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What can go wrong when useRef is used with useSelector?

While I can access the store using thunks and/or global stores, I don't want to bind my component to the Redux store. Because the component will be used with other stores both inside and outside the project. The component has a number of children and passes props to them via the context API.
Since context APIs cause rerendering on child components, we employ the below approach to pass data from redux store to context APIs. This way, I can use dataRef.current whenever I need to access data in the redux store:
export const useData = () => {
const dataRef = React.useRef();
useSelector((state) => {
dataRef.current = state;
});
return dataRef;
};
This works perfectly fine, But my concern is whether it will cause memory leaks or any other unknown problems.

It will likely cause things to not rerender in the right moment - since you leave the whole world of "letting React rerender" at that point in time. A change in the Redux store will not rerender your component any more.
I think you need to reevaluate why you are doing this. This technique seems to bind your component to the Redux store just as much as just calling useSeletor would do. As long as your state structure has the value in the path you expect, you could also just call useSelector directly without any weird workarounds. That would work with any Redux store being present at that point in time.

What are the pros and cons on using React Context API with the useReducer() hook?

I'm working on a web application and I'm using the React Context without using the useReducer() hook. This is a simple example of how I'm using the Context in my app:
const [stateValue, setStateValue] = useState("");
const [stateValue1, setStateValue1] = useState("");
const contextValue : MainContext = {
stateValue: stateValue,
setStateValue: setStateValue,
stateValue1: stateValue1,
setStateValue1: setStateValue1
}
So I pass to my Context Provider the contextValue and every time a child component has to change the stateValuex just calls the setStateValuex so that it triggers the re-rendering of the stateValuex inside all the child components.
What would the pros and cons be on using instead the Context with the useReducer() hook?

I'd approach it as two issues: 1) pros/cons of useState vs useReducer 2) pros/cons of props vs context. Then stick those answers together.
useReducer can be useful if you have a complicated state that you want to make sure all your update logic is in one centralized location. useState on the other hand is good for simple state where you don't need that kind of control.
props is the standard way to pass values from one component to its child. If you're passing it a short distances, this is the simplest and best approach. context is useful if you need to pass values a long way down the component tree. If you have a lot of cases where a component receives a prop not for itself, but just so it can forward it to a child, then this may indicate context would be better than props.
const contextValue : MainContext = {
stateValue: stateValue,
setStateValue: setStateValue,
stateValue1: stateValue1,
setStateValue1: setStateValue1
}
P.S: if your context value is an object, don't forget to memoize it. If you don't, you'll be creating a brand new object every time you render, and that will force any components consuming the context to render too
const contextValue: MainContext = useMemo(() => {
return {
stateValue: stateValue,
setStateValue: setStateValue,
stateValue1: stateValue1,
setStateValue1: setStateValue1
}
}, [stateValue, stateValue1])

when you use hooks or custom hooks, the states from them are indivisual.
which means suppose you used useReducer in Component A and B. state from useReducer in A, B is totally different whereas if you use contextAPI the state is same.

