QReact · Interview preparation
How do you choose a useEffect dependency array?
List every reactive value used inside an effect, such as props and state.
The idea to remember.
List every reactive value used inside an effect, such as props and state. If an effect runs too often, improve the effect's structure rather than removing required dependencies.
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 how do you choose a useEffect dependency array 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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