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useSelector not updating even after dispatch

I'm trying to display products using the fetched axios result from reducer, but the useSelector value just won't change and is still empty even after dispatch. I have checked the axios result and the response has correct data. Does it have something to do with this line on redux documentation?
With useSelector(), returning a new object every time will always force a re-render by default.
reducer
import axios from "axios";
export const products = (state = [], action) => {
switch (action.type) {
case "FETCH_PRODUCTS": {
const uri = "/products";
axios.get(uri).then(function (response) {
if (response.status == 200) {
console.log(response.data.products); // ===> correct new value
return { state: response.data.products };
}
});
}
App.js
import React, { useEffect } from "react";
import { shallowEqual, useSelector, useDispatch } from "react-redux";
import "../css/App.css";
import { Products, Navbar, Cart } from "../components";
function App() {
const dispatch = useDispatch();
const products = useSelector((state) => state.products, shallowEqual);
const cart = useSelector((state) => state.cart, shallowEqual);
useEffect(() => {
dispatch({
type: "FETCH_PRODUCTS",
});
console.log(products); // ===> still empty array
}, []);
return (
<div className="App">
<Navbar />
<Cart cart={cart} />
<Products products={products} />
</div>
);
}
export default App;

You should first create your action
import axios from 'axios';
export const fetchProducts = () => async (dispatch) => {
try {
const { data } = await axios.get('...');
dispatch({ type: "FETCH_PRODUCTS", payload: data.result });
} catch (error) {
console.log(error);
}
};
Then, Use dispatch and action together
import { useEffect } from 'react';
import { useDispatch, useSelector } from 'react-redux';
import { fetchProducts } from './actions';
const getSelectors = state => ({ cart: state.cart, products: state.products });
const App = () => {
const dispatch = useDispatch();
const {cart, products} = useSelector(getSelectors);
useEffect(() => {
dispatch(fetchProducts());
}, []);
return (
<div className="App">
<Navbar />
<Cart cart={cart} />
<Products products={products} />
</div>
);
};

React useEffect and React-Query useQuery issue?

I'm still new to React so forgive me if this is a silly approach to this problem.
My goal: Global error handling using a context provider and a custom hook.
The Problem: I can't remove errors without them immediately being re-added.
I display my errors via this component in the shell...
import React, { useState, useEffect } from 'react'
import Alert from '#mui/material/Alert'
import Collapse from '#mui/material/Collapse'
import { useAlertContext } from '#/context/alert-context/alert-context'
export default function AppAlert () {
const [show, setShow] = useState(false)
const alertContext = useAlertContext()
const handleClose = () => {
alertContext.remove()
setShow(false)
}
useEffect(() => {
if (alertContext.alert) {
setShow(true)
}
}, [alertContext.alert])
return (
<Collapse in={show}>
<Alert severity='error' onClose={handleClose}>
{alertContext.alert}
</Alert>
</Collapse>
)
}
I have a provider setup that also exposes a custom hook...
import React, { useState, createContext, useContext } from 'react'
const AlertContext = createContext()
const AlertProvider = ({ children }) => {
const [alert, setAlert] = useState(null)
const removeAlert = () => setAlert(null)
const addAlert = (message) => setAlert(message)
return (
<AlertContext.Provider value={{
alert,
add: addAlert,
remove: removeAlert
}}
>
{children}
</AlertContext.Provider>
)
}
const useAlertContext = () => {
return useContext(AlertContext)
}
export {
AlertProvider as default,
useAlertContext
}
And finally I have a hook setup to hit an API and call throw errors if it any occur while fetching the data. I'm purposely triggering a 404 by passing a bad API path.
import { useEffect } from 'react'
import { useQuery } from 'react-query'
import ApiV4 from '#/services/api/v4/base'
import { useAlertContext } from '#/context/alert-context/alert-context'
export const useAccess = () => {
const alertContext = useAlertContext()
const route = '/accessx'
const query = useQuery(route, async () => await ApiV4.get(route), {
retry: 0
})
useEffect(() => {
if (query.isError) {
alertContext.add(query.error.toString())
}
}, [alertContext, query.isError, query.error])
return query
}
This code seems to be the issue. Because alertContext.remove() triggers useEffect here and query.error still exists, it immediately re-adds the error to the page on remove. Removing alertContext from the array works, but it is not a real fix and linter yells.
useEffect(() => {
if (query.isError) {
alertContext.add(query.error.toString())
}
}, [alertContext, query.isError, query.error])

