Rules of Hooks explained: why they exist, what breaks when violated, and how to structure custom hooks correctly.
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Hooks rely on a stable call order between renders. Violating the rules causes bugs that are difficult to diagnose.
Do not call hooks inside loops, conditions, or nested functions.
// BAD — conditional hook
function Profile({ userId }: { userId?: string }) {
if (userId) {
const [user, setUser] = useState(null); // breaks!
}
}
// GOOD — unconditional hook, conditional logic inside
function Profile({ userId }: { userId?: string }) {
const [user, setUser] = useState(null);
useEffect(() => {
if (userId) fetchUser(userId).then(setUser);
}, [userId]);
}Why: React tracks hooks by their call index. If a hook is conditionally skipped, all subsequent hooks shift positions, causing React to return the wrong state for each hook.
Call hooks only from:
use)// BAD — hook in a regular function
function getUser() {
const [user] = useState(null); // not a component or hook!
return user;
}
// GOOD — custom hook
function useUser() {
const [user, setUser] = useState(null);
useEffect(() => { fetchUser().then(setUser); }, []);
return user;
}| Violation | Symptom |
|---|---|
Hook inside if |
State values swap between hooks unpredictably |
| Hook inside loop | Crash on re-render when array length changes |
| Hook in regular function | React error: hooks called outside component |
| Hook after early return | Fewer hooks than expected error |
function useDebounce<T>(value: T, delay: number): T {
const [debouncedValue, setDebouncedValue] = useState(value);
useEffect(() => {
const timer = setTimeout(() => setDebouncedValue(value), delay);
return () => clearTimeout(timer);
}, [value, delay]);
return debouncedValue;
}Best practices:
useuseEffect returnInstall the official plugin:
{
"plugins": ["react-hooks"],
"rules": {
"react-hooks/rules-of-hooks": "error",
"react-hooks/exhaustive-deps": "warn"
}
}This catches most violations at lint time rather than runtime.