What is React StrictMode effects?

Question

Accepted Answer

**StrictMode double-effects:**
- Effects run twice in development
- Cleanup runs before second effect
- Helps detect missing cleanup

```tsx
useEffect(() => {
  const sub = api.subscribe(handler);
  return () => sub.unsubscribe();
}, []);
// In Strict Mode: subscribe -> unsubscribe -> subscribe
```

**What it catches:** Missing cleanup functions, impure effects, race conditions.

**No production impact:** Effects run once in production.

**Best practice:** Always include cleanup in useEffect.

React: What is React StrictMode effects?