React Hooks Expert

Priority: P0 (CRITICAL)

Effects sync external systems. Everything else should stay in render, event handlers, or custom hooks.

Rules

• Effects: use useEffect only for subscriptions, timers, network lifecycle, DOM APIs, or other external systems.
• Dependency churn: objects recreated each render cause unstable dependencies; move to primitives, refs, or measured memoization.
• Effect events: use useEffectEvent when effect logic needs current props/state without widening dependencies.
• Refs: store mutable, non-render state in useRef.
• Memoization: add useMemo / useCallback only for measured hotspots or memoized child contracts.
• Concurrency: use startTransition / useDeferredValue for non-urgent updates.
• Fetch cleanup: use AbortController when an effect owns request cancellation.

Verify

• Hooks stay at top level.
• Effects have cleanup when they subscribe, attach, or schedule.
• No effect exists only to derive render state from other render state.
• Dependency arrays reflect real external dependencies.
• Custom hooks hide internal wiring and expose a small API.

Performance Checklist (Mandatory)

• Rules of Hooks: Called at top level? No loops/conditions?
• Dependencies: object/function deps exist for a reason, not by accident?
• Cleanup: useEffect subscriptions return cleanup functions?
• Render Count: component re-render excessively?

Anti-Patterns

• No Missing Deps: Fix logic, don't disable linter.
• No giant effects: Split unrelated sync concerns.
• No Derived State: Compute during render, don't useEffect to sync state.
• No Heavy Init: Use lazy state initialization useState(() => heavy()).

References

• Framework Map
• Optimization Patterns

Canonical response anchors

When this skill applies, preserve the following domain terminology or equivalent concrete examples in the answer when relevant:

• cleanup
• recreated