React Patterns

Platform: Web and Mobile (shared React patterns). For React Native-specific patterns (Pressable, ScrollView, FlashList, safe areas), see the react-native-patterns skill.

Overview

Patterns for building maintainable React applications with TypeScript, leveraging React 19 features and composition patterns.

API Reference: Use Context7 MCP for full React API reference (mcp__context7__resolve-library-id → react).

Workflows

• Choose appropriate component composition pattern
• Apply TypeScript types for props and events
• Implement custom hooks for shared logic
• Add performance optimizations where needed
• Handle loading and error states with Suspense/boundaries
• Validate component render behavior

Feedback Loops

• Components render without TypeScript errors
• Props are properly typed and validated
• Custom hooks have clear return types
• No unnecessary re-renders (use React DevTools Profiler)
• Error boundaries catch component errors
• Loading states work with Suspense

Reference Implementation

1. Component Composition

Compound Components

// Shares implicit state between parent and children
const TabsContext = createContext<{ activeTab: string; setActiveTab: (id: string) => void } | null>(null);

function Tabs({ children, defaultTab }: { children: ReactNode; defaultTab: string }) {
  const [activeTab, setActiveTab] = useState(defaultTab);
  return <TabsContext.Provider value={{ activeTab, setActiveTab }}>{children}</TabsContext.Provider>;
}

function Tab({ id, children }: { id: string; children: ReactNode }) {
  const ctx = use(TabsContext);
  if (!ctx) throw new Error('Tab must be used within Tabs');
  return (
    <button role="tab" aria-selected={ctx.activeTab === id} onClick={() => ctx.setActiveTab(id)}>
      {children}
    </button>
  );
}

Slot Pattern

// Named slots for flexible composition
interface CardProps { header?: ReactNode; footer?: ReactNode; children: ReactNode; }

function Card({ header, footer, children }: CardProps) {
  return (
    <div className="card">
      {header && <div className="card-header">{header}</div>}
      <div className="card-body">{children}</div>
      {footer && <div className="card-footer">{footer}</div>}
    </div>
  );
}

2. React 19 Features

use() Hook

// Unwrap promises and context — simpler than useContext
function UserProfile({ userPromise }: { userPromise: Promise<User> }) {
  const user = use(userPromise); // Suspends until resolved
  return <div>{user.name}</div>;
}

useActionState

async function updateUser(prevState: { error?: string }, formData: FormData) {
  'use server';
  return { error: undefined };
}

function UserForm() {
  const [state, formAction, isPending] = useActionState(updateUser, {});
  return (
    <form action={formAction}>
      <input name="name" disabled={isPending} />
      {state.error && <p className="error">{state.error}</p>}
      <button disabled={isPending}>{isPending ? 'Saving...' : 'Save'}</button>
    </form>
  );
}

useOptimistic

function TodoList({ todos }: { todos: Todo[] }) {
  const [optimisticTodos, addOptimisticTodo] = useOptimistic(
    todos,
    (state, newTodo: Todo) => [...state, newTodo]
  );
  async function handleAdd(formData: FormData) {
    const todo = { id: crypto.randomUUID(), text: formData.get('text') as string };
    addOptimisticTodo(todo);
    await saveTodo(todo);
  }
  return (
    <form action={handleAdd}>
      {optimisticTodos.map(todo => <li key={todo.id}>{todo.text}</li>)}
      <input name="text" /><button>Add</button>
    </form>
  );
}

3. Custom Hooks

// Object return for multiple named values
function useAuth() {
  const [user, setUser] = useState<User | null>(null);
  const [loading, setLoading] = useState(true);
  const login = async (credentials: Credentials) => { setUser(await api.login(credentials)); };
  return { user, loading, login };
}

// Tuple return for positional access (like useState)
function useToggle(initial = false): [boolean, () => void] {
  const [value, setValue] = useState(initial);
  return [value, useCallback(() => setValue(v => !v), [])];
}

// Composing hooks
function useLocalStorage<T>(key: string, initial: T) {
  const [value, setValue] = useState<T>(() => {
    const stored = localStorage.getItem(key);
    return stored ? JSON.parse(stored) : initial;
  });
  useEffect(() => { localStorage.setItem(key, JSON.stringify(value)); }, [key, value]);
  return [value, setValue] as const;
}

