to abstract this logic using an HOC, but I don't know how to do it using react hooks, so that every time pageA, pageB or pageC mounts, an action to setPageActive is dispatched and formField is set to blank if userInteractionBegun is false.
For instance, I would do in pageA.js
import usePageActive from 'path/to/usePageActive';
const [pageActive, setPageActive] = useReducer(props.pageActive);
usePageActive(pageActive);
Then in usePageActive.js
export default usePageActive(pageActive) {
const [state, setState] = useState(pageActive);
setState(// dispatch an action //)
}

I haven't had much time to dip my toes into react hooks yet, but after reading the docs and playing with it for a minute, I think this will do what you're asking. I'm using built-in state here, but you could use redux or whatever else you like in the effect. You can see a working example of this code here The trick is using a hook creator to create the custom hook. That way the parent and children can keep a reference to the same state without the useEffect affecting the parent.
import React, { useState, useEffect } from 'react';
import ReactDOM from "react-dom";
const activePageFactory = (setActivePage) => (activePage) => {
useEffect(() => {
setActivePage(activePage)
return () => {
setActivePage('')
}
}, [activePage])
return activePage
}
function App() {
const [activePage, setActivePage] = useState('');
const [localPage, setLocalPage] = useState('Not Selected');
const selectedPage = () => {
switch(localPage) {
case 'A':
return <PageA useActivePage={activePageFactory(setActivePage)} />
case 'B':
return <PageB useActivePage={activePageFactory(setActivePage)} />
default:
return null;
}
}
return (
<div>
<p>Active page is {activePage}</p>
<button onClick={() => setLocalPage('A')}>
Make A Active
</button>
<button onClick={() => setLocalPage('B')}>
Make B Active
</button>
{
selectedPage()
}
</div>
);
}
function PageA({useActivePage}) {
useActivePage('A');
return (
<div>
<p>I am Page A</p>
</div>
)
}
function PageB({useActivePage}) {
useActivePage('B');
return (
<div>
<p>I am Page B</p>
</div>
)
}