This is a perfectly fine approach to the problem. You've also accurately identified the problem. The solution is to create a second hook with access to the methods that will modify the context. AppAlert needs access to the data in the context, and needs to update when AlertContext.alert changes. UseAccess only needs to be able to call AlertContext.add, and that method wont change and trigger a re-render. This can be done with a second Context. You can just expose one Provider and bake the actions provider into the outer context provider.
import React, { useState, createContext, useContext } from 'react'
const AlertContext = createContext()
const AlertContextActions = createContext()
const AlertProvider = ({ children }) => {
const [alert, setAlert] = useState(null)
const removeAlert = () => setAlert(null)
const addAlert = (message) => setAlert(message)
return (
<AlertContext.Provider value={{ alert }}>
<AlertContextActions.Provider value={{ addAlert, removeAlert }}>
{children}
</AlertContextActions.Provider>
</AlertContext.Provider>
)
}
const useAlertContext = () => {
return useContext(AlertContext)
}
export {
AlertProvider as default,
useAlertContext
}
Now, where you need access to the alert you use one hook and where you need access to the actions you use the other.
// in AppAlert
import { useAlertContext, useAlertContextActions } from '#/context/alert-context/alert-context'
...
const { alert } = useAlertContext()
const { removeAlert } = useAlertContextActions()
And finally
// in useAccess
import { useAlertContextActions } from '#/context/alert-context/alert-context'
...
const { addAlert } = useAlertContextActions()

So I found a solution that seems to work for my purposes. I got a hint from this article. https://mortenbarklund.com/blog/react-architecture-provider-pattern/
Note the use of useCallback above. It ensures minimal re-renders of components using this context, as the function is guaranteed to be stable (as its memoized without dependencies).
So with this I tried the following and it solved the problem.
import React, { useState, createContext, useContext, useCallback } from 'react'
const AlertContext = createContext()
const AlertProvider = ({ children }) => {
const [alert, setAlert] = useState(null)
const removeAlert = useCallback(() => setAlert(null), [])
const addAlert = useCallback((message) => setAlert(message), [])
return (
<AlertContext.Provider value={{
alert,
add: addAlert,
remove: removeAlert
}}
>
{children}
</AlertContext.Provider>
)
}
const useAlertContext = () => {
return useContext(AlertContext)
}
export {
AlertProvider as default,
useAlertContext
}

My goal: Global error handling
One problem with the above useEffect approach is that every invocation of useAccess will run their own effects. So if you have useAccess twice on the page, and it fails, you will get two alerts, so it's not really "global".
I would encourage you to look into the global callbacks on the QueryCache in react-query. They are made for this exact use-case: To globally handle errors. Note that to use context, you would need to create the queryClient inside the Application, and make it "stable" with either useRef or useState:
function App() {
const alertContext = useAlertContext()
const [queryClient] = React.useState(() => new QueryClient({
queryCache: new QueryCache({
onError: (error) =>
alertContext.add(error.toString())
}),
}))
return (
<QueryClientProvider client={queryClient}>
<RestOfMyApp />
</QueryClientProvider>
)
}
I also have some examples in my blog.

Enzime: Wait that context is updated to start execute tests

I have a React app that I need to test. It's using the useContext() hook to create Provider that are using in most of my components. I have a dedicated component to handle a Context (lets say UserContext for the example) that look like that:
UserContext.jsx:
import React from 'react'
export const UserContext = React.createContext(undefined)
export const UserProvider = (props) => {
const [currentUser, setCurrentUser] = React.useState(undefined)
const context = {
currentUser,
setCurrentUser,
}
return (
<UserContext.Provider value={context}>
{props.children}
</UserContext.Provider>
)
}
So you can use the Provider like that:
import { UserProvider } from './context/UserContext'
<UserProvider>
{ ... }
</UserProvider>
Now I need to test a component that use this UserContext so let's say UserModal:
UserModal.test.jsx
import React from 'react'
import { mount } from 'enzyme'
import { BrowserRouter as Router } from 'react-router-dom'
import { UserProvider, UserContext } from '../context/UserContext'
import UserModal from '../components/UserModal'
// D A T A
import exampleUser from '../data/user.json' // Load user's data from a json file
describe('<UserModal />', () => {
let wrapper
const Wrapper = () => {
const { setCurrentUser } = React.useContext(UserContext)
React.useEffect(() => {
// Init UserContext value
setCurrentUser(exampleUser)
}, [])
return (
<UserProvider>
<UserModal />
</UserProvider>
)
}
beforeEach(() => {
wrapper = mount(<Wrapper />)
})
})
Problem is that when <UserModal /> is mounted inside of the <UserProvider>, I get an error that the currentUser in the UserContext is undefined. This error make sense because I call setCurrentUser() when the component is mounted once using React.useEffect(() => { }, []).
So have you an idea how I can mount() my <UserModal /> component inside of a context's provider in the way that the context is not undefined?

