Frontend Patterns Skill

Modern React patterns and best practices for building scalable applications.

Component Patterns

1. Compound Components

// Parent provides context, children consume it
const Tabs = ({ children, defaultTab }) => {
  const [activeTab, setActiveTab] = useState(defaultTab);

  return (
    <TabsContext.Provider value={{ activeTab, setActiveTab }}>
      <div className="tabs">{children}</div>
    </TabsContext.Provider>
  );
};

Tabs.List = ({ children }) => <div className="tabs-list">{children}</div>;
Tabs.Tab = ({ id, children }) => {
  const { activeTab, setActiveTab } = useTabsContext();
  return (
    <button
      className={activeTab === id ? 'active' : ''}
      onClick={() => setActiveTab(id)}
    >
      {children}
    </button>
  );
};
Tabs.Panel = ({ id, children }) => {
  const { activeTab } = useTabsContext();
  return activeTab === id ? <div>{children}</div> : null;
};

// Usage
<Tabs defaultTab="tab1">
  <Tabs.List>
    <Tabs.Tab id="tab1">Tab 1</Tabs.Tab>
    <Tabs.Tab id="tab2">Tab 2</Tabs.Tab>
  </Tabs.List>
  <Tabs.Panel id="tab1">Content 1</Tabs.Panel>
  <Tabs.Panel id="tab2">Content 2</Tabs.Panel>
</Tabs>

2. Render Props

interface MousePosition {
  x: number;
  y: number;
}

const MouseTracker = ({
  render,
}: {
  render: (pos: MousePosition) => ReactNode;
}) => {
  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: {x}, {y}</div>
  )}
/>

3. Custom Hooks

// Reusable logic extraction
function useLocalStorage<T>(key: string, initialValue: T) {
  const [storedValue, setStoredValue] = useState<T>(() => {
    try {
      const item = window.localStorage.getItem(key);
      return item ? JSON.parse(item) : initialValue;
    } catch {
      return initialValue;
    }
  });

  const setValue = (value: T | ((val: T) => T)) => {
    try {
      const valueToStore = value instanceof Function ? value(storedValue) : value;
      setStoredValue(valueToStore);
      window.localStorage.setItem(key, JSON.stringify(valueToStore));
    } catch (error) {
      console.error(error);
    }
  };

  return [storedValue, setValue] as const;
}

4. Higher-Order Components (HOC)

function withAuth<P extends object>(Component: ComponentType<P>) {
  return function AuthenticatedComponent(props: P) {
    const { user, isLoading } = useAuth();

    if (isLoading) return <Spinner />;
    if (!user) return <Navigate to="/login" />;

    return <Component {...props} />;
  };
}

// Usage
const ProtectedDashboard = withAuth(Dashboard);

State Management Patterns

Zustand Store Pattern

interface Store {
  // State
  items: Item[];
  isLoading: boolean;

  // Actions
  fetchItems: () => Promise<void>;
  addItem: (item: Item) => void;
  removeItem: (id: string) => void;
}

const useStore = create<Store>((set, get) => ({
  items: [],
  isLoading: false,

  fetchItems: async () => {
    set({ isLoading: true });
    try {
      const items = await api.getItems();
      set({ items, isLoading: false });
    } catch {
      set({ isLoading: false });
    }
  },

  addItem: (item) => set((state) => ({
    items: [...state.items, item],
  })),

  removeItem: (id) => set((state) => ({
    items: state.items.filter((i) => i.id !== id),
  })),
}));

// Selectors for performance
const useItems = () => useStore((s) => s.items);
const useIsLoading = () => useStore((s) => s.isLoading);

React Query Pattern

// Query keys factory
const todoKeys = {
  all: ['todos'] as const,
  lists: () => [...todoKeys.all, 'list'] as const,
  list: (filters: string) => [...todoKeys.lists(), { filters }] as const,
  details: () => [...todoKeys.all, 'detail'] as const,
  detail: (id: string) => [...todoKeys.details(), id] as const,
};

// Queries
export const useTodos = (filters?: string) =>
  useQuery({
    queryKey: todoKeys.list(filters ?? ''),
    queryFn: () => fetchTodos(filters),
  });

// Mutations with optimistic updates
export const useUpdateTodo = () => {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: updateTodo,
    onMutate: async (newTodo) => {
      await queryClient.cancelQueries({ queryKey: todoKeys.lists() });
      const previous = queryClient.getQueryData(todoKeys.lists());
      queryClient.setQueryData(todoKeys.lists(), (old) =>
        old?.map((t) => (t.id === newTodo.id ? newTodo : t))
      );
      return { previous };
    },
    onError: (err, newTodo, context) => {
      queryClient.setQueryData(todoKeys.lists(), context?.previous);
    },
    onSettled: () => {
      queryClient.invalidateQueries({ queryKey: todoKeys.lists() });
    },
  });
};

Performance Patterns

Memoization

// Memoize expensive calculations
const expensiveValue = useMemo(() => {
  return items.filter(x => x.active).reduce((acc, x) => acc + x.value, 0);
}, [items]);

// Memoize callbacks
const handleClick = useCallback((id: string) => {
  setSelected(id);
}, []);

// Memoize components
const MemoizedList = memo(({ items }: { items: Item[] }) => (
  <ul>
    {items.map(item => <li key={item.id}>{item.name}</li>)}
  </ul>
));

Code Splitting

// Lazy load routes
const Dashboard = lazy(() => import('./pages/Dashboard'));
const Settings = lazy(() => import('./pages/Settings'));

// With loading fallback
<Suspense fallback={<PageSkeleton />}>
  <Routes>
    <Route path="/dashboard" element={<Dashboard />} />
    <Route path="/settings" element={<Settings />} />
  </Routes>
</Suspense>

Virtual Lists

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

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

  const virtualizer = useVirtualizer({
    count: items.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 50,
  });

  return (
    <div ref={parentRef} style={{ height: 400, overflow: 'auto' }}>
      <div style={{ height: virtualizer.getTotalSize() }}>
        {virtualizer.getVirtualItems().map((virtualItem) => (
          <div
            key={virtualItem.key}
            style={{
              position: 'absolute',
              top: virtualItem.start,
              height: virtualItem.size,
            }}
          >
            {items[virtualItem.index].name}
          </div>
        ))}
      </div>
    </div>
  );
}

Error Handling

Error Boundaries

class ErrorBoundary extends Component<Props, State> {
  state = { hasError: false, error: null };

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

  componentDidCatch(error: Error, info: ErrorInfo) {
    console.error('Error caught:', error, info);
  }

  render() {
    if (this.state.hasError) {
      return <ErrorFallback error={this.state.error} />;
    }
    return this.props.children;
  }
}

Accessibility Patterns

// Focus management
const modalRef = useRef<HTMLDivElement>(null);
useEffect(() => {
  modalRef.current?.focus();
}, []);

// ARIA attributes
<button
  aria-label="Close modal"
  aria-expanded={isOpen}
  aria-controls="modal-content"
>
  <CloseIcon />
</button>

// Keyboard navigation
const handleKeyDown = (e: KeyboardEvent) => {
  switch (e.key) {
    case 'ArrowDown':
      setFocusedIndex(i => Math.min(i + 1, items.length - 1));
      break;
    case 'ArrowUp':
      setFocusedIndex(i => Math.max(i - 1, 0));
      break;
    case 'Enter':
      handleSelect(focusedIndex);
      break;
  }
};