No useEffect

Never call useEffect directly. Use derived state, event handlers, data-fetching libraries, or useMountEffect instead.

Quick Reference

This skill is dervied from the React docs: You Might Not Need an Effect. IMPORTANT: when this skill is invoked you MUST read this post in entirety.

Instead of useEffect for...	Use
Deriving state from other state/props	Inline computation (Rule 1)
Fetching data	useQuery / data-fetching library (Rule 2)
Responding to user actions	Event handlers (Rule 3)
One-time external sync on mount	useMountEffect (Rule 4)
Resetting state when a prop changes	key prop on parent (Rule 5)

When to Use This Skill

• Writing new React components
• Refactoring existing useEffect calls
• Reviewing PRs that introduce useEffect
• An agent adds useEffect "just in case"

Workflow

1. Identify the useEffect

Determine what the effect is doing -- deriving state, fetching data, responding to an event, syncing with an external system, or resetting state.

2. Apply the Correct Replacement Pattern

Use the five rules below to pick the right replacement.

3. Verify

bun --bun run typecheck -- --filter=<package>
bun --bun run test -- --filter=<package>

The Escape Hatch: useMountEffect

For the rare case where you need to sync with an external system on mount:

The implementation wraps useEffect with an empty dependency array to make intent explicit:

export function useMountEffect(effect: () => void | (() => void)) {
  /* eslint-disable no-restricted-syntax */
  useEffect(effect, []);
}

Replacement Patterns

Rule 1: Derive state, do not sync it

Most effects that set state from other state are unnecessary and add extra renders.

// BAD: Two render cycles - first stale, then filtered
function ProductList() {
  const [products, setProducts] = useState([]);
  const [filteredProducts, setFilteredProducts] = useState([]);

  useEffect(() => {
    setFilteredProducts(products.filter((p) => p.inStock));
  }, [products]);
}

// GOOD: Compute inline in one render
function ProductList() {
  const [products, setProducts] = useState([]);
  const filteredProducts = products.filter((p) => p.inStock);
}

Smell test: You are about to write useEffect(() => setX(deriveFromY(y)), [y]), or you have state that only mirrors other state or props.

Rule 2: Use data-fetching libraries

Effect-based fetching creates race conditions and duplicated caching logic.

// BAD: Race condition risk
function ProductPage({ productId }) {
  const [product, setProduct] = useState(null);

  useEffect(() => {
    fetchProduct(productId).then(setProduct);
  }, [productId]);
}

// GOOD: Query library handles cancellation/caching/staleness
function ProductPage({ productId }) {
  const { data: product } = useQuery(['product', productId], () =>
    fetchProduct(productId)
  );
}

Smell test: Your effect does fetch(...) and then setState(...), or you are re-implementing caching, retries, cancellation, or stale handling.

Rule 3: Event handlers, not effects

If a user clicks a button, do the work in the handler.

// BAD: Effect as an action relay
function LikeButton() {
  const [liked, setLiked] = useState(false);

  useEffect(() => {
    if (liked) {
      postLike();
      setLiked(false);
    }
  }, [liked]);

  return <button onClick={() => setLiked(true)}>Like</button>;
}

// GOOD: Direct event-driven action
function LikeButton() {
  return <button onClick={() => postLike()}>Like</button>;
}

Smell test: State is used as a flag so an effect can do the real action, or you are building "set flag -> effect runs -> reset flag" mechanics.

Rule 4: useMountEffect for one-time external sync

Good uses: DOM integration (focus, scroll), third-party widget lifecycles, browser API subscriptions.

// BAD: Guard inside effect
function VideoPlayer({ isLoading }) {
  useEffect(() => {
    if (!isLoading) playVideo();
  }, [isLoading]);
}

// GOOD: Mount only when preconditions are met
function VideoPlayerWrapper({ isLoading }) {
  if (isLoading) return <LoadingScreen />;
  return <VideoPlayer />;
}

function VideoPlayer() {
  useMountEffect(() => playVideo());
}

Use useMountEffect for stable dependencies (singletons, refs, context values that never change):

// BAD: useEffect with dependency that never changes
useEffect(() => {
  connectionManager.on('connected', handleConnect);
  return () => connectionManager.off('connected', handleConnect);
}, [connectionManager]); // connectionManager is a singleton from context

// GOOD: useMountEffect for stable dependencies

useMountEffect(() => {
  connectionManager.on('connected', handleConnect);
  return () => connectionManager.off('connected', handleConnect);
});

Smell test: You are synchronizing with an external system, and the behavior is naturally "setup on mount, cleanup on unmount."

Rule 5: Reset with key, not dependency choreography

// BAD: Effect attempts to emulate remount behavior
function VideoPlayer({ videoId }) {
  useEffect(() => {
    loadVideo(videoId);
  }, [videoId]);
}

// GOOD: key forces clean remount
function VideoPlayer({ videoId }) {
  useMountEffect(() => {
    loadVideo(videoId);
  });
}

function VideoPlayerWrapper({ videoId }) {
  return <VideoPlayer key={videoId} videoId={videoId} />;
}

Smell test: You are writing an effect whose only job is to reset local state when an ID/prop changes, or you want the component to behave like a brand-new instance for each entity.

Component Structure Convention

Computed values come after hooks and local state, never via useEffect:

export function FeatureComponent({ featureId }: ComponentProps) {
  // Hooks first
  const { data, isLoading } = useQueryFeature(featureId);

  // Local state
  const [isOpen, setIsOpen] = useState(false);

  // Computed values (NOT useEffect + setState)
  const displayName = user?.name ?? 'Unknown';

  // Event handlers
  const handleClick = () => { setIsOpen(true); };

  // Early returns
  if (isLoading) return <Loading />;

  // Render
  return <Flex direction="column" gap="lg">...</Flex>;
}