What is React useEffect dependency array?

Question

Accepted Answer

**Dependency array** — controls when effect runs.

```tsx
// Runs on every render (no deps)
useEffect(() => { console.log("rendered"); });

// Runs once on mount (empty deps)
useEffect(() => { initApp(); }, []);

// Runs when userId changes
useEffect(() => { fetchUser(userId); }, [userId]);
```

**Rules:**
- All outer-scope values used inside must be in deps
- Use exhaustive-deps ESLint rule
- Avoid suppressing warnings

**Common mistakes:** Missing dependencies, object/array dependencies (new reference each render), functions created in render.

**Fix:** Use useCallback for functions, useMemo for objects.

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