Develop Reference

Exporting a variable set in a function to another component React

I have some variables that are set within a function in one component of my react application that I need to reuse in other components.
I set the variables in component 1 like so (this is a much simplified version but captures the structure)
export default function Example() {
const [market, setMarket] = useState('');
return (
<button onClick={setMarket('1')}>Click 1</button>
<button onClick={setMarket('2')}>Click 2</button>
<button onClick={setMarket('3')}>Click 3</button> )}
How can I export the 'market' variable specifically, so that I can import it into another component (in a separate jsx file) and render as necessary. I know that I can just import the whole component, or set a variable outside of this function in component 1 and export it but I do not know how I would then conditionally set it based on which button is clicked.
Thank you

Hey #Milo there are different ways to use state value in another component.
First is props-
Create a component that passes values like-
const passValue = () => {
const [ value, setValue ] = useState("")
return (
)
}
While in the second component we get the value like-
const SecondComponent = ({value})=>{
return(
<div>
{value}
</div>
)
}
While Second method is to pass value using state and get it by useLocation in another component-
First Component like-
const FirstComponent = () =>{
return(
<div>
<Link to="/secondpage" state={{value:yourValue/state}}>Click Here</Link>
</div>
)
}
Second Component Like-
const Second Component = () => {
const {state} = useLocation()
return(
<div>{state}</div>
)
}
Hope these solution helps to solve your problem. If you still facing issue lemme know, i will help you.
Thanks

Variable passed trough state in Link can't update

I am updating my theme in my App per useState. This is passed to Topbar-Component per prop. console.log() gets triggered every time it changes. From Topbar theme is passed into a link to AboutMe-Copmponent as state, which works, but when i now change the state of theme it only updates in Topbar. I even tried Useeffect. Only when I refresh the site the change is noticed. I read hours about this but I cant solve it somehow.
AppComponent (not all code just the necessary):
function App() {
const [theme, setTheme] = useState('dark')
return (
<Topbar theme={theme}></Topbar>
<ToggleButton variant='light' onClick={() => setTheme('light')}>Light</ToggleButton>
<ToggleButton variant='dark' onClick={() => setTheme('dark')}>Dark</ToggleButton>
TopbarComponent:
export default function Topbar({theme}) {
console.log('Topbar',theme)
React.useEffect(()=>{
console.log('changed')
},[theme])
Output when I press the buttons:
Topbar light
changed
Topbar dark
changed
AboutMeComponent:
export default function AboutMe() {
const location = useLocation()
console.log(location.state)
React.useEffect(() => {
console.log('About-Me',location.state)
},[location])
Initial output:
dark
About-Me dark
When I now press the other Button I only get the Topbar Output
Only when refreshing I get the AboutMe Outputs again.
PS
The theme is changed anyway from dark to light but i need this state to change fonts etc.

I would suggest sticking with documentation's recommendation which is to use useContext for very this example of setting theme using context.
Check out: https://beta.reactjs.org/apis/react/useContext
Usage : Passing data deeply into the tree
import { useContext } from 'react';
function Button() {
const theme = useContext(ThemeContext);
useContext returns the context value for the context you passed. To determine the context value, React searches the component tree and finds the closest context provider above for that particular context.
To pass context to a Button, wrap it or one of its parent components into the corresponding context provider:
function MyPage() {
return (
<ThemeContext.Provider value="dark">
<Form />
</ThemeContext.Provider>
);
}
function Form() {
// ... renders buttons inside ...
}
It doesn’t matter how many layers of components there are between the provider and the Button. When a Button anywhere inside of Form calls useContext(ThemeContext), it will receive "dark" as the value.

I have it working now with the useContext hook. Thank you i somehow forgot about it.
App:
export const ThemeContext = React.createContext()
function App() {
const [theme, setTheme] = useState('black')
console.log(theme)
return (
<ThemeContext.Provider value={{backgroundColor:theme}}>
<BrowserRouter>
<div className='App' id={theme}>
<Topbar/>
<div className="position-absolute top-0 start-0">
<ToggleButton variant='light' onClick={() => setTheme('white')}>Light</ToggleButton>
<ToggleButton variant='dark' onClick={() => setTheme('black')}>Dark</ToggleButton>
</div>
Topbar:
export default function Topbar() {
const {user,logout} = UserAuth()
const [error, setError] = useState('')
const navigate = useNavigate()
const style = useContext(ThemeContext)
console.log(style)
AboutMe:
export default function AboutMe() {
const style = useContext(ThemeContext)
console.log(style)
return (
<>
<div className='d-flex' style={style}>
I had to move my Routing from Index.js to App.js because it had to be wrapped in the Context provider, but now my theme gets passed into every single component.

How to use state value in another js file

I have a file named Card.js. It contains two states called status and view .
Every time a button is clicked, my status state changes & according to this state my card is hidden.
At the same time, each of my cards has a view state. This increases by 1 according to the click event.
src/components/Card.js
const Card = (props) => {
const [status, setStatus] = useState(false);
const [view, setView] = useState(0);
const handleClick = () => {
setStatus(!status);
setView(view + 1);
};
return (
<div>
<Button onClick={handleClick}>{status ? "Show Component" : "Hide Component"}</Button>
{status ? null : <div className="card">{props.children} </div>}
</div>
);
};
export default Card;
In my app.js file, I return the data in my JSON file with the map method, I print this data to my card component.
src/App.js
const App= () => {
return (
<div>
{post.map((value, index) => {
return (
<Card key={index}>
{// here I want to print the "view" state in my Card.js file.}
</Card>
);
})}
</div>
);
};
export default App;
In App.js, I tried to create the view state using the useEffect hook, but every time I click the button, the view state of both my cards is updated at the same time. I don't want it to happen this way.

