Component Patterns

Implement React component patterns for building reusable, maintainable components.

Quick Start

Use composition for most cases, custom hooks for shared logic, and compound components for flexible APIs.

Instructions

Composition Pattern

The default pattern for building flexible components.

Basic composition:

function Card({ children }) {
  return <div className="card">{children}</div>;
}

function CardHeader({ children }) {
  return <div className="card-header">{children}</div>;
}

function CardBody({ children }) {
  return <div className="card-body">{children}</div>;
}

// Usage
<Card>
  <CardHeader>Title</CardHeader>
  <CardBody>Content</CardBody>
</Card>

With props:

interface ButtonProps {
  variant?: 'primary' | 'secondary';
  size?: 'sm' | 'md' | 'lg';
  children: React.ReactNode;
}

function Button({ variant = 'primary', size = 'md', children }: ButtonProps) {
  return (
    <button className={`btn btn-${variant} btn-${size}`}>
      {children}
    </button>
  );
}

Custom Hooks Pattern

Extract and reuse stateful logic across components.

Basic custom hook:

function useToggle(initialValue = false) {
  const [value, setValue] = useState(initialValue);
  
  const toggle = useCallback(() => {
    setValue(v => !v);
  }, []);
  
  return [value, toggle] as const;
}

// Usage
function Component() {
  const [isOpen, toggleOpen] = useToggle();
  return <button onClick={toggleOpen}>{isOpen ? 'Close' : 'Open'}</button>;
}

Data fetching hook:

function useFetch<T>(url: string) {
  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 { data, loading, error };
}

Form hook:

function useForm<T>(initialValues: T) {
  const [values, setValues] = useState(initialValues);
  
  const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    setValues(prev => ({
      ...prev,
      [e.target.name]: e.target.value
    }));
  };
  
  const reset = () => setValues(initialValues);
  
  return { values, handleChange, reset };
}

Compound Components Pattern

Create flexible component APIs with implicit state sharing.

Basic compound component:

interface TabsContextValue {
  activeTab: string;
  setActiveTab: (tab: string) => void;
}

const TabsContext = createContext<TabsContextValue | null>(null);

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

function TabList({ children }: { children: React.ReactNode }) {
  return <div className="tab-list">{children}</div>;
}

function Tab({ id, children }: { id: string; children: React.ReactNode }) {
  const context = useContext(TabsContext);
  if (!context) throw new Error('Tab must be used within Tabs');
  
  const { activeTab, setActiveTab } = context;
  
  return (
    <button
      className={activeTab === id ? 'active' : ''}
      onClick={() => setActiveTab(id)}
    >
      {children}
    </button>
  );
}

function TabPanel({ id, children }: { id: string; children: React.ReactNode }) {
  const context = useContext(TabsContext);
  if (!context) throw new Error('TabPanel must be used within Tabs');
  
  if (context.activeTab !== id) return null;
  return <div className="tab-panel">{children}</div>;
}

// Attach sub-components
Tabs.List = TabList;
Tabs.Tab = Tab;
Tabs.Panel = TabPanel;

// Usage
<Tabs defaultTab="home">
  <Tabs.List>
    <Tabs.Tab id="home">Home</Tabs.Tab>
    <Tabs.Tab id="profile">Profile</Tabs.Tab>
  </Tabs.List>
  <Tabs.Panel id="home">Home content</Tabs.Panel>
  <Tabs.Panel id="profile">Profile content</Tabs.Panel>
</Tabs>

Render Props Pattern

Provide render flexibility through function props.

Basic render prop:

interface MouseTrackerProps {
  render: (position: { x: number; y: number }) => React.ReactNode;
}

function MouseTracker({ render }: MouseTrackerProps) {
  const [position, setPosition] = useState({ x: 0, y: 0 });
  
  useEffect(() => {
    const handleMove = (e: MouseEvent) => {
      setPosition({ x: e.clientX, y: e.clientY });
    };
    
    window.addEventListener('mousemove', handleMove);
    return () => window.removeEventListener('mousemove', handleMove);
  }, []);
  
  return <>{render(position)}</>;
}

// Usage
<MouseTracker
  render={({ x, y }) => (
    <div>Mouse at {x}, {y}</div>
  )}
/>

Children as function:

interface DataProviderProps<T> {
  url: string;
  children: (data: T | null, loading: boolean) => React.ReactNode;
}

function DataProvider<T>({ url, children }: DataProviderProps<T>) {
  const { data, loading } = useFetch<T>(url);
  return <>{children(data, loading)}</>;
}

// Usage
<DataProvider url="/api/users">
  {(users, loading) => (
    loading ? <Spinner /> : <UserList users={users} />
  )}
</DataProvider>

Higher-Order Component (HOC) Pattern

Wrap components to add functionality (legacy pattern, prefer hooks).

