frontend-patterns

# Frontend Development Patterns Modern frontend patterns for React, Next.js, and performant user interfaces. ## UI/UX Design Consultation Tactics Use this method when the Frontend Designer specialist is asked to consult on a Spec or Plan: 1. **Component spec**: define purpose, public props, content rules, interaction model, and integration boundary without writing production code. 2. **States and variants**: cover default, loading, empty, success, error, disabled, and permission-sensitive states plus every intentional visual variant. 3. **Accessibility**: document semantic structure, names and labels, keyboard behavior, focus order and visibility, contrast, and assistive-technology expectations. 4. **Design system**: reuse established tokens, components, spacing, typography, and interaction patterns; identify any genuinely new primitive explicitly. 5. **Responsive behavior**: describe layout changes at project-defined breakpoints and verify that content, touch targets, and reading order remain usable. 6. **Handoff**: return component decisions, states, accessibility requirements, responsive constraints, design-system impact, risks, and unresolved questions to the requesting Spec or Plan owner. ## Component Patterns ### Composition Over Inheritance ```typescript // ✅ GOOD: Component composition interface CardProps { children: React.ReactNode variant?: 'default' | 'outlined' } export function Card({ children, variant = 'default' }: CardProps) { return <div className={`card card-${variant}`}>{children}</div> } export function CardHeader({ children }: { children: React.ReactNode }) { return <div className="card-header">{children}</div> } export function CardBody({ children }: { children: React.ReactNode }) { return <div className="card-body">{children}</div> } // Usage <Card> <CardHeader>Title</CardHeader> <CardBody>Content</CardBody> </Card> ``` ### Compound Components ```typescript interface TabsContextValue { activeTab: string setActiveTab: (tab: string) => void } const TabsContext = createContext<TabsContextValue | undefined>(undefined) export function Tabs({ children, defaultTab }: { children: React.ReactNode defaultTab: string }) { const [activeTab, setActiveTab] = useState(defaultTab) return ( <TabsContext.Provider value={{ activeTab, setActiveTab }}> {children} </TabsContext.Provider> ) } export function TabList({ children }: { children: React.ReactNode }) { return <div className="tab-list">{children}</div> } export function Tab({ id, children }: { id: string, children: React.ReactNode }) { const context = useContext(TabsContext) if (!context) throw new Error('Tab must be used within Tabs') return ( <button className={context.activeTab === id ? 'active' : ''} onClick={() => context.setActiveTab(id)} > {children} </button> ) } // Usage <Tabs defaultTab="overview"> <TabList> <Tab id="overview">Overview</Tab> <Tab id="details">Details</Tab> </TabList> </Tabs> ``` ### Render Props Pattern ```typescript interface DataLoaderProps<T> { url: string children: (data: T | null, loading: boolean, error: Error | null) => React.ReactNode } export function DataLoader<T>({ url, children }: DataLoaderProps<T>) { const [data, setData] = useState<T | null>(null) const [loading, setLoading] = useState(true) const [error, setError] = useState<Error | null>(null) useEffect(() => { fetch(url) .then(res => res.json()) .then(setData) .catch(setError) .finally(() => setLoading(false)) }, [url]) return <>{children(data, loading, error)}</> } // Usage <DataLoader<Market[]> url="/api/markets"> {(markets, loading, error) => { if (loading) return <Spinner /> if (error) return <Error error={error} /> return <MarketList markets={markets!} /> }} </DataLoader> ``` ## Custom Hooks Patterns ### State Management Hook ```typescript export function useToggle(initialValue = false): [boolean, () => void] { const [value, setValue] = useState(initialValue) const toggle = useCallback(() => { setValue(v => !v) }, []) return [value, toggle] } // Usage const [isOpen, toggleOpen] = useToggle() ``` ### Async Data Fetching Hook ```typescript interface UseQueryOptions<T> { onSuccess?: (data: T) => void onError?: (error: Error) => void enabled?: boolean } export function useQuery<T>( key: string, fetcher: () => Promise<T>, options?: UseQueryOptions<T> ) { const [data, setData] = useState<T | null>(null) const [error, setError] = useState<Error | null>(null) const [loading, setLoading] = useState(false) const refetch = useCallback(async () => { setLoading(true) setError(null) try { const result = await fetcher() setData(result) options?.onSuccess?.(result) } catch (err) { const error = err as Error setError(error) options?.onError?.(error) } finally { setLoading(false) } }, [fetcher, options]) useEffect(() => { if (options?.enabled !