You can pass the data in your card as props so that the data is available in your Card module.
{
post.map((value, index) => {
return (
<Card key={index} status={false}>
{// here I want to print the "view" state in my Card.js file.}
</Card>
);
})
}
And in card module, pass the prop value to useState. I hope this will solve your problem.
const [status, setStatus] = useState(props.status);

Can't update parent component state with React UseState

im facing this weird behavior when trying to update the parent component with an set function to the child with props
this hook is to open and close the modal to edit an element
//PARENT FILE
//hook
const [isEditModalOpen, setEditModalOpen] = useState(false)
//more code...
//modal
{isEditModalOpen && <EditExcerciseModal setEditModalOpen={setEditModalOpen} isEditModalOpen={isEditModalOpen} />}
and this is the child code
//CHILD FILE
export const EditExcerciseModal = ({setEditModalOpen, excerciseInfo,fetchExcercisesFromRoutine})
//more code etc etc
<div className="addExcerciseModalContainer">
<span onClick={() =>{ setEditModalOpen(false) }} className="xModal">X</span>
i checked and the onClick is working. if i change the parent state manually the Modal works fine and closes.
the weird case when it its working is when instead of calling the set function i create a function with a setTimeout without time like this:
function closeModal(){
setTimeout(() => { setEditModalOpen(false)}, 0);
}
any ideas?
thanks for the help

You need to create a separation of concern. A Modal consists of three parts
The Modal of its Self.
The Content of the Modal.
And the container of the two.
You should be using the useState() hook and calling setEditModalOpen in the same containing component.
You need to make sure that you're declaring and setting state inside the same component.
// children would be the content of the modal
const Modal = ({ children, selector, open, setOpen }) => {
// we need the useEffect hook so that when we change open to false
// the modal component will re-render and the portal will not be created
useEffect(() => {
setOpen(false);
//provide useEffect hook with clean up.
return () => setOpen(true);
}, [selector]);
return open ? createPortal(children, selector) : null;
};
export const EditExerciseModal = ({ close }) => {
return (
<div>
{/* Instead of creating a handler inside this component we can create it in it's parent element */}
<span onClick={close}>X</span>
{/* Content */}
</div>
);
};
export const ModalBtn = () => {
const [isEditModalOpen, setEditModalOpen] = useState(false);
// this is where it all comes together,
// our button element will keep track of the isEditModalOpen variable,
// which in turn will update both child elements
// when true useEffect hook will re-render Modal Component only now it "will" createPortal()
// when our EditExerciseModal alls close it will set change the isEditModalOpen to false
// which will be passed to the Modal component which
// will then cause the component to re-render and not call createPortal()
return (
<>
<button onClick={() => setEditModalOpen(true)}>EditExerciseModal</button>
{setEditModalOpen && (
<Modal
open={isEditModalOpen}
setOpen={setEditModalOpen}
selector={'#portal'}>
<div className='overlay'>
<EditExerciseModal close={() => setEditModalOpen(false)} />
</div>
</Modal>
)}
</>
);
};

How to avoid re-renders at the App level when I make a change to a child component (like a searchbar)?

I'm trying to make a searchbar React component that doesn't trigger an App-wide re-render when I type, yet allows me to use the query in other components/to make an API call.
Background:
I learned that stateless input components are good for reusability and creating controlled components. So state stays at parent (or App) level and the component's value gets passed in via props.
On the other hand, tracking the query's state at the App level causes ALL components to re-render (when the input's handleChange calls setQuery) and feels like a needless use of resources.
What am I missing here? Do I leave the query piece of state at the SearchBar level instead? Should I use React.memo or useCallback?
SearchBar component:
import React from 'react';
const Searchbar = ({ query, handleQueryChange }) => {
return (
<div className="field">
<label>Enter search term</label>
<input type="text" onChange={handleQueryChange} value={query}></input>
</div>
);
};
export default Searchbar;
And the App component
const App = () => {
const [query, setQuery] = useState('');
const [results, setResults] = useState([]);
const handleQueryChange = (e) => {
setQuery(e.currentTarget.value);
};
useEffect(() => {
function search() {
...makeAPIcallwith(query).then((result) => {setResults(result)})
};
if (query) {
const timer = setTimeout(() => {
search()}, 1000);
return () => clearTimeout(timer);
}
}, [query]);
return (
<div className="content-container">
<SearchBar query={query} handleQueryChange={handleQueryChange} />
<...Other React component not needing to re-render... />
</div>
);
};
export default App;

The tiniest optimization that you could make here is this:
const handleQueryChange = useCallback((e) => {
setQuery(e.currentTarget.value);
},[]);
It's not worth making. What you've shown is good idomatic react code.
I guess the other thing that you could do, if you haven't already because you haven't shown the code, is to help React out by encapsulating the results in a component like this:
return (
<div className="content-container">
<SearchBar query={query} handleQueryChange={handleQueryChange} />
<ListOfThings results={results}/>
</div>
);
Super tiny components, so tiny they seem almost trivially simple, is the name of the game in React. If your components are over 30-40 lines long, then 👎