/frontend-patterns

name: frontend-patterns
description: Frontend development patterns for React, Next.js, state management, performance optimization, and UI best practices.
origin: ECC

Frontend Development Patterns

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

When to Activate

• Building React components (composition, props, rendering)
• Managing state (useState, useReducer, Zustand, Context)
• Implementing data fetching (SWR, React Query, server components)
• Optimizing performance (memoization, virtualization, code splitting)
• Working with forms (validation, controlled inputs, Zod schemas)
• Handling client-side routing and navigation
• Building accessible, responsive UI patterns

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: a