React Patterns

Best practices for React development: functional components, hooks, state management, performance, and accessibility.

Component Rules

1. Always use functional components -- never class components for new code
2. One component per file -- name the file the same as the component (PascalCase)
3. Export components as named exports -- default exports only for pages/routes
4. Colocate related files -- keep Component.tsx, Component.test.tsx, and Component.module.css together
5. Props interface above the component -- name it ComponentNameProps

// Good
interface UserCardProps {
  user: User;
  onSelect: (id: string) => void;
  variant?: "compact" | "full";
}

export function UserCard({ user, onSelect, variant = "full" }: UserCardProps) {
  return ( /* ... */ );
}

// Bad -- default export, inline props, class component
export default class UserCard extends React.Component<{user: any}> { /* ... */ }

Hooks Patterns

Custom Hooks

1. Prefix with use -- useAuth, useDebounce, useLocalStorage
2. Extract shared logic into custom hooks -- if two components share stateful logic, extract it
3. Return tuples for simple hooks, objects for complex ones

// Simple: return tuple
function useToggle(initial = false): [boolean, () => void] {
  const [value, setValue] = useState(initial);
  const toggle = useCallback(() => setValue((v) => !v), []);
  return [value, toggle];
}

// Complex: return object
function useApi<T>(url: string) {
  const [data, setData] = useState<T | null>(null);
  const [error, setError] = useState<Error | null>(null);
  const [loading, setLoading] = useState(true);
  // ... fetch logic
  return { data, error, loading, refetch };
}

Hook Rules

1. Never call hooks conditionally -- all hooks must run on every render
2. Use useCallback for functions passed to children -- prevents unnecessary re-renders
3. Use useMemo only for expensive computations -- don't wrap everything
4. Prefer useReducer over useState when state transitions are complex

useEffect Guidelines

1. Always specify dependencies -- never use // eslint-disable-next-line
2. Return a cleanup function for subscriptions, timers, and listeners
3. Avoid setting state in useEffect when you can derive it -- computed values don't need effects

// Bad -- unnecessary effect
const [fullName, setFullName] = useState("");
useEffect(() => {
  setFullName(`${firstName} ${lastName}`);
}, [firstName, lastName]);

// Good -- derived value
const fullName = `${firstName} ${lastName}`;

State Management Tiers

Use the simplest tier that solves the problem:

Context Guidelines

1. Split contexts by domain -- AuthContext, ThemeContext, not AppContext
2. Keep context values stable -- use useMemo on the provider value
3. Don't put frequently changing values in context -- it re-renders all consumers

// Good -- stable context value
function AuthProvider({ children }: { children: ReactNode }) {
  const [user, setUser] = useState<User | null>(null);

  const value = useMemo(() => ({ user, setUser }), [user]);

  return <AuthContext.Provider value={value}>{children}</AuthContext.Provider>;
}

Performance

1. Use React.lazy + Suspense for route-level code splitting
2. Memoize expensive list items with React.memo -- include a custom comparator if props are objects
3. Virtualize long lists -- use react-window or @tanstack/virtual for 100+ items
4. Avoid anonymous functions in JSX when passing to memoized children
5. Use key correctly -- stable, unique identifiers, never array index for dynamic lists

// Good -- code splitting
const Dashboard = lazy(() => import("./pages/Dashboard"));

function App() {
  return (
    <Suspense fallback={<Loading />}>
      <Routes>
        <Route path="/dashboard" element={<Dashboard />} />
      </Routes>
    </Suspense>
  );
}

Accessibility

1. Use semantic HTML -- <button> not <div onClick>, <nav> not <div className="nav">
2. All images need alt text -- empty alt="" for decorative images
3. Form inputs need labels -- use <label htmlFor> or aria-label
4. Manage focus on route changes -- focus the main heading after navigation
5. Support keyboard navigation -- all interactive elements must be reachable via Tab
6. Use ARIA attributes when semantic HTML is insufficient -- aria-expanded, aria-live, role

// Good -- accessible modal
function Modal({ isOpen, onClose, title, children }: ModalProps) {
  const closeRef = useRef<HTMLButtonElement>(null);

  useEffect(() => {
    if (isOpen) closeRef.current?.focus();
  }, [isOpen]);

  if (!isOpen) return null;

  return (
    <div role="dialog" aria-modal="true" aria-labelledby="modal-title">
      <h2 id="modal-title">{title}</h2>
      {children}
      <button ref={closeRef} onClick={onClose}>
        Close
      </button>
    </div>
  );
}

Error Boundaries

1. Wrap route-level components in error boundaries
2. Provide a meaningful fallback UI -- not just "Something went wrong"
3. Log errors to your monitoring service in the boundary

import { ErrorBoundary } from "react-error-boundary";

function ErrorFallback({ error, resetErrorBoundary }: FallbackProps) {
  return (
    <div role="alert">
      <h2>Something went wrong</h2>
      <pre>{error.message}</pre>
      <button onClick={resetErrorBoundary}>Try again</button>
    </div>
  );
}

// Usage
<ErrorBoundary FallbackComponent={ErrorFallback}>
  <Dashboard />
</ErrorBoundary>

Anti-patterns

• Prop drilling more than 2 levels -- use composition or context instead
• Giant useEffect blocks -- split into multiple focused effects
• Storing derived state -- compute it during render
• Premature optimization -- profile before adding useMemo/useCallback everywhere
• String-based refs -- use useRef only
• Direct DOM manipulation -- use refs only when React APIs are insufficient