No useEffect

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

Quick Reference

• Repo linting: bun lint --format json <package-path>
• Escape hatch hook: useMountEffect from @onequery/ui/hooks/use-mount-effect
• React docs: You Might Not Need an Effect
• Origin: https://x.com/alvinsng/status/2033969062834045089

Comment: the repo currently has useMountEffect, but it does not appear to have a global lint rule that already bans useEffect. Treat this skill as the policy source unless that restriction is added later.

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 lint --format json <package-path>
bunx turbo typecheck --filter=<package> --json
bunx turbo test --filter=<package> --json

Use bunx turbo check --json when the change spans multiple packages or needs a whole-repo lint/format pass.

The Escape Hatch: useMountEffect

For the rare case where you need to sync with an external system on mount, use the shared wrapper from @onequery/ui/hooks/use-mount-effect.

The implementation keeps the intent explicit:

import { useEffect } from "react";

export function useMountEffect(effect: () => void | (() => void)) {
  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 }: { productId: string }) {
  const [product, setProduct] = useState(null);

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

// GOOD: Query library handles cancellation, caching, and staleness
function ProductPage({ productId }: { productId: string }) {
  const { data: product } = useQuery({
    queryKey: ["product", productId],
    queryFn: () => 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 }: { isLoading: boolean }) {
  useEffect(() => {
    if (!isLoading) playVideo();
  }, [isLoading]);
}

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

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

Use useMountEffect for stable dependencies such as singletons, refs, or context values that do not change over the component lifetime:

// BAD: useEffect with dependency that never changes
useEffect(() => {
  connectionManager.on("connected", handleConnect);
  return () => connectionManager.off("connected", handleConnect);
}, [connectionManager]);

// 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 }: { videoId: string }) {
  useEffect(() => {
    loadVideo(videoId);
  }, [videoId]);
}

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

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

Smell test: You are writing an effect whose only job is to reset local state when an ID or 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>;
}