== false) { refetch() } }, [key, refetch, options?.enabled]) return { data, error, loading, refetch } } // Usage const { data: markets, loading, error, refetch } = useQuery( 'markets', () => fetch('/api/markets').then(r => r.json()), { onSuccess: data => console.log('Fetched', data.length, 'markets'), onError: err => console.error('Failed:', err) } ) ``` ### Debounce Hook ```typescript export function useDebounce<T>(value: T, delay: number): T { const [debouncedValue, setDebouncedValue] = useState<T>(value) useEffect(() => { const handler = setTimeout(() => { setDebouncedValue(value) }, delay) return () => clearTimeout(handler) }, [value, delay]) return debouncedValue } // Usage const [searchQuery, setSearchQuery] = useState('') const debouncedQuery = useDebounce(searchQuery, 500) useEffect(() => { if (debouncedQuery) { performSearch(debouncedQuery) } }, [debouncedQuery]) ``` ## State Management Patterns ### Context + Reducer Pattern ```typescript interface State { markets: Market[] selectedMarket: Market | null loading: boolean } type Action = | { type: 'SET_MARKETS'; payload: Market[] } | { type: 'SELECT_MARKET'; payload: Market } | { type: 'SET_LOADING'; payload: boolean } function reducer(state: State, action: Action): State { switch (action.type) { case 'SET_MARKETS': return { ...state, markets: action.payload } case 'SELECT_MARKET': return { ...state, selectedMarket: action.payload } case 'SET_LOADING': return { ...state, loading: action.payload } default: return state } } const MarketContext = createContext<{ state: State dispatch: Dispatch<Action> } | undefined>(undefined) export function MarketProvider({ children }: { children: React.ReactNode }) { const [state, dispatch] = useReducer(reducer, { markets: [], selectedMarket: null, loading: false }) return ( <MarketContext.Provider value={{ state, dispatch }}> {children} </MarketContext.Provider> ) } export function useMarkets() { const context = useContext(MarketContext) if (!context) throw new Error('useMarkets must be used within MarketProvider') return context } ``` ## Performance Optimization ### Memoization ```typescript // ✅ useMemo for expensive computations const sortedMarkets = useMemo(() => { return markets.sort((a, b) => b.volume - a.volume) }, [markets]) // ✅ useCallback for functions passed to children const handleSearch = useCallback((query: string) => { setSearchQuery(query) }, []) // ✅ React.memo for pure components export const MarketCard = React.memo<MarketCardProps>(({ market }) => { return ( <div className="market-card"> <h3>{market.name}</h3> <p>{market.description}</p> </div> ) }) ``` ### Code Splitting & Lazy Loading ```typescript import { lazy, Suspense } from 'react' // ✅ Lazy load heavy components const HeavyChart = lazy(() => import('./HeavyChart')) const ThreeJsBackground = lazy(() => import('./ThreeJsBackground')) export function Dashboard() { return ( <div> <Suspense fallback={<ChartSkeleton />}> <HeavyChart data={data} /> </Suspense> <Suspense fallback={null}> <ThreeJsBackground /> </Suspense> </div> ) } ``` ### Virtualization for Long Lists ```typescript import { useVirtualizer } from '@tanstack/react-virtual' export function VirtualMarketList({ markets }: { markets: Market[] }) { const parentRef = useRef<HTMLDivElement>(null) const virtualizer = useVirtualizer({ count: markets.length, getScrollElement: () => parentRef.current, estimateSize: () => 100, // Estimated row height overscan: 5 // Extra items to render }) return ( <div ref={parentRef} style={{ height: '600px', overflow: 'auto' }}> <div style={{ height: `${virtualizer.getTotalSize()}px`, position: 'relative' }} > {virtualizer.getVirtualItems().map(virtualRow => ( <div key={virtualRow.index} style={{ position: 'absolute', top: 0, left: 0, width: '100%', height: `${virtualRow.size}px`, transform: `translateY(${virtualRow.start}px)` }} > <MarketCard market={markets[virtualRow.index]} /> </div> ))} </div> </div> ) } ``` ## Form Handling Patterns ### Controlled Form with Validation ```typescript interface FormData { name: string description: string endDate: string } interface FormErrors { name?: string description?: string endDate?: string } export function CreateMarketForm() { const [formData, setFormData] = useState<FormData>({ name: '', description: '', endDate: '' }) const [errors, setErrors] = useState<FormErrors>({}) const validate = (): boolean => { const newErrors: FormErrors = {} if (!formData.name.trim()) { newErrors.name = 'Name is required' } else if (formData.name.length > 200) { newErrors.name = 'Name must be under 200 characters' } if (!formData.description.trim()) { newErrors.description = 'Description is required' } if (!formData.endDate) { newErrors.endDate = 'End date is required' } setErrors(newErrors) return Object.keys(newErrors).length === 0 } const handleSubmit = async (e: React.FormEvent) => { e.preventDefault() if (!validate()) return try { await createMarket(formData) // Success handling } catch (error) { // Error handling } } return ( <form onSubmit={handleSubmit}> <input