Unique storage for every function call inside React HOC

I want to create a helper that generates some data and saves it in some variable and on the next execution, it should use the memoized value for calculation.
Basically it's a helper for the High Order wrapper. It means that the storage should be created for every HOC but it shouldn't be re-created on the next re-render.
Now it looks like:
pseudo code
var storage; // I want to create this storage for every HOC only once.
function createDynamicStyles(theme, stylesCreator, status) {
// create a styles registry only once. This function can be re-called by the same HOC
// on each re-render so I want to use the memoized registry.
if (!storage) {
storage = stylesCreator(theme);
};
return storage[status];
}
const styleCreator = theme => ({
disabled: { color: theme.disabled },
success: { color: theme.success }
})
const Component_1 = componentHOC((props) => {
const { theme, status } = props;
// I'd like to keep this helper call as simple as possible. It already has 3 arguments.
const finalStyle = createDynamicStyles(theme, stylesCreator, status);
})(AwesomeComponent)
// these props can be changed during runtime
<Component_1 disabled={false} success={true} />
The functionality flow of this helper can be divided into 2 steps.
1) The first HOC call. It creates the styles based on the theme and saves them in the storage
2) Next Re-render of the HOC. It should fetch the previously created styles and return memoized value. This value should be unique for each HOC.
The problem is that the same helper can be used for other Components as well and it means that we can't use the same storage because it will be overwritten but the 'latest' HOC.
The possible ways how to solve it:
1) Create a class that will contain storage itself and creates a new Instance for each HOC.
To be honest, I'd like to avoid it because it looks too complicated for me in this case.
2) Create some Shared Registry and pass the UUID for every HOC.
It'd be nice but I don't know how to automatically do it. I don't want to manually pass the UUID on each HOC. I'd like to have this functionality under the hood to keep HOC calls, lightweight.
I was thinking about the new Map, and saving the created styles as Key-Value pair but it simply doesn't work as we don't have the generated KEY reference in the HOC. So we can't use it as a key.
Is it possible to do such a thing in the case of plain functions only?
Maybe I missed some other interesting variants.
Thanks for any help and suggestion.
Kind Regards.

Should I use useselector/useDispatch instead of mapStateToProps

When creating a React app, if I use the hook useSelector, I need to adhere to the hooks invoking rules (Only call it from the top level of a functional component). If I use the mapStateToProps, I get the state in the props and I can use it anywhere without any issues... Same issue for useDispatch
What are the benefits of using the hook besides saving lines of code compared to mapStateToProps?

Redux store state can be read and changed from anywhere in the component, including callbacks. Whenever the store state is changed the component rerenders. When the component rerenders, useSelector runs again, and gives you the updated data, later to be used wherever you want. Here is an example of that and a usage of useDispatch inside a callback (after an assignment in the root level):
function Modal({ children }) {
const isOpen = useSelector(state => state.isOpen);
const dispatch = useDispatch();
function handleModalToggeled() {
// using updated data from store state in a callback
if(isOpen) {
// writing to state, leading to a rerender
dispatch({type: "CLOSE_MODAL"});
return;
}
// writing to state, leading to a rerender
dispatch({type: "OPEN_MODAL"});
}
// using updated data from store state in render
return (isOpen ? (
<div>
{children}
<button onClick={handleModalToggeled}>close modal</button>
</div>
) : (
<button onClick={handleModalToggeled}>open modal</button>
);
);
}
There is nothing you can do with mapStateToProps/mapDispatchToProps that you can't do with the useSelector and useDispatch hooks as well.
With that said, there are a couple of differences between the two methods that are worth considering:
Decoupling: with mapStateToProps, container logic (the way store data is injected into the component) is separate from the view logic (component rendering).
useSelector represents a new and different way of thinking about connected components, arguing that the decoupling is more important between components and that components are self contained. Which is better? Verdict: no clear winner. source
DX (Developer experience): using the connect function usually means there should be another additional container component for each connected component, where using the useSelector and useDispatch hooks is quite straightforward. Verdict: hooks have better DX.
"Stale props" and "Zombie child": there are some weird edge cases with useSelector, if it depends on props, where useSelector can run before the newest updated props come in. These are mostly rare and avoidable edge cases, but they had been already worked out in the older connect version. verdict: connect is slightly more stable than hooks. source
Performance optimizations: both support performance optimizations in different ways: connect has some advanced techniques, using merge props and other options hidden in the connect function. useSelector accepts a second argument - an equality function to determine if the state has changed. verdict: both are great for performance in advanced situations.
Types: using typescript with connect is a nightmare. I remember myself feverishly writing three props interfaces for each connected component (OwnProps, StateProps, DispatchProps). Redux hooks support types in a rather straightforward way. verdict: types are significantly easier to work with using hooks.
The future of React: Hooks are the future of react. This may seam like an odd argument, but change to the ecosystem is right around the corner with "Concurrent mode" and "Server components". While class components will still be supported in future React versions, new features may rely solely on hooks. This change will of course also affect third party libraries in the eco system, such as React-Redux. verdict: hooks are more future proof.
TL;DR - Final verdict: each method has its merits. connect is more mature, has less potential for weird bugs and edge cases, and has better separation of concerns. Hooks are easier to read and write, as they are collocated near the place where they are used (all in one self contained component). Also, they are easier to use with TypeScript. Finally, they will easily be upgradable for future react versions.

