frontend-patterns

Frontend Development Patterns

Modern frontend patterns for React, Next.js, and performant user interfaces.

Component Patterns

Composition Over Inheritance

// ✅ 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

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

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

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

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

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

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

// ✅ 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

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>

  &#x3C;Suspense fallback={null}>
    &#x3C;ThreeJsBackground />
  &#x3C;/Suspense>
&#x3C;/div>

) }

Virtualization for Long Lists

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

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 value={formData.name} onChange={e => setFormData(prev => ({ ...prev, name: e.target.value }))} placeholder="Market name" /> {errors.name && <span className="error">{errors.name}</span>}

  {/* Other fields */}

  &#x3C;button type="submit">Create Market&#x3C;/button>
&#x3C;/form>

) }

Error Boundary Pattern

interface ErrorBoundaryState { hasError: boolean error: Error | null }

export class ErrorBoundary extends React.Component< { children: React.ReactNode }, ErrorBoundaryState

{ state: ErrorBoundaryState = { hasError: false, error: null }

static getDerivedStateFromError(error: Error): ErrorBoundaryState { return { hasError: true, error } }

componentDidCatch(error: Error, errorInfo: React.ErrorInfo) { console.error('Error boundary caught:', error, errorInfo) }

render() { if (this.state.hasError) { return ( <div className="error-fallback"> <h2>Something went wrong</h2> <p>{this.state.error?.message}</p> <button onClick={() => this.setState({ hasError: false })}> Try again </button> </div> ) }

return this.props.children

} }

// Usage <ErrorBoundary> <App /> </ErrorBoundary>

Animation Patterns

Framer Motion Animations

import { motion, AnimatePresence } from 'framer-motion'

// ✅ List animations export function AnimatedMarketList({ markets }: { markets: Market[] }) { return ( <AnimatePresence> {markets.map(market => ( <motion.div key={market.id} initial={{ opacity: 0, y: 20 }} animate={{ opacity: 1, y: 0 }} exit={{ opacity: 0, y: -20 }} transition={{ duration: 0.3 }} > <MarketCard market={market} /> </motion.div> ))} </AnimatePresence> ) }

// ✅ Modal animations export function Modal({ isOpen, onClose, children }: ModalProps) { return ( <AnimatePresence> {isOpen && ( <> <motion.div className="modal-overlay" initial={{ opacity: 0 }} animate={{ opacity: 1 }} exit={{ opacity: 0 }} onClick={onClose} /> <motion.div className="modal-content" initial={{ opacity: 0, scale: 0.9, y: 20 }} animate={{ opacity: 1, scale: 1, y: 0 }} exit={{ opacity: 0, scale: 0.9, y: 20 }} > {children} </motion.div> </> )} </AnimatePresence> ) }

Accessibility Patterns

Keyboard Navigation

export function Dropdown({ options, onSelect }: DropdownProps) { const [isOpen, setIsOpen] = useState(false) const [activeIndex, setActiveIndex] = useState(0)

const handleKeyDown = (e: React.KeyboardEvent) => { switch (e.key) { case 'ArrowDown': e.preventDefault() setActiveIndex(i => Math.min(i + 1, options.length - 1)) break case 'ArrowUp': e.preventDefault() setActiveIndex(i => Math.max(i - 1, 0)) break case 'Enter': e.preventDefault() onSelect(options[activeIndex]) setIsOpen(false) break case 'Escape': setIsOpen(false) break } }

return ( <div role="combobox" aria-expanded={isOpen} aria-haspopup="listbox" onKeyDown={handleKeyDown} > {/* Dropdown implementation */} </div> ) }

Focus Management

export function Modal({ isOpen, onClose, children }: ModalProps) { const modalRef = useRef<HTMLDivElement>(null) const previousFocusRef = useRef<HTMLElement | null>(null)

useEffect(() => { if (isOpen) { // Save currently focused element previousFocusRef.current = document.activeElement as HTMLElement

  // Focus modal
  modalRef.current?.focus()
} else {
  // Restore focus when closing
  previousFocusRef.current?.focus()
}

}, [isOpen])

return isOpen ? ( <div ref={modalRef} role="dialog" aria-modal="true" tabIndex={-1} onKeyDown={e => e.key === 'Escape' && onClose()} > {children} </div> ) : null }

Remember: Modern frontend patterns enable maintainable, performant user interfaces. Choose patterns that fit your project complexity.

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