4. TypeScript + React

// Extend HTML element props
interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
  variant?: 'primary' | 'secondary';
  loading?: boolean;
}
function Button({ variant = 'primary', loading, children, ...props }: ButtonProps) {
  return <button className={variant} disabled={loading} {...props}>{loading ? 'Loading...' : children}</button>;
}

// Generic components with full type inference
interface ListProps<T> { items: T[]; renderItem: (item: T) => ReactNode; keyExtractor: (item: T) => string; }
function List<T>({ items, renderItem, keyExtractor }: ListProps<T>) {
  return <ul>{items.map(item => <li key={keyExtractor(item)}>{renderItem(item)}</li>)}</ul>;
}

// Refs as props (React 19+) — forwardRef is deprecated
interface InputProps extends React.InputHTMLAttributes<HTMLInputElement> {
  label: string;
  ref?: React.Ref<HTMLInputElement>;
}
function Input({ label, ref, ...props }: InputProps) {
  return <label>{label}<input ref={ref} {...props} /></label>;
}

5. State Management

useReducer for Complex State

type Action =
  | { type: 'FETCH_START' }
  | { type: 'FETCH_SUCCESS'; payload: User[] }
  | { type: 'FETCH_ERROR'; payload: string };

function reducer(state: { data: User[]; loading: boolean; error: string | null }, action: Action) {
  switch (action.type) {
    case 'FETCH_START': return { ...state, loading: true, error: null };
    case 'FETCH_SUCCESS': return { ...state, loading: false, data: action.payload };
    case 'FETCH_ERROR': return { ...state, loading: false, error: action.payload };
  }
}

6. Performance Patterns

// memo + useCallback prevent unnecessary re-renders
const ListItem = memo(function ListItem({ item, onDelete }: ItemProps) {
  return <li>{item.name}<button onClick={() => onDelete(item.id)}>Delete</button></li>;
});

function List() {
  const [items, setItems] = useState<Item[]>([]);
  const handleDelete = useCallback((id: string) => {
    setItems(items => items.filter(item => item.id !== id));
  }, []);
  return <>{items.map(item => <ListItem key={item.id} item={item} onDelete={handleDelete} />)}</>;
}

// Lazy loading with Suspense
const Dashboard = lazy(() => import('./Dashboard'));
function App() {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <Routes><Route path="/dashboard" element={<Dashboard />} /></Routes>
    </Suspense>
  );
}

7. Error Handling

class ErrorBoundary extends Component<{ children: ReactNode; fallback?: ReactNode }, { hasError: boolean }> {
  state = { hasError: false };
  static getDerivedStateFromError(): { hasError: boolean } { return { hasError: true }; }
  componentDidCatch(error: Error, info: ErrorInfo) { console.error(error, info); }
  render() {
    return this.state.hasError ? (this.props.fallback ?? <div>Something went wrong</div>) : this.props.children;
  }
}

// Wrap async data loading with Suspense
function UserProfile({ userId }: { userId: string }) {
  const user = use(fetchUser(userId)); // Suspends
  return <div>{user.name}</div>;
}

Best Practices

• Composition over inheritance - Use composition patterns for flexibility
• Type everything - Leverage TypeScript for compile-time safety
• Colocate state - Keep state as close to where it's used as possible
• Extract custom hooks - Share logic across components with hooks
• Stable references - Use useCallback/useMemo to prevent unnecessary re-renders
• Error boundaries - Wrap component trees to catch rendering errors
• Suspense for loading - Use Suspense instead of manual loading states
• Avoid prop drilling - Use context or composition for deeply nested props

Anti-Patterns

• Using forwardRef in React 19 - Use ref as a regular prop instead
• Class components for new code - Use function components and hooks
• Mutating state directly - Always use setState or reducer actions
• Missing dependency arrays - Include all dependencies in useEffect/useMemo/useCallback
• Overusing useMemo/useCallback - Only optimize when necessary (profile first)
• Context for everything - Use context sparingly; prefer props or state management library
• Derived state in useState - Compute derived values during render instead
• useEffect for derived state - Use useMemo or compute directly in render
• Index as key - Use stable unique IDs for list keys
• Ignoring TypeScript errors - Never use 'any' or '// @ts-ignore' as shortcuts