I think you misunderstand what "top level" is. It merely means that, inside a functional component, useSelector() cannot be placed inside loops, conditions and nested functions. It doesn't have anything to do with root component or components structure
// bad
const MyComponent = () => {
if (condition) {
// can't do this
const data = useSelector(mySelector);
console.log(data);
}
return null;
}
---
// good
const MyComponent = () => {
const data = useSelector(mySelector);
if (condition) {
console.log(data); // using data in condition
}
return null;
}
If anything, mapStateToPtops is located at even higher level than a hook call
the rules of hooks make it very hard to use that specific hook. You still need to somehow access a changing value from the state inside callbacks
To be fair you almost never have to access changing value inside a callback. I can't remember last time I needed that. Usually if your callback needs the latest state, you are better off just dispatching an action and then handler for that action (redux-thunk, redux-saga, redux-observable etc) will itself access the latest state
This is just specifics of hooks in general (not just useSelector) and there are tons of ways to go around it if you really want to, for example
const MyComponent = () => {
const data = useSelector(mySelector);
const latestData = useRef()
latestData.current = data
return (
<button
onClick={() => {
setTimeout(() => {
console.log(latestData.current) // always refers to latest data
}, 5000)
}}
/>
)
}
What are the benefits of using the hook besides saving lines of code compared to mapStateToProps?
You save time by not writing connect function any time you need to access store, and removing it when you no longer need to access store. No endless wrappers in react devtools
You have clear distinction and no conflicts between props coming from connect, props coming from parent and props injected by wrappers from 3rd party libraries
Sometimes you (or fellow developers you work with) would choose unclear names for props in mapStateToProps and you will have to scroll all the way to mapStateToProps in the file to find out which selector is used for this specific prop. This is not the case with hooks where selectors and variables with data they return are coupled on the same line
By using hooks you get general advantages of hooks, the biggest of which is being able couple together and reuse related stateful logic in multiple components
With mapStateToProps you usually have to deal with mapDispatchToProps which is even more cumbersome and easier to get lost in, especially reading someone else's code (object form? function form? bindActionCreators?). Prop coming from mapDispatchToProps can have same name as it's action creator but different signature because it was overridden in mapDispatchToprops. If you use one action creator in a number of components and then rename that action creator, these components will keep using old name coming from props. Object form easily breaks if you have a dependency cycle and also you have to deal with shadowing variable names
.
import { getUsers } from 'actions/user'
class MyComponent extends Component {
render() {
// shadowed variable getUsers, now you either rename it
// or call it like this.props.getUsers
// or change import to asterisk, and neither option is good
const { getUsers } = this.props
// ...
}
}
const mapDispatchToProps = {
getUsers,
}
export default connect(null, mapDispatchToProps)(MyComponent)

See EDIT 2 at the end for the final answer
Since no one knows how to answer, it seems like the best answer is that you should NOT be using useselector when you need information in other places other than the root level of your component. Since you don't know if the component will change in the future, just don't use useselector at all.
If someone has a better answer than this, I'll change the accepted answer.
Edit: Some answers were added, but they just emphasize why you shouldn't be using useselector at all, until the day when the rules of hooks will change, and you'll be able to use it in a callback as well. That being said, if you don't want to use it in a callback, it could be a good solution for you.
EDIT 2: An answer with examples of all that I wanted was added and showed how useSelector and useDispatch are easier to use.

