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Converting code from functional React hooks to class components

I'm currently trying to convert some hooks over to being inside class components in React. I'm attempting to recreate the code that allows for session storage so that the app doesn't need to fetch from the API so much. Here is the code:
useEffect(() => {
if(sessionStorage.homeState){
// console.log("Getting from session storage");
setState(JSON.parse(sessionStorage.homeState));
setLoading(false);
}else{
// console.log("Getting from API");
fetchMovies(POPULAR_BASE_URL);
}
}, [])
useEffect(() => {
if(!searchTerm){
// console.log("Writing to session storage");
sessionStorage.setItem('homeState', JSON.stringify(state));
}
}, [searchTerm, state])
These are two useEffect hooks so it makes sense to me to go with a regular componentDidMount to get from the session storage. The only thing that I can't seem to figure out is how to recreate the second useEffect that sets session storage and fires only when searchTerm or state changes. searchTerm and state are simply two properties of the state. How could I implement this since componentDidMount only fires once when the app first mounts? Thanks

The only thing that I can't seem to figure out is how to recreate the second useEffect that sets session storage and fires only when searchTerm or state changes. searchTerm and state are simply two properties of the state.
componentDidMount() is only one of methods used by the class components you can recreate the second hook with componentWillUpdate() or shouldComponentUpdate().
For example:
componentWillUpdate(nextProps, nextState) {
if (this.props.searchTerm !== prevProps.searchTerm) {
...
}
}
You can check what lifecycle methods are available in class components by Googling "class component lifecycle".
But as you can read in the comment to your questions Hooks can offer you more than class components, and recreating them is not trivial. It is easier to move from the class component to the Hooks.

Fetch graphql query result on render and re-renders BUT LAZILY

I have a React Apollo app and what I am trying to do is that I have a component that renders some data using charts. For this data, I have some filters that I save in the local state of the component (Using hooks)
const [filters, setFilters] = useState(defaultFilters);
Now what I want is that whenever the component mounts, fetch the data using the default filters. But I also want to re-fetch data when the user updates the filters AND CLICKS ON SUBMIT and I'd fetch the results using new filters.
Since I also want to fetch the results on filter update, I am using useLazyQuery hook provided by apollo
const [getData, {data}] = useLazyQuery(GET_DATA_QUERY, { variables: {filters} });
useEffect(getData, []); // this useEffect runs only when the component mounts and never again
But, what happens is whenever my state, filters, updates the getData function is automatically run! ALWAYS! (BEHIND THE SCENE)
How do I handle such cases, where I want to fetch results on mounting and re-rendering.
I have tried using useQuery and refetch provided by it but I get the same problem there, whenever I update the state, the component rerenders and the useQuery hooks is run and makes the call. (That's how I believe it runs)
How do I fix my current code. Calling the getData function inside the useEffect function makes it run on every re-render.
I think I the problem defined in this stackoverflow-question is somewhat similar to mine.

Part of the problem is that you really have two different states that you're trying to utilize a single hook for. You have state that represents your inputs' values in the UI, and then you have state that represents the filters you want to actually apply to your charts. These are two separate bits of state.
The simplest solution is to just do something like this:
const [inputFilters, setInputFilters] = useState(defaultFilters)
const [appliedFilters, setAppliedFilters] = useState(inputFilters)
const { data } = useQuery(GET_DATA_QUERY, { variables: { filters: appliedFilters } })
const handleSubmit = () => setAppliedFilters(inputFilters)
const handleSomeInputChange = event => setInputFilters(...)
This way, you use inputFilters/setInputFilters only to manage your inputs' state. When the user clicks your submit button, the appliedFilters are set to whatever the inputFilters are at the time, and your query will update to reflect the new variables.

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)

Gatsby: Context update causes infinite render loop

I am trying to update context once a Gatsby page loads.
The way I did it, the context is provided to all pages, and once the page loads the context is updated (done with useEffect to ensure it only happens when the component mounts).
Unfortunately, this causes an infinite render loop (perhaps not in Firefox, but at least in Chrome).
Why does this happen? I mean, the context update means all the components below the provider are re-rendered, but the useEffect should only run once, and thats when the component mounts.
Here is the code: https://codesandbox.io/s/6l3337447n
The infinite loop happens when you go to page two (link at bottom of page one).
What is the solution here, if I want to update the context whenever a page loads?