Basic HOC:

function withLoading<P extends object>(
  Component: React.ComponentType<P>
) {
  return function WithLoadingComponent(props: P & { loading: boolean }) {
    const { loading, ...rest } = props;
    
    if (loading) return <Spinner />;
    return <Component {...(rest as P)} />;
  };
}

// Usage
const UserListWithLoading = withLoading(UserList);
<UserListWithLoading users={users} loading={loading} />

HOC with additional props:

function withAuth<P extends object>(
  Component: React.ComponentType<P & { user: User }>
) {
  return function WithAuthComponent(props: P) {
    const { user, loading } = useAuth();
    
    if (loading) return <Spinner />;
    if (!user) return <Navigate to="/login" />;
    
    return <Component {...props} user={user} />;
  };
}

Performance Optimization Patterns

React.memo

Prevent unnecessary re-renders of expensive components.

const ExpensiveComponent = React.memo(function ExpensiveComponent({ data }) {
  // Expensive rendering logic
  return <div>{/* Complex UI */}</div>;
});

// With custom comparison
const MemoizedComponent = React.memo(
  function Component({ user }) {
    return <div>{user.name}</div>;
  },
  (prevProps, nextProps) => prevProps.user.id === nextProps.user.id
);

useMemo

Memoize expensive calculations.

function Component({ items }) {
  const sortedItems = useMemo(() => {
    return items.sort((a, b) => a.value - b.value);
  }, [items]);
  
  const total = useMemo(() => {
    return items.reduce((sum, item) => sum + item.price, 0);
  }, [items]);
  
  return <div>{/* Use sortedItems and total */}</div>;
}

useCallback

Memoize functions to prevent child re-renders.

function Parent() {
  const [count, setCount] = useState(0);
  
  const handleClick = useCallback(() => {
    setCount(c => c + 1);
  }, []);
  
  return <MemoizedChild onClick={handleClick} />;
}

const MemoizedChild = React.memo(function Child({ onClick }) {
  return <button onClick={onClick}>Click</button>;
});

Code Splitting

Split code to reduce initial bundle size.

const LazyComponent = React.lazy(() => import('./HeavyComponent'));

function App() {
  return (
    <Suspense fallback={<Spinner />}>
      <LazyComponent />
    </Suspense>
  );
}

// Route-based splitting
const Dashboard = React.lazy(() => import('./pages/Dashboard'));
const Profile = React.lazy(() => import('./pages/Profile'));

<Routes>
  <Route path="/dashboard" element={
    <Suspense fallback={<Spinner />}>
      <Dashboard />
    </Suspense>
  } />
</Routes>

Virtual Scrolling

Handle large lists efficiently.

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

function VirtualList({ items }) {
  const parentRef = useRef<HTMLDivElement>(null);
  
  const virtualizer = useVirtualizer({
    count: items.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 50,
  });
  
  return (
    <div ref={parentRef} style={{ height: '400px', overflow: 'auto' }}>
      <div style={{ height: `${virtualizer.getTotalSize()}px`, position: 'relative' }}>
        {virtualizer.getVirtualItems().map(virtualItem => (
          <div
            key={virtualItem.key}
            style={{
              position: 'absolute',
              top: 0,
              left: 0,
              width: '100%',
              height: `${virtualItem.size}px`,
              transform: `translateY(${virtualItem.start}px)`,
            }}
          >
            {items[virtualItem.index].name}
          </div>
        ))}
      </div>
    </div>
  );
}

Common Patterns

Container/Presentational Pattern

Separate logic from presentation.

// Presentational component
interface UserListProps {
  users: User[];
  onDelete: (id: string) => void;
}

function UserList({ users, onDelete }: UserListProps) {
  return (
    <ul>
      {users.map(user => (
        <li key={user.id}>
          {user.name}
          <button onClick={() => onDelete(user.id)}>Delete</button>
        </li>
      ))}
    </ul>
  );
}

// Container component
function UserListContainer() {
  const { data: users, isLoading } = useQuery(['users'], fetchUsers);
  const deleteMutation = useMutation(deleteUser);
  
  if (isLoading) return <Spinner />;
  
  return <UserList users={users} onDelete={deleteMutation.mutate} />;
}

Provider Pattern

Share data across component tree.

interface ThemeContextValue {
  theme: 'light' | 'dark';
  toggleTheme: () => void;
}

const ThemeContext = createContext<ThemeContextValue | null>(null);

export function ThemeProvider({ children }: { children: React.ReactNode }) {
  const [theme, setTheme] = useState<'light' | 'dark'>('light');
  
  const toggleTheme = useCallback(() => {
    setTheme(t => t === 'light' ? 'dark' : 'light');
  }, []);
  
  return (
    <ThemeContext.Provider value={{ theme, toggleTheme }}>
      {children}
    </ThemeContext.Provider>
  );
}

export function useTheme() {
  const context = useContext(ThemeContext);
  if (!context) throw new Error('useTheme must be used within ThemeProvider');
  return context;
}

Controlled vs Uncontrolled Components

Controlled (recommended):

function ControlledInput() {
  const [value, setValue] = useState('');
  
  return (
    <input
      value={value}
      onChange={e => setValue(e.target.value)}
    />
  );
}

Uncontrolled:

function UncontrolledInput() {
  const inputRef = useRef<HTMLInputElement>(null);
  
  const handleSubmit = () => {
    console.log(inputRef.current?.value);
  };
  
  return <input ref={inputRef} />;
}

Troubleshooting

Unnecessary re-renders:

• Use React DevTools Profiler to identify
• Wrap with React.memo
• Use useMemo/useCallback appropriately
• Check if state is lifted too high

Props drilling:

• Use Context API for deeply nested props
• Consider component composition
• Extract intermediate components

Stale closures in hooks:

• Add dependencies to useEffect/useCallback
• Use functional updates: setState(prev => prev + 1)
• Use useRef for mutable values

Memory leaks:

• Clean up subscriptions in useEffect
• Cancel pending requests on unmount
• Remove event listeners