The redux state returned from the useSelector hook can be passed around anywhere else just like its done for mapStateToProps. Example: It can be passed to another function too. Only constraint being that the hook rules has to be followed during its declaration:
It has to be declared only within a functional component.
During declaration, it can not be inside any conditional block . Sample code below
function test(displayText) {
return (<div>{displayText}</div>);
}
export function App(props) {
const displayReady = useSelector(state => {
return state.readyFlag;
});
const displayText = useSelector(state => {
return state.displayText;
});
if(displayReady) {
return
(<div>
Outer
{test(displayText)}
</div>);
}
else {
return null;
}
}
EDIT: Since OP has asked a specific question - which is about using it within a callback, I would like to add a specific code.In summary, I do not see anything that stops us from using useSelector hook output in a callback. Please see the sample code below, its a snippet from my own code that demonstrates this particular use case.
export default function CustomPaginationActionsTable(props) {
//Read state with useSelector.
const searchCriteria = useSelector(state => {
return state && state.selectedFacets;
});
//use the read state in a callback invoked from useEffect hook.
useEffect( ()=>{
const postParams = constructParticipantListQueryParams(searchCriteria);
const options = {
headers: {
'Content-Type': 'application/json'
},
validateStatus: () => true
};
var request = axios.post(PORTAL_SEARCH_LIST_ALL_PARTICIPANTS_URI, postParams, options)
.then(function(response)
{
if(response.status === HTTP_STATUS_CODE_SUCCESS) {
console.log('Accessing useSelector hook output in axios callback. Printing it '+JSON.stringify(searchCriteria));
}
})
.catch(function(error) {
});
}, []);
}

For callback functions you can use the value returned from useSelector the same way you would use the value from useState.
const ExampleComponent = () => {
// use hook to get data from redux state.
const stateData = useSelector(state => state.data);
// use hook to get dispatch for redux store.
// this allows actions to be dispatched.
const dispatch = useDispatch();
// Create a non-memoized callback function using stateData.
// This function is recreated every rerender, a change in
// state.data in the redux store will cause a rerender.
const callbackWithoutMemo = (event) => {
// use state values.
if (stateData.condition) {
doSomething();
}
else {
doSomethingElse();
}
// dispatch some action to the store
// can pass data if needed.
dispatch(someActionCreator());
};
// Create a memoized callback function using stateData.
// This function is recreated whenever a value in the
// dependency array changes (reference comparison).
const callbackWithMemo = useCallback((event) => {
// use state values.
if (stateData.condition) {
doSomething();
}
else {
doSomethingElse();
}
// dispatch some action to the store
// can pass data if needed.
dispatch(someActionCreator());
}, [stateData, doSomething, doSomethingElse]);
// Use the callbacks.
return (
<>
<div onClick={callbackWithoutMemo}>
Click me
</div>
<div onClick={callbackWithMemo}>
Click me
</div>
</>
)
};
Rules of hooks says you must use it at the root of your component, meaning you CANT use it anywhere.
As Max stated in his answer just means that the hook statement itself must not be dynamic / conditional. This is because the order of the base hooks (react's internal hooks: useState, etc) is used by the backing framework to populate the stored data each render.
The values from hooks can be used where ever you like.
While I doubt this will be close to answering your complete question, callbacks keep coming up and no examples had been posted.

not the answer but this hook can be very helpful if you want to get decoupled nature of mapDispatchToProps while keeping simplicity and dev experience of hooks:
https://gist.github.com/ErAz7/1bffea05743440d6d7559afc9ed12ddc
the reason I don't mention one for mapStatesToProps is that useSelector itself is more store-logic-decoupling than mapStatesToProps so don't see any advantage for mapStatesToProps. Of course I dont mean using useSelector directly but instead create a wrapper on it in your store files (e.g. in reducer file) and import from there, like this:
// e.g. userReducer.js
export const useUserProfile = () => useSelector(state => state.user.profile)

ReactJS - how to pass "global" data to deeply nested child components?

