langgenius/mosoo-skills/no-use-effect

Avoid Unnecessary useEffect

Prefer render-time derivation, event handlers, the existing query layer, and component identity over effect choreography. Use React useEffect directly when a component must synchronize with a system outside React.

Decision Table

Workflow

1. State what external system, if any, the component must synchronize with.
2. If there is none, move the work to render, an event handler, the query

layer, or component identity.

1. If there is one, keep the effect narrow, declare every reactive dependency,

and return cleanup when setup creates a resource or subscription.

1. Verify the affected web package with the repository's focused commands.

1. Derive During Render

Do not store a value that can be calculated from current props or state.

// Avoid: an extra render and a temporarily stale value.
const [visibleItems, setVisibleItems] = useState<Item[]>([]);
useEffect(() => {
  setVisibleItems(items.filter((item) => item.visible));
}, [items]);
// Prefer: one source of truth.
const visibleItems = items.filter((item) => item.visible);

Use useMemo only when the calculation is measurably expensive or stable identity is required by a consumer. It is not a replacement for every effect.

2. Keep User Actions In Event Handlers

Do not turn state into a command queue for an effect.

// Avoid: click -> flag -> effect -> reset flag.
const [shouldSave, setShouldSave] = useState(false);
useEffect(() => {
  if (!shouldSave) return;
  void saveDraft();
  setShouldSave(false);
}, [shouldSave]);
// Prefer: the event owns the action.
function handleSave() {
  void saveDraft();
}

If multiple callers need the action, extract a normal function or mutation hook. Do not route it through render state.

3. Use The Existing Data Layer

Avoid ad hoc fetch effects that recreate cancellation, caching, retry, loading, and stale-data behavior. Use mosoo's generated GraphQL access and existing TanStack Query patterns when they fit the boundary.

Keep request parameters in the query key or mutation input. Handle user-triggered mutations from the event that caused them.

4. Synchronize External Systems Directly

Effects are appropriate for browser APIs, timers, sockets, subscriptions, third-party widgets, DOM integration, and other resources whose lifecycle must follow the mounted component.

useEffect(() => {
  const unsubscribe = sessionStream.subscribe(sessionId, handleEvent);
  return unsubscribe;
}, [sessionId, handleEvent, sessionStream]);

Requirements for a necessary effect:

• setup and cleanup are safe under React Strict Mode's development replay;
• every reactive value used by the effect is represented in its dependency

list;

• callbacks or objects are stabilized only when their identity is part of the

external subscription contract;

• cleanup releases listeners, timers, observers, sockets, or pending work;
• the effect does not also mirror derived state or relay a user action.

Do not hide dependencies behind a mount-only wrapper. If synchronization is truly mount-scoped, the direct effect and its cleanup should make that contract visible.

5. Reset With Component Identity

When a component should behave as a new instance for a different entity, put the identity in a parent key instead of resetting several state values in an effect.

function AgentEditorRoute({ agentId }: { agentId: string }) {
  return <AgentEditor key={agentId} agentId={agentId} />;
}

Use this only when a full local subtree reset is the intended user experience. Preserve state explicitly when users expect drafts or selections to survive an identity change.

Review Questions

• What external system requires synchronization?
• Could the value be computed during render?
• Did a user event already provide the correct execution boundary?
• Does the existing query or mutation layer own this request?
• Would a key express the intended reset more directly?
• For a necessary effect, are dependencies, cleanup, and replay safety clear?

Verification

Run from the mosoo repository root:

just tc-package @mosoo/web
just test-package @mosoo/web

Use just test-file <path> when one focused test proves the behavior. Add a browser or manual flow when the effect changes a user-visible lifecycle such as reconnection, notifications, autosave, or navigation.