Your test should look like this:
import React from 'react'
import { mount } from 'enzyme'
import { BrowserRouter as Router } from 'react-router-dom'
import { UserProvider, UserContext } from '../context/UserContext'
import UserModal from '../components/UserModal'
// D A T A
import exampleUser from '../data/user.json' // Load user's data from a json file
describe('<UserModal />', () => {
let wrapper
const Wrapper = () => {
const { setCurrentUser } = React.useContext(UserContext)
React.useEffect(() => {
// Init UserContext value
setCurrentUser(exampleUser)
}, [])
return (
<UserModal />
)
}
beforeEach(() => {
wrapper = mount(<UserProvider><Wrapper /></UserProvider>)
})
})
Or see codesandbox here - simple test passes.
Note that UserProvider wraps Wrapper and not is used inside. It's like this because if you are using it inside, there is no UserContext to get with useContext hook, therefore there is no setCurrentUser function.

React context not updating

I have set a basic sample project that use Context to store the page title, but when I set it the component is not rerendered.
Principal files:
Context.js
import React from 'react'
const Context = React.createContext({})
export default Context
AppWrapper.js
import React from 'react'
import App from './App'
import Context from './Context'
function AppWrapper () {
return (
<Context.Provider value={{page: {}}}>
<App />
</Context.Provider>
)
}
export default AppWrapper
App.js
import React, { useContext } from 'react';
import Context from './Context';
import Home from './Home';
function App() {
const { page } = useContext(Context)
return (
<>
<h1>Title: {page.title}</h1>
<Home />
</>
);
}
export default App;
Home.js
import React, { useContext } from 'react'
import Context from './Context'
function Home () {
const { page } = useContext(Context)
page.title = 'Home'
return (
<p>Hello, World!</p>
)
}
export default Home
full code
What am I doing wrong?

Think about React context just like you would a component, if you want to update a value and show it then you need to use state. In this case your AppWrapper where you render the context provider is where you need to track state.
import React, {useContext, useState, useCallback, useEffect} from 'react'
const PageContext = React.createContext({})
function Home() {
const {setPageContext, page} = useContext(PageContext)
// essentially a componentDidMount
useEffect(() => {
if (page.title !== 'Home')
setPageContext({title: 'Home'})
}, [setPageContext])
return <p>Hello, World!</p>
}
function App() {
const {page} = useContext(PageContext)
return (
<>
<h1>Title: {page.title}</h1>
<Home />
</>
)
}
function AppWrapper() {
const [state, setState] = useState({page: {}})
const setPageContext = useCallback(
newState => {
setState({page: {...state.page, ...newState}})
},
[state, setState],
)
const getContextValue = useCallback(
() => ({setPageContext, ...state}),
[state, updateState],
)
return (
<PageContext.Provider value={getContextValue()}>
<App />
</PageContext.Provider>
)
}
Edit - Updated working solution from linked repository
I renamed a few things to be a bit more specific, I wouldn't recommend passing setState through the context as that can be confusing and conflicting with a local state in a component. Also i'm omitting chunks of code that aren't necessary to the answer, just the parts I changed
src/AppContext.js
export const updatePageContext = (values = {}) => ({ page: values })
export const updateProductsContext = (values = {}) => ({ products: values })
export const Pages = {
help: 'Help',
home: 'Home',
productsList: 'Products list',
shoppingCart: 'Cart',
}
const AppContext = React.createContext({})
export default AppContext
src/AppWrapper.js
const getDefaultState = () => {
// TODO rehydrate from persistent storage (localStorage.getItem(myLastSavedStateKey)) ?
return {
page: { title: 'Home' },
products: {},
}
}
function AppWrapper() {
const [state, setState] = useState(getDefaultState())
// here we only re-create setContext when its dependencies change ([state, setState])
const setContext = useCallback(
updates => {
setState({ ...state, ...updates })
},
[state, setState],
)
// here context value is just returning an object, but only re-creating the object when its dependencies change ([state, setContext])
const getContextValue = useCallback(
() => ({
...state,
setContext,
}),
[state, setContext],
)
return (
<Context.Provider value={getContextValue()}>
...
src/App.js
...
import AppContext, { updateProductsContext } from './AppContext'
function App() {
const [openDrawer, setOpenDrawer] = useState(false)
const classes = useStyles()
const {
page: { title },
setContext,
} = useContext(Context)
useEffect(() => {
fetch(...)
.then(...)
.then(items => {
setContext(updateProductsContext({ items }))
})
}, [])
src/components/DocumentMeta.js
this is a new component that you can use to update your page names in a declarative style reducing the code complexity/redundancy in each view
import React, { useContext, useEffect } from 'react'
import Context, { updatePageContext } from '../Context'
export default function DocumentMeta({ title }) {
const { page, setContext } = useContext(Context)
useEffect(() => {
if (page.title !== title) {
// TODO use this todo as a marker to also update the actual document title so the browser tab name changes to reflect the current view
setContext(updatePageContext({ title }))
}
}, [title, page, setContext])
return null
}
aka usage would be something like <DocumentMeta title="Whatever Title I Want Here" />
src/pages/Home.js
each view now just needs to import DocumentMeta and the Pages "enum" to update the title, instead of pulling the context in and manually doing it each time.
import { Pages } from '../Context'
import DocumentMeta from '../components/DocumentMeta'
function Home() {
return (
<>
<DocumentMeta title={Pages.home} />
<h1>WIP</h1>
</>
)
}
Note: The other pages need to replicate what the home page is doing
Remember this isn't how I would do this in a production environment, I'd write up a more generic helper to write data to your cache that can do more things in terms of performance, deep merging.. etc. But this should be a good starting point.

