Frontend Dev Guidelines

---
name: frontend-dev-guidelines
description: Frontend development guidelines for React/TypeScript applications. Modern patterns including Suspense, lazy loading, useSuspenseQuery, file organization with features directory, styling best practices, routing, performance optimization, and TypeScript best practices. Use when creating components, pages, features, fetching data, styling, routing, or working with frontend code.
---

# Frontend Development Guidelines

## Project Structure

```
src/
├── features/           # Feature-based organization
│   ├── auth/
│   │   ├── components/
│   │   ├── hooks/
│   │   ├── api/
│   │   └── index.ts
│   ├── dashboard/
│   └── settings/
├── components/         # Shared/common components
│   ├── ui/            # Base UI components
│   └── layout/        # Layout components
├── hooks/             # Shared hooks
├── utils/             # Utility functions
├── types/             # TypeScript types
├── api/               # API layer
└── styles/            # Global styles
```

## Component Patterns

### Functional Components with TypeScript

```tsx
interface Props {
  title: string;
  onAction: (id: string) => void;
  isLoading?: boolean;
}

export function MyComponent({ title, onAction, isLoading = false }: Props) {
  // Component logic
  return (
    <div>
      {isLoading ? <Spinner /> : <Content title={title} />}
    </div>
  );
}
```

### Suspense for Data Fetching

```tsx
import { Suspense } from 'react';

function ParentComponent() {
  return (
    <Suspense fallback={<LoadingSkeleton />}>
      <DataComponent />
    </Suspense>
  );
}

function DataComponent() {
  // useSuspenseQuery automatically suspends
  const { data } = useSuspenseQuery({
    queryKey: ['items'],
    queryFn: fetchItems,
  });
  
  return <ItemList items={data} />;
}
```

### Lazy Loading Routes

```tsx
import { lazy, Suspense } from 'react';

const Dashboard = lazy(() => import('./features/dashboard'));
const Settings = lazy(() => import('./features/settings'));

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

## State Management

### Local State
```tsx
const [value, setValue] = useState<string>('');
const [items, setItems] = useState<Item[]>([]);
```

### Server State (TanStack Query)
```tsx
// Queries
const { data, isLoading, error } = useQuery({
  queryKey: ['users', userId],
  queryFn: () => fetchUser(userId),
});

// Mutations
const mutation = useMutation({
  mutationFn: updateUser,
  onSuccess: () => {
    queryClient.invalidateQueries({ queryKey: ['users'] });
  },
});
```

### Global State (Zustand or Context)
```tsx
// Simple global state with Zustand
const useStore = create<Store>((set) => ({
  theme: 'light',
  setTheme: (theme) => set({ theme }),
}));
```

## Styling Best Practices

### Tailwind CSS
```tsx
<div className="flex items-center gap-4 p-4 bg-white rounded-lg shadow-md">
  <span className="text-lg font-semibold text-gray-900">
    {title}
  </span>
</div>
```

### CSS Modules (when needed)
```tsx
import styles from './Component.module.css';

<div className={styles.container}>
  <span className={styles.title}>{title}</span>
</div>
```

### Styled Components / Emotion (if used)
```tsx
const Container = styled.div`
  display: flex;
  align-items: center;
  padding: ${({ theme }) => theme.spacing.md};
`;
```

## Performance Optimization

### Memoization
```tsx
// Memoize expensive computations
const expensiveValue = useMemo(() => {
  return computeExpensiveValue(data);
}, [data]);

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

// Memoize components
const MemoizedComponent = memo(function Component({ data }: Props) {
  return <div>{data.value}</div>;
});
```

### Code Splitting
```tsx
// Route-based splitting
const Page = lazy(() => import('./Page'));

// Component-based splitting
const HeavyComponent = lazy(() => import('./HeavyComponent'));
```

### Virtual Lists
```tsx
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: '400px', 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>
  );
}
```

## TypeScript Best Practices

### Type Definitions
```tsx
// Props interfaces
interface ButtonProps {
  variant: 'primary' | 'secondary' | 'danger';
  size?: 'sm' | 'md' | 'lg';
  onClick: () => void;
  children: React.ReactNode;
}

// API response types
interface ApiResponse<T> {
  data: T;
  meta: {
    page: number;
    total: number;
  };
}

// Utility types
type PartialUser = Partial<User>;
type RequiredUser = Required<User>;
type UserKeys = keyof User;
```

### Strict Null Checks
```tsx
// Handle potentially null values
const user = useUser();

if (!user) {
  return <NotFound />;
}

// Now TypeScript knows user is defined
return <UserProfile user={user} />;
```

## Error Handling

### Error Boundaries
```tsx
import { ErrorBoundary } from 'react-error-boundary';

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

<ErrorBoundary FallbackComponent={ErrorFallback}>
  <MyComponent />
</ErrorBoundary>
```

## Testing

### Component Tests
```tsx
import { render, screen, fireEvent } from '@testing-library/react';

describe('Button', () => {
  it('calls onClick when clicked', () => {
    const onClick = vi.fn();
    render(<Button onClick={onClick}>Click me</Button>);
    
    fireEvent.click(screen.getByRole('button'));
    
    expect(onClick).toHaveBeenCalledOnce();
  });
});
```

### Hook Tests
```tsx
import { renderHook, act } from '@testing-library/react';

describe('useCounter', () => {
  it('increments count', () => {
    const { result } = renderHook(() => useCounter());
    
    act(() => {
      result.current.increment();
    });
    
    expect(result.current.count).toBe(1);
  });
});
```

## Resource Files

For detailed patterns, see:
- [component-patterns.md](resources/component-patterns.md)
- [state-management.md](resources/state-management.md)
- [performance.md](resources/performance.md)
- [testing.md](resources/testing.md)