Use Effect

Use Effect

Inspect the local React patterns before changing code. Prefer the project's existing data-fetching library, lifecycle wrapper, and lint setup over introducing new abstractions.

Treat direct useEffect as a code smell by default. Replace it with clearer control flow unless the code is genuinely synchronizing with an external system on mount or unmount.

Workflow

1. Locate every direct useEffect in the relevant scope and classify why it exists.
2. Decide whether the effect is:
   • deriving state from props or state
   • fetching data
   • relaying an action that should happen in response to a user event
   • synchronizing with an external system on mount
   • resetting local state when an identity changes
3. Replace the effect with the narrowest alternative pattern.
4. Preserve existing project conventions. If the codebase already has useMountEffect, use it instead of raw useEffect(..., []).
5. Verify behavior by running the relevant tests or targeted checks. Pay special attention to loops, duplicate requests, stale state, and remount semantics.

Replacement Rules

Derive State, Do Not Sync It

If an effect sets state from other props or state, compute the value during render instead.

Common smell:

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

Preferred direction:

const filteredProducts = products.filter((p) => p.inStock);

Also collapse multi-step derived values instead of chaining effects through intermediary state.

Use Data-Fetching Abstractions

If an effect fetches data and then writes it into local state, prefer the project's query or loader abstraction. Reuse the codebase's existing library when present.

Common smell:

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

Preferred direction:

const { data: product } = useQuery({
  queryKey: ["product", productId],
  queryFn: () => fetchProduct(productId),
});

If the project does not already use a client-side query library, check whether the fetch belongs in framework loaders, server components, or route-level data APIs before adding one.

Use Event Handlers, Not Effect Relays

If state is only used as a flag to make an effect do work later, move that work into the event handler that caused it.

Common smell:

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

Preferred direction:

const handleLike = () => {
  postLike();
};

Use Mount-Only Effects Only for External Sync

For true setup and cleanup with external systems, use the project's mount-only wrapper if it exists. Keep this category narrow: DOM integration, subscriptions, widget lifecycle, imperative browser APIs.

If a precondition gates the mount-only effect, prefer conditional rendering so the component mounts only when ready.

if (isLoading) return <LoadingScreen />;
return <VideoPlayer />;

Then let VideoPlayer run mount-only setup once.

Reset with key, Not Dependency Choreography

If the goal is "treat this as a new instance when identity changes", remount with key instead of writing effect logic that tries to reset local state.

return <VideoPlayer key={videoId} videoId={videoId} />;

Use this when the desired behavior is a fresh lifecycle boundary.

Code Review Heuristics

Flag direct useEffect when you see:

• setState driven by props or other state
• fetch(...).then(setState) or async loading logic inside an effect
• state used as an action flag
• dependency arrays that are being "fixed" incrementally
• effect chains where one effect updates state that triggers another effect
• logic that really wants remount semantics

Do not remove mount-only effects that are genuinely integrating with external systems unless you replace them with an equivalent lifecycle boundary.

Adoption Guidance

When the user asks for a broader rollout:

• Add or update lint rules that ban direct useEffect.
• Add a local wrapper such as useMountEffect only if the codebase wants an explicit exception path.
• Update agent guidance or contributor docs so future edits do not reintroduce direct effects.
• Prefer incremental refactors around touched files unless the user asks for a full migration.

References

Read patterns.md when you need detailed smell tests, larger before/after examples, or help choosing between replacement patterns.