Here is a working version of what you need.
import React, { useState, useContext, useEffect } from "react";
import "./styles.css";
const Context = React.createContext({});
export default function AppWrapper() {
// creating a local state
const [state, setState] = useState({ page: {} });
return (
<Context.Provider value={{ state, setState }}> {/* passing state to in provider */}
<App />
</Context.Provider>
);
}
function App() {
// getting the state from Context
const { state } = useContext(Context);
return (
<>
<h1>Title: {state.page.title}</h1>
<Home />
</>
);
}
function Home() {
// getting setter function from Context
const { setState } = useContext(Context);
useEffect(() => {
setState({ page: { title: "Home" } });
}, [setState]);
return <p>Hello, World!</p>;
}
Read more on Hooks API Reference.

You may put useContext(yourContext) at wrong place.
The right position is inner the <Context.Provider>:
// Right: context value will update
<Context.Provider>
<yourComponentNeedContext />
</Context.Provider>
// Bad: context value will NOT update
<yourComponentNeedContext />
<Context.Provider>
</Context.Provider>

React jest pass data to mounted component

I am trying to test a component that use context. After I mount it (shallow does not work with useContext apparently) I am trying to set default values for the component data.
I was expecting const contextValues = { text: 'mock', msg: 'SUCCESS' }; and passing that to the AlertContextProvider to set a state for that component but I am probably looking at this the wrong way.
AlertContext.js:
import React, { createContext, useState, useContext } from 'react';
export const AlertContext = createContext();
const AlertContextProvider = props => {
const [alert, setAlert] = useState({
text: '',
msg: ''
});
const updateAlert = (text, msg) => {
setAlert({
text,
msg
});
};
return (
<AlertContext.Provider value={{ alert, updateAlert }}>
{props.children}
</AlertContext.Provider>
);
};
export default AlertContextProvider;
Alert.js (component):
import React, { useContext } from 'react';
import './Alert.scss';
import { AlertContext } from '../context/AlertContext';
const Alert = () => {
const { alert } = useContext(AlertContext);
return (
<div className='alert'>
<p className="alert-para">{alert.text}</p>
</div>
);
};
export default Alert;
Alert.js(text)
import React from 'react';
import { mount } from 'enzyme';
import Alert from '../components/Alert';
import AlertContextProvider from '../context/AlertContext';
describe('Alert', () => {
let wrapper;
beforeEach(() => {
const contextValues = { text: 'mock', msg: 'SUCCESS' };
// Below mounting is needed as Enzyme does not yet support shallow mocks
wrapper = mount(
<AlertContextProvider value={contextValues}>
<Alert />
</AlertContextProvider>
);
});
test('Should render a paragraph', () => {
const element =wrapper.find('.alert-para');
expect(element.length).toBe(1); // this is correct
expect(element.text()).toEqual('mock'); // THIS FAILS AS THE VALUE OF THE ELEMENT IS AN EMPTY STRING WHILE I WAS EXPECTING 'mock'
});
});

You are passing your contextValues through value prop on <AlertContextProvider /> but you are never using that prop to initialize data inside your context provider.
In this example, I used useEffect hook as componentDidMount to initialize your state AlertContext.js`
const AlertContextProvider = props => {
const [alert, setAlert] = useState({
text: '',
msg: ''
});
// The same as component did mount
useEffect(() => {
setAlert({
text: props.value.text,
msg: props.value.msg
})
}, [])
const updateAlert = (text, msg) => {
setAlert({
text,
msg
});
};
return (
<AlertContext.Provider value={{ alert, updateAlert }}>
{props.children}
</AlertContext.Provider>
);
};
You should use useCallback hook for your updateAlert function to memoize it.