Frontend Patterns

Frontend Development Patterns

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

Methodology Integration

These patterns complement .claude/docs/patterns/FRONTEND_APP.md with implementation details.

Component Patterns

Composition Over Inheritance

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

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 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' : ''}
      => context.setActiveTab(id)}
    >
      {children}
    </button>
  )
}

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]
}

Async Data Fetching Hook

export function useQuery<T>(
  key: string,
  fetcher: () => Promise<T>,
  options?: { onSuccess?: (data: T) => void; enabled?: boolean }
) {
  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) {
      setError(err as Error)
    } finally {
      setLoading(false)
    }
  }, [fetcher, options])

  useEffect(() => {
    if (options?.enabled !== false) {
      refetch()
    }
  }, [key, refetch, options?.enabled])

  return { data, error, loading, refetch }
}

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
}

State Management Patterns

Context + Reducer Pattern

interface State {
  items: Item[]
  selectedItem: Item | null
  loading: boolean
}

type Action =
  | { type: 'SET_ITEMS'; payload: Item[] }
  | { type: 'SELECT_ITEM'; payload: Item }
  | { type: 'SET_LOADING'; payload: boolean }

function reducer(state: State, action: Action): State {
  switch (action.type) {
    case 'SET_ITEMS':
      return { ...state, items: action.payload }
    case 'SELECT_ITEM':
      return { ...state, selectedItem: action.payload }
    case 'SET_LOADING':
      return { ...state, loading: action.payload }
    default:
      return state
  }
}

const ItemContext = createContext<{
  state: State
  dispatch: Dispatch<Action>
} | undefined>(undefined)

export function ItemProvider({ children }: { children: React.ReactNode }) {
  const [state, dispatch] = useReducer(reducer, {
    items: [],
    selectedItem: null,
    loading: false
  })

  return (
    <ItemContext.Provider value={{ state, dispatch }}>
      {children}
    </ItemContext.Provider>
  )
}

Performance Optimization

Memoization

// useMemo for expensive computations
const sortedItems = useMemo(() => {
  return items.sort((a, b) => b.value - a.value)
}, [items])

// useCallback for functions passed to children
const handleSearch = useCallback((query: string) => {
  setSearchQuery(query)
}, [])

// React.memo for pure components
export const ItemCard = React.memo<ItemCardProps>(({ item }) => {
  return (
    <div className="item-card">
      <h3>{item.name}</h3>
    </div>
  )
})

Code Splitting & Lazy Loading

import { lazy, Suspense } from 'react'

const HeavyChart = lazy(() => import('./HeavyChart'))

export function Dashboard() {
  return (
    <Suspense fallback={<ChartSkeleton />}>
      <HeavyChart data={data} />
    </Suspense>
  )
}

Virtualization for Long Lists

import { useVirtualizer } from '@tanstack/react-virtual'

export function VirtualList({ items }: { items: Item[] }) {
  const parentRef = useRef<HTMLDivElement>(null)

  const virtualizer = useVirtualizer({
    count: items.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 100,
    overscan: 5
  })

  return (
    <div ref={parentRef} style={{ height: '600px', overflow: 'auto' }}>
      <div style={{ height: `${virtualizer.getTotalSize()}px`, position: 'relative' }}>
        {virtualizer.getVirtualItems().map(virtualRow => (
          <div
            key={virtualRow.index}
            style={{
              position: 'absolute',
              top: 0,
              transform: `translateY(${virtualRow.start}px)`,
              height: `${virtualRow.size}px`,
              width: '100%'
            }}
          >
            <ItemCard item={items[virtualRow.index]} />
          </div>
        ))}
      </div>
    </div>
  )
}

Form Handling Patterns

Controlled Form with Validation

export function CreateForm() {
  const [formData, setFormData] = useState({ name: '', description: '' })
  const [errors, setErrors] = useState<Record<string, string>>({})

  const validate = (): boolean => {
    const newErrors: Record<string, string> = {}

    if (!formData.name.trim()) {
      newErrors.name = 'Name is required'
    }

    setErrors(newErrors)
    return Object.keys(newErrors).length === 0
  }

  const handleSubmit = async (e: React.FormEvent) => {
    e.preventDefault()
    if (!validate()) return

    try {
      await createResource(formData)
    } catch (error) {
      // Handle error
    }
  }

  return (
    <form
      <input
        value={formData.name}
        => setFormData(prev => ({ ...prev, name: e.target.value }))}
      />
      {errors.name && <span className="error">{errors.name}</span>}
      <button type="submit">Create</button>
    </form>
  )
}

Error Boundary Pattern

export class ErrorBoundary extends React.Component<
  { children: React.ReactNode },
  { hasError: boolean; error: Error | null }
> {
  state = { hasError: false, error: null }

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

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

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}
     
    >
      {/* Implementation */}
    </div>
  )
}

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

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