senior-frontend

# Senior Frontend Developer

Expert-level frontend development for modern web applications.

## Core Competencies

- Component architecture
- State management
- Performance optimization
- Accessibility (a11y)
- CSS architecture
- Build optimization
- Testing strategies
- Design system implementation

## Framework Comparison

| Feature | React | Vue | Svelte |
|---------|-------|-----|--------|
| Learning Curve | Medium | Low | Low |
| Bundle Size | Medium | Small | Smallest |
| Ecosystem | Largest | Large | Growing |
| Performance | Excellent | Excellent | Best |
| Enterprise Use | Highest | High | Medium |

## React Patterns

### Component Patterns

**Presentational Component:**
```typescript
interface ButtonProps {
  variant: 'primary' | 'secondary';
  size: 'sm' | 'md' | 'lg';
  disabled?: boolean;
  children: React.ReactNode;
  onClick?: () => void;
}

export function Button({
  variant,
  size,
  disabled,
  children,
  onClick
}: ButtonProps) {
  return (
    <button
      className={cn(
        'button',
        `button--${variant}`,
        `button--${size}`,
        disabled && 'button--disabled'
      )}
      disabled={disabled}
      onClick={onClick}
    >
      {children}
    </button>
  );
}
```

**Container Component:**
```typescript
export function UserListContainer() {
  const { data: users, isLoading, error } = useUsers();

  if (isLoading) return <Skeleton count={5} />;
  if (error) return <ErrorMessage error={error} />;

  return <UserList users={users} />;
}
```

**Higher-Order Component:**
```typescript
function withAuth<P extends object>(Component: React.ComponentType<P>) {
  return function AuthenticatedComponent(props: P) {
    const { user, isLoading } = useAuth();

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

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

### Custom Hooks

**Data Fetching Hook:**
```typescript
function useApi<T>(url: string, options?: RequestInit) {
  const [data, setData] = useState<T | null>(null);
  const [error, setError] = useState<Error | null>(null);
  const [isLoading, setIsLoading] = useState(true);

  useEffect(() => {
    const controller = new AbortController();

    async function fetchData() {
      try {
        setIsLoading(true);
        const response = await fetch(url, {
          ...options,
          signal: controller.signal,
        });
        if (!response.ok) throw new Error('Request failed');
        const json = await response.json();
        setData(json);
      } catch (err) {
        if (err.name !== 'AbortError') {
          setError(err as Error);
        }
      } finally {
        setIsLoading(false);
      }
    }

    fetchData();
    return () => controller.abort();
  }, [url]);

  return { data, error, isLoading };
}
```

**Local Storage Hook:**
```typescript
function useLocalStorage<T>(key: string, initialValue: T) {
  const [storedValue, setStoredValue] = useState<T>(() => {
    if (typeof window === 'undefined') return initialValue;
    try {
      const item = window.localStorage.getItem(key);
      return item ? JSON.parse(item) : initialValue;
    } catch {
      return initialValue;
    }
  });

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

  return [storedValue, setValue] as const;
}
```

## State Management

### Zustand

```typescript
import { create } from 'zustand';
import { persist } from 'zustand/middleware';

interface CartStore {
  items: CartItem[];
  addItem: (item: CartItem) => void;
  removeItem: (id: string) => void;
  clearCart: () => void;
  total: () => number;
}

export const useCartStore = create<CartStore>()(
  persist(
    (set, get) => ({
      items: [],
      addItem: (item) =>
        set((state) => ({
          items: [...state.items, item],
        })),
      removeItem: (id) =>
        set((state) => ({
          items: state.items.filter((item) => item.id !== id),
        })),
      clearCart: () => set({ items: [] }),
      total: () =>
        get().items.reduce((sum, item) => sum + item.price * item.quantity, 0),
    }),
    { name: 'cart-storage' }
  )
);
```

### React Query

```typescript
import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';

// Query
function useProducts(category: string) {
  return useQuery({
    queryKey: ['products', category],
    queryFn: () => fetchProducts(category),
    staleTime: 5 * 60 * 1000,
  });
}

// Mutation
function useCreateProduct() {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: createProduct,
    onSuccess: () => {
      queryClient.invalidateQueries({ queryKey: ['products'] });
    },
  });
}

// Optimistic Update
function useUpdateProduct() {
  const queryClient = useQueryClient();

  return useMutation({
    mutationFn: updateProduct,
    onMutate: async (newProduct) => {
      await queryClient.cancelQueries({ queryKey: ['products', newProduct.id] });
      const previous = queryClient.getQueryData(['products', newProduct.id]);
      queryClient.setQueryData(['products', newProduct.id], newProduct);
      return { previous };
    },
    onError: (err, newProduct, context) => {
      queryClient.setQueryData(['products', newProduct.id], context?.previous);
    },
    onSettled: (data, error, variables) => {
      queryClient.invalidateQueries({ queryKey: ['products', variables.id] });
    },
  });
}
```

## CSS Architecture

### Tailwind CSS Best Practices

**Component Classes:**
```typescript
const buttonVariants = cva(
  'inline-flex items-center justify-center rounded-md font-medium transition-colors',
  {
    variants: {
      variant: {
        primary: 'bg-blue-600 text-white hover:bg-blue-700',
        secondary: 'bg-gray-200 text-gray-900 hover:bg-gray-300',
        outline: 'border border-gray-300 hover:bg-gray-100',
      },
      size: {
        sm: 'h-8 px-3 text-sm',
        md: 'h-10 px-4 text-base',
        lg: 'h-12 px-6 text-lg',
      },
    },
    defaultVariants: {
      variant: 'primary',
      size: 'md',
    },
  }
);
```

### CSS Modules

