React Frontend

Verify before implementing: For App Router patterns, React 19 APIs, or version-specific behavior, search current docs via search_docs before writing code. Training data may lag current releases.

Component TypeScript

• Extend native elements with ComponentPropsWithoutRef<'button'>, add custom props via intersection
• Use React.ReactNode for children, React.ReactElement for single element, render prop (data: T) => ReactNode
• Discriminated unions for variant props -- TypeScript narrows automatically in branches
• Generic components: <T> with keyof T for column keys, T extends { id: string } for constraints
• Event types: React.MouseEvent<HTMLButtonElement>, FormEvent<HTMLFormElement>, ChangeEvent<HTMLInputElement>
• as const for custom hook tuple returns
• useRef<HTMLInputElement>(null) for DOM (use ?.), useRef<number>(0) for mutable values
• Explicit useState<User | null>(null) for unions/null
• useReducer actions as discriminated unions: { type: 'set'; payload: number } | { type: 'reset' }
• useContext null guard: throw in custom useX() hook if context is null

Effects Decision Tree

Effects are escape hatches -- most logic should NOT use effects.

Need	Solution
Derived value from props/state	Calculate during render (useMemo if expensive)
Reset state on prop change	key prop on component
Respond to user event	Event handler
Notify parent of state change	Call onChange in event handler, or fully controlled component
Chain of state updates	Calculate all next state in one event handler
Sync with external system	Effect with cleanup

Effect rules:

• Never suppress the linter -- fix the code instead
• Use updater functions (setItems(prev => [...prev, item])) to remove state dependencies
• Move objects/functions inside effects to stabilize dependencies
• useEffectEvent for non-reactive values (e.g., theme in a connection effect)
• Always return cleanup for subscriptions, connections, listeners
• Data fetching cancellation (pick by situation): AbortController for fetch; ignore flag for non-cancellable promises; React Query handles both automatically

Concurrency & Race Classes

Frontend bugs that survive type-checking and unit tests usually land in one of five race classes. Hunt each one explicitly during review:

1. Lifecycle cleanup gaps -- in-production signal: "Can't perform state update on an unmounted component" warnings, slow-burn memory leaks under rapid navigation, duplicate event handlers firing. Root cause: useEffect registered a listener/timer/observer without returning cleanup (see Effect rules above for the rule).
2. Remount-timing mistakes -- async callbacks mutate DOM or state after swap / disconnect / route change. Classic cases: a fetch().then(setData) resolves after navigation to a different route; a requestAnimationFrame fires after the parent unmounts. See "Data fetching" in Effect rules above for the cancellation hierarchy.
3. Boolean-as-state for UI that isn't binary -- isLoading: boolean can't represent idle | loading | success | error | retry without creating inconsistent combinations (isLoading: true, error: Error is contradictory). Prefer an explicit state constant ('idle' | 'loading' | 'success' | 'error') with a transition function so invalid states are unreachable.
4. Stale promises and timers with no cancel path -- a promise chain or setTimeout holds a reference to setState after the component's moved on. Bind every async operation to a cancel mechanism per the cancellation hierarchy above, and verify the cleanup path is exercised by a test.
5. Per-element handlers where delegation would be safer -- attaching onClick to every row in a list creates N closures and N subscriptions; delegated listeners (single handler on the parent reading event.target.closest(...)) are safer under rapid re-renders, avoid stale-closure bugs, and scale to large lists. Use delegation when the list exceeds ~50 items or updates frequently.

These classes produce bugs that are intermittent, environment-dependent, and invisible to type-checking -- exactly the ones that reach production. Review for them deliberately, not just as "subscriptions need cleanup."

State Management

Local UI state       → useState, useReducer
Shared client state  → Zustand (simple) | Redux Toolkit (complex)
Atomic/granular      → Jotai
Server/remote data   → React Query (TanStack Query)
URL state            → nuqs, router search params
Form state           → React Hook Form

Key patterns:

• Zustand: create<State>()(devtools(persist((set) => ({...})))) -- use slices for scale, selective subscriptions to prevent re-renders
• React Query: query keys factory (['users', 'detail', id] as const), staleTime/gcTime, optimistic updates with onMutate/onError rollback
• Separate client state (Zustand) from server state (React Query) -- never duplicate server data in client store
• Colocate state close to where it's used; don't over-globalize

Performance

Critical -- eliminate waterfalls:

• Promise.all() for independent async operations
• Move await into branches where actually needed
• Suspense boundaries to stream slow content

Critical -- bundle size:

• Import directly from modules, avoid barrel files (index.ts re-exports)
• next/dynamic or React.lazy() for heavy components
• Defer third-party scripts (analytics, logging) until after hydration
• Preload on hover/focus for perceived speed
• content-visibility: auto + contain-intrinsic-size on long lists -- skips off-screen layout/paint

Re-render optimization:

• Derive state during render, not in effects
• Subscribe to derived booleans, not raw objects (state.items.length > 0 not state.items)
• Functional setState for stable callbacks: setCount(c => c + 1)
• Lazy state init: useState(() => expensiveComputation())
• useTransition for non-urgent updates (search filtering)
• useDeferredValue for expensive derived UI
• Don't subscribe to searchParams/state if only read in callbacks -- read on demand instead
• Use ternary (condition ? <A /> : <B />), not && for conditionals
• React.memo only for expensive subtrees with stable props
• Hoist static JSX outside components

React Compiler (React 19): auto-memoizes -- write idiomatic React, remove manual useMemo/useCallback/memo. Enable via framework config (Next.js: reactCompiler: true in next.config). Non-framework: install babel-plugin-react-compiler. Keep components pure.

