QReact · Interview preparation
When should you avoid useEffect?
Effects are for synchronization with external systems, not a default place for application logic.
The idea to remember.
Avoid an effect when a value can be derived during render, when an action belongs directly in an event handler, or when you are using an effect only to keep two pieces of React state in sync. Effects are most appropriate for synchronizing React with systems outside React, such as subscriptions, network lifecycles, timers, or browser APIs.
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 avoid useEffect 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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