Compound Eng React Frontend

---
name: ia-react-frontend
class: language
description: >-
  React architecture patterns, TypeScript, Next.js, hooks, and testing. Use when
  working with React component structure, state management, Next.js routing,
  Vitest, React Testing Library, or reviewing React code. For visual design and
  aesthetic direction, use frontend-design instead.
paths: "**/*.tsx,**/*.jsx"
---

# 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](../writing-tests/SKILL.md) skill.
See [testing patterns and examples](./references/testing.md) for component, hook, and mocking examples.
See [e2e testing](./references/e2e-testing.md) for Playwright patterns.

## Tailwind Integration

For Tailwind v4 configuration, utility patterns, dark mode, and component variants, see [ia-tailwind-css](../tailwind-css/SKILL.md) 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](./references/testing.md) -- Component, hook, and mocking test examples
- [e2e-testing.md](./references/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)