How do people typically approach having "global" data in a React application?
For example, say I have the following data for a user once they're logged into my app.
user: {
email: 'test#user.com',
name: 'John Doe'
}
This is data that almost any component in my app might like to know about - so it could either render in a logged in or logged out state, or perhaps display the users email address if logged in.
From my understanding, the React way of accessing this data in a child component is for a top level component to own the data, and pass it to child components using properties, for example:
<App>
<Page1/>
<Page2>
<Widget1/>
<Widget2 user={user}/>
</Page2>
</App>
But this seems unwieldy to me, as that would mean I'd have to pass the data through each composite, just to get it to the child that needed it.
Is there a React way of managing this type of data?
Note: This example is very simplified - I like to wrap intents up as composites so implementation details of entire UI features can be drastically changed as I see fit.
EDIT: I'm aware that by default, calling setState on my top level component would cause all child components to be re-rendered, and that in each child component I can render using whatever data I like (e.g. global data, not just state or props). But how are people choosing to notify only certain child components that they should be rendered?

Since I originally answered this question, it's become apparent to me that React itself doesn't support "global" data in any sense - it is truly meant to manage the UI and that's it. The data of your app needs to live somewhere else. Having said that, it does now support accessing global context data as detailed in this other answer on this page. Here's a good article by Kent Dodds on how the context api has evolved, and is now officially supported in React.
The context approach should only be used for truly global data. If your data falls into any other category, then you should do as follows:
Facebook themselves solve this problem using their own Flux library.
Mobx and Redux are similar to Flux, but seem to have more popular appeal. They do the same thing, but in a cleaner, more intuitive way.
I'm leaving my original edits to this answer below, for some history.
OLD ANSWER:
The best answer I've found for this so far are these 2 React mixins, which I haven't had a chance to try, but they sound like they'll address this problem:
https://github.com/dustingetz/react-cursor
and this similar library:
https://github.com/mquan/cortex
MAJOR NOTE: I think this is a job for Facebook's Flux, or something similar (which the above are). When the data flow gets too complex, another mechanism is required to communicate between components other than callbacks, and Flux and it's clones seem to be it....

Use the React Context Property This is specifically for passing global data sets down the chain without explicitly forwarding them. It does complicate your Component lifecycle functions though, and note the cautions offered on the page I've linked.

You can use the React Context API for passing global data down to deeply nested child components. Kent C. Dodds wrote an extensive article on it on Medium React’s ⚛️ new Context API. It'll help in getting a better understanding of how to use the API.

I think React.createContext() is perfect solution for your purpose.
React will re-render only components, that listen context changes with useContext hook.
Here is a simple snippet for your code:
export const CurrentUser = React.createContext({})
const App = () =>
{
const User = getUser() // any authorisation method
return <>
<CurrentUser.Provider value={User}>
<App>
<Page1/>
<Page2>
<Widget1/>
<Widget2/>
</Page2>
</App>
</CurrentUser.Provider>
</>
}
const Widget2 = () =>
{
const User = useContext(CurrentUser)
return <>{User?.name}</>
}
In case if you want to control re-renders directly, you can use React.memo in nested components. For example, if you need re-render component only after specific attribute change.
Also, with nesting context values, you can reach good flexibility of your app. You can pass different context values for different part of your application.
export const CurrentUser = React.createContext({})
const App = () =>
{
const User = getUser() // any authorisation method
const AnotherUser = getAnotherUser() // any authorisation method
return <>
<CurrentUser.Provider value={User}>
<App>
<Page1/>
<CurrentUser.Provider value={AnotherUser}>
<Page2>
<Widget1/>
<Widget2/>
</Page2>
</CurrentUser.Provider>
</App>
</CurrentUser.Provider>
</>
}
const Widget2 = () =>
{
const User = useContext(CurrentUser)
return <>{User?.name}</>
}

What's wrong with just passing data all the way down the component chain via rendering all children with {...restOfProps}?
render(){
const {propIKnowAbout1, propIKnowAbout2, ...restOfProps} = this.props;
return <ChildComponent foo={propIKnowAbout1} bar={propIKnowAbout2} {...restOfProps}/>
}

There is Reactn https://www.npmjs.com/package/reactn
You use this.global and this.setGlobal to get and set the global state same as you do with the local state.
To be able to do so you only need to
import React from 'reactn';