```css
/* Button.module.css */
.button {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  border-radius: 0.375rem;
  font-weight: 500;
  transition: all 0.2s;
}

.primary {
  background-color: var(--color-primary);
  color: white;
}

.primary:hover {
  background-color: var(--color-primary-dark);
}
```

## Accessibility

### ARIA Patterns

**Modal Dialog:**
```typescript
function Modal({ isOpen, onClose, title, children }) {
  const modalRef = useRef<HTMLDivElement>(null);

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

  if (!isOpen) return null;

  return (
    <div
      role="dialog"
      aria-modal="true"
      aria-labelledby="modal-title"
      ref={modalRef}
      tabIndex={-1}
      onKeyDown={(e) => e.key === 'Escape' && onClose()}
    >
      <h2 id="modal-title">{title}</h2>
      {children}
      <button onClick={onClose} aria-label="Close modal">
        Close
      </button>
    </div>
  );
}
```

**Tab Panel:**
```typescript
function Tabs({ tabs, activeTab, onChange }) {
  return (
    <div>
      <div role="tablist" aria-label="Content tabs">
        {tabs.map((tab, index) => (
          <button
            key={tab.id}
            role="tab"
            aria-selected={activeTab === index}
            aria-controls={`panel-${tab.id}`}
            id={`tab-${tab.id}`}
            tabIndex={activeTab === index ? 0 : -1}
            onClick={() => onChange(index)}
          >
            {tab.label}
          </button>
        ))}
      </div>
      {tabs.map((tab, index) => (
        <div
          key={tab.id}
          role="tabpanel"
          id={`panel-${tab.id}`}
          aria-labelledby={`tab-${tab.id}`}
          hidden={activeTab !== index}
        >
          {tab.content}
        </div>
      ))}
    </div>
  );
}
```

### Accessibility Checklist

- [ ] Keyboard navigation works
- [ ] Focus indicators visible
- [ ] Color contrast meets WCAG AA
- [ ] Images have alt text
- [ ] Form inputs have labels
- [ ] Error messages are announced
- [ ] Skip links present
- [ ] Heading hierarchy correct
- [ ] ARIA labels for icons
- [ ] Motion respects user preferences

## Performance Optimization

### Code Splitting

```typescript
// Route-based splitting
const Dashboard = lazy(() => import('./pages/Dashboard'));
const Settings = lazy(() => import('./pages/Settings'));

// Component-based splitting
const HeavyChart = lazy(() => import('./components/HeavyChart'));

// Named exports
const { DataTable } = lazy(() =>
  import('./components/DataTable').then((module) => ({
    default: module.DataTable,
  }))
);
```

### Memoization

```typescript
// Memoize expensive component
const ExpensiveList = memo(function ExpensiveList({ items, onSelect }) {
  return (
    <ul>
      {items.map((item) => (
        <li key={item.id} onClick={() => onSelect(item.id)}>
          {item.name}
        </li>
      ))}
    </ul>
  );
});

// Memoize callback
function Parent() {
  const [selected, setSelected] = useState<string | null>(null);

  const handleSelect = useCallback((id: string) => {
    setSelected(id);
  }, []);

  return <ExpensiveList items={items} onSelect={handleSelect} />;
}

// Memoize computed value
function DataDisplay({ data }) {
  const processedData = useMemo(() => {
    return data.map(expensiveTransform).filter(expensiveFilter);
  }, [data]);

  return <Chart data={processedData} />;
}
```

### Virtual Lists

```typescript
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,
    overscan: 5,
  });

  return (
    <div ref={parentRef} style={{ height: '400px', overflow: 'auto' }}>
      <div
        style={{
          height: `${virtualizer.getTotalSize()}px`,
          position: 'relative',
        }}
      >
        {virtualizer.getVirtualItems().map((virtualItem) => (
          <div
            key={virtualItem.key}
            style={{
              position: 'absolute',
              top: 0,
              transform: `translateY(${virtualItem.start}px)`,
              height: `${virtualItem.size}px`,
            }}
          >
            {items[virtualItem.index].name}
          </div>
        ))}
      </div>
    </div>
  );
}
```

## Testing

### Component Testing

```typescript
import { render, screen, fireEvent } from '@testing-library/react';

describe('Button', () => {
  it('renders with correct text', () => {
    render(<Button>Click me</Button>);
    expect(screen.getByRole('button', { name: 'Click me' })).toBeInTheDocument();
  });

  it('calls onClick when clicked', () => {
    const onClick = jest.fn();
    render(<Button onClick={onClick}>Click me</Button>);
    fireEvent.click(screen.getByRole('button'));
    expect(onClick).toHaveBeenCalledTimes(1);
  });

  it('is disabled when disabled prop is true', () => {
    render(<Button disabled>Click me</Button>);
    expect(screen.getByRole('button')).toBeDisabled();
  });
});
```

### Hook Testing

```typescript
import { renderHook, act } from '@testing-library/react';

describe('useCounter', () => {
  it('increments counter', () => {
    const { result } = renderHook(() => useCounter(0));

    act(() => {
      result.current.increment();
    });

    expect(result.current.count).toBe(1);
  });
});
```

## Reference Materials

- `references/react_patterns.md` - Advanced React patterns
- `references/css_architecture.md` - CSS organization
- `references/accessibility.md` - A11y guidelines
- `references/performance.md` - Performance optimization

## Scripts

```bash
# Component scaffolder
python scripts/component_gen.py --name Button --type atom

# Bundle analyzer
python scripts/bundle_analyzer.py --build-dir ./dist

# Accessibility audit
python scripts/a11y_audit.py --url http://localhost:3000

# Performance report
python scripts/perf_report.py --lighthouse-json report.json
```