QReact · Interview preparation
When should you extract a custom React Hook?
Extract a custom Hook when several components share related stateful behavior.
The idea to remember.
Extract a custom Hook when several components share related stateful behavior. A custom Hook shares logic, but each call has its own state unless it connects to an external shared store.
01Learn by doing
From first example to real project.
Start with the smallest working idea, examine a more detailed example, then look at a real application pattern. Adapt dependencies, error handling and data models to your project.
Basic: local state
01 / BEGINNERUpdate React state without mutating the previous value.
function Counter() {
const [count, setCount] = React.useState(0);
return <button onClick={() => setCount(n => n + 1)}>{count}</button>;
}Intermediate: synchronize external data
02 / INTERMEDIATEAn effect subscribes to something outside React and cleans it up.
function OnlineStatus() {
const [online, setOnline] = React.useState(navigator.onLine);
React.useEffect(() => {
const update = () => setOnline(navigator.onLine);
window.addEventListener('online', update);
window.addEventListener('offline', update);
return () => {
window.removeEventListener('online', update);
window.removeEventListener('offline', update);
};
}, []);
return <p>{online ? 'Online' : 'Offline'}</p>;
}Real scenario: cancel stale requests
03 / REAL SCENARIOCancel the previous request when a search term changes.
React.useEffect(() => {
const controller = new AbortController();
fetch('/api/search?q=' + encodeURIComponent(query), { signal: controller.signal })
.then(response => { if (!response.ok) throw new Error('Request failed'); return response.json(); })
.then(setResults)
.catch(error => { if (error.name !== 'AbortError') setError(error.message); });
return () => controller.abort();
}, [query]);02Check your understanding
Try it in your own words.
Explain when should you extract a custom React Hook without looking at the code. Then modify the intermediate example, describe one trade-off and identify when the real-world pattern fits.
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