The correct answer for this issue is not to pass an empty dependency array to useEffect but to wrap your context's mergeData in a useCallback hook. I'm unable to edit your code but you may also need to add dependencies to your useCallback like in my example below
import React, { useState, useCallback } from "react"
const defaultContextValue = {
data: {
// set initial data shape here
menuOpen: false,
},
mergeData: () => {},
}
const Context = React.createContext(defaultContextValue)
const { Provider } = Context
function ContextProviderComponent({ children }) {
const [data, setData] = useState({
...defaultContextValue,
mergeData, // shorthand method name
})
const mergeData = useCallback((newData) {
setData(oldData => ({
...oldData,
data: {
...oldData.data,
...newData,
},
}))
}, [setData])
return <Provider value={data}>{children}</Provider>
}
export { Context as default, ContextProviderComponent }
The selected answer is incorrect because the react docs explicitly say not to omit dependencies that are used within the effect which the current selected answer is suggesting.
If you use es-lint with the eslint-plugin-react-hooks it will tell you this is incorrect.
Note
If you use this optimization, make sure the array includes all values
from the component scope (such as props and state) that change over
time and that are used by the effect. Otherwise, your code will
reference stale values from previous renders. Learn more about how to
deal with functions and what to do when the array changes too often.
https://reactjs.org/docs/hooks-effect.html
Is it safe to omit functions from the list of dependencies? Generally
speaking, no. It’s difficult to remember which props or state are used
by functions outside of the effect. This is why usually you’ll want to
declare functions needed by an effect inside of it. Then it’s easy to
see what values from the component scope that effect depends on:
https://reactjs.org/docs/hooks-faq.html#is-it-safe-to-omit-functions-from-the-list-of-dependencies

By default, useEffect runs every render. In your example, useEffect updates the context every render, thus trigger an infinite loop.
There's this bit in the React doc:
If you want to run an effect and clean it up only once (on mount and unmount), you can pass an empty array ([]) as a second argument. This tells React that your effect doesn’t depend on any values from props or state, so it never needs to re-run. This isn’t handled as a special case — it follows directly from how the dependencies array always works.
So applies to your example:
useEffect(() => {
console.log("CONTEXT DATA WHEN PAGE 2 LOADS:", data)
mergeData({
location,
})
- }, [location, mergeData, data])
+ }, [])
This way, useEffect only runs on first mount. I think you can also leave location in there, it will also prevent the infinite loop since useEffect doesn't depend on the value from context.

React Router 4 navigation pattern in a Redux app

I am currently working on a simple React app with a very common workflow where users trigger Redux actions that, in turn, request data from an API. But since I would like to make the results of these actions persistent in the URL, I have opted for React Router v4 to help me with the job.
I have gone through the Redux integration notes in the React Router documentation but the idea of passing the history object to Redux actions just doesn't feel like the most elegant pattern to me. Since both Redux and Router state changes cause React components to be re-rendered, I'm a little worried the component updates could go a bit out of control in this scenario.
So in order to make the re-rendering a bit more predictable and sequential, I have come up with the following pattern that attempts to follow the single direction data flow principle:
Where I used to trigger Redux actions as a result of users' interactions with the UI, I am now calling React Router's props.history.push to update the URL instead. The actual change is about updating a URL parameter rather than the whole route but that's probably not that relevant here.
Before:
// UserSelector.jsx
handleUserChange = ({ target: selectElement }) => {
// Some preliminary checks here...
const userId = selectElement.value
// Fire a Redux action
this.props.setUser(userId)
}
After:
// UserSelector.jsx
handleUserChange = ({ target: selectElement }) => {
// Some preliminary checks here...
const userId = selectElement.value
// Use React Router to update the URL
this.props.history.push(`/user-selector/${userId}`)
}
The userId change in the URL causes React Router to trigger a re-render of the current route.
Route definition in App.jsx:
<Route path="/user-selector/:userId?" component={UserSelector} />
During that re-render, a componentDidUpdate lifecycle hook gets invoked. In there I am comparing the previous and current values of the URL parameter via the React Router's props.match.params object. If a change is detected, a Redux action gets fired to fetch new data.
Modified UserSelector.jsx:
componentDidUpdate (prevProps) {
const { match: { params: { userId: prevUserId } } } = prevProps
const { match: { params: { userId } } } = this.props
if (prevUserId === userId) {
return
}
// Fire a Redux action (previously this sat in the onChange handler)
this.props.setUser(userId)
}
When the results are ready, all React components subscribed to Redux get re-rendered.
And this is my attempt to visualise how the code's been structured:
If anyone could verify if this pattern is acceptable, I would be really grateful.

For step 3, I suggest a different approach which should be more in line with react-router:
react-router renders a component based on a route
this component should act as the handler based on the particular route it matches (think of this as a container or page component)
when this component is mounted, you can use componentWillMount to fetch (or isomorphic-fetch) to load up the data for itself/children
this way, you do not need to use componentDidUpdate to check the URL/params
Don't forget to use componentWillUnmount to cancel the fetch request so that it doesn't cause an action to trigger in your redux state
Don't use the App level itself to do the data fetching, it needs to be done at the page/container level
From the updated code provided in the question:
I suggest moving the logic out, as you would most likely need the same logic for componentDidMount (such as the case when you first hit that route, componentDidUpdate will only trigger on subsequent changes, not the first render)

I think it's worth considering whether you need to store information about which user is selected in your Redux store and as part of URL - do you gain anything by structuring the application like this? If you do, is it worth the added complexity?