/no-use-effect

name: no-use-effect
description: |
  Enforce the no-useEffect rule when writing or reviewing React code.
  ACTIVATE when writing React components, refactoring existing useEffect calls,
  reviewing PRs with useEffect, or when an agent adds useEffect "just in case."
  Provides the five replacement patterns and the useMountEffect escape hatch.

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]($SKILLBASE/references/you-might-not-need-an-effect.md). 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 lo