React 19

• ref as prop -- forwardRef deprecated. Accept ref?: React.Ref<HTMLElement> as regular prop
• useActionState -- replaces useFormState: const [state, formAction, isPending] = useActionState(action, initialState)
• use() -- unwrap Promise or Context during render (not in callbacks/effects). Enables conditional context reads
• useOptimistic -- const [optimistic, addOptimistic] = useOptimistic(state, mergeFn) for instant UI feedback
• useFormStatus -- const { pending } = useFormStatus() in child of <form action={...}>
• Server Components -- default in App Router. Async, access DB/secrets directly. No hooks, no event handlers
• Server Actions -- 'use server' directive. Validate inputs (Zod), revalidateTag/revalidatePath after mutations. Server Actions are public endpoints -- always verify auth/authz inside each action, not just in middleware or layout guards
• <Activity mode='visible'|'hidden'> -- preserves state/DOM for toggled components (experimental)

Next.js App Router

File conventions: page.tsx (route UI), layout.tsx (shared wrapper), template.tsx (re-mounted on navigation, unlike layout), loading.tsx (Suspense), error.tsx (error boundary), not-found.tsx (404), default.tsx (parallel route fallback), route.ts (API endpoint)

Rendering modes: Server Components (default) | Client ('use client') | Static (build) | Dynamic (request) | Streaming (progressive)

Decision: Server Component unless it needs hooks, event handlers, or browser APIs. Split: server parent + client child. Isolate interactive components as 'use client' leaf components -- keep server components static with no global state or event handlers.

Routing patterns:

• Route groups (name) -- organize without affecting URL
• Parallel routes @slot -- independent loading states in same layout
• Intercepting routes (.) -- modal overlays with full-page fallback

Caching:

• fetch(url, { cache: 'force-cache' }) -- static
• fetch(url, { next: { revalidate: 60 } }) -- ISR
• fetch(url, { cache: 'no-store' }) -- dynamic
• Tag-based: fetch(url, { next: { tags: ['products'] } }) then revalidateTag('products')

Data fetching: Fetch in Server Components where data is used. Use Suspense boundaries for slow queries. React.cache() for per-request dedup. generateStaticParams for static generation. generateMetadata for dynamic SEO. Static metadata with title: { default: 'App', template: '%s | App' } for cascading page titles. after() for non-blocking side effects (logging, analytics) -- runs after response is sent. Hoist static I/O (fonts, config) to module level -- runs once, not per request.

Testing (Vitest + React Testing Library)

• Component tests: Vitest + RTL, co-located *.test.tsx. Default for React components.
• Hook tests: renderHook + act, co-located *.test.ts
• Unit tests: Vitest for pure functions, utilities, services
• E2E: Playwright for user flows and critical paths
• Query priority: getByRole > getByLabelText > getByPlaceholderText > getByText > getByTestId
• Mock API services and external providers; render child components real for integration confidence
• One behavior per test with AAA structure. Name: should <behavior> when <condition>
• Use userEvent over fireEvent for realistic interactions
• findBy* for async elements, waitFor after state-triggering actions
• vi.clearAllMocks() in beforeEach. Recreate state per test. General testing discipline (anti-patterns, rationalization resistance): see ia-writing-tests skill. See testing patterns and examples for component, hook, and mocking examples. See e2e testing for Playwright patterns.

Tailwind Integration

For Tailwind v4 configuration, utility patterns, dark mode, and component variants, see ia-tailwind-css skill.

Class sorting in JSX: when using clsx, cva, cn, tv, or tw utility functions, keep Tailwind classes in canonical order. Configure eslint-plugin-better-tailwindcss with useSortedClasses and functions: ["clsx", "cva", "cn", "tv", "tw"] to enforce this automatically across JSX attributes and helper calls.

Discipline

• Simplicity first -- every change as simple as possible, impact minimal code
• Only touch what's necessary -- avoid introducing unrelated changes
• No hacky workarounds -- if a fix feels wrong, step back and implement the clean solution
• Before adding a new abstraction, verify it appears in 3+ places

References

• testing.md -- Component, hook, and mocking test examples
• e2e-testing.md -- Playwright E2E patterns

Verify

• TypeScript compiles with zero errors
• No suppressed lint rules (eslint-disable, @ts-ignore) in new code
• useEffect dependency arrays not manually overridden
• No forwardRef usage in React 19+ projects (use ref prop directly)