senior-frontend

What this skill does

# Senior Frontend Engineer

## Overview

Deliver production-grade frontend code following a structured three-phase workflow: context discovery, development, and handoff. This skill enforces strict quality standards including atomic design component architecture, comprehensive state management patterns, SSR/SSG/ISR optimization, and mandatory >85% test coverage with Vitest, React Testing Library, and Playwright.

**Announce at start:** "I'm using the senior-frontend skill for production-grade React/TypeScript development."

---

## Phase 1: Context Discovery

**Goal:** Understand the existing codebase before writing any code.

### Actions

1. Analyze existing codebase structure and conventions
2. Identify the tech stack version (React 18/19, Next.js 14/15, TypeScript version)
3. Review existing component library and design system
4. Check state management approach already in use
5. Understand build tooling and CI pipeline
6. Map existing test infrastructure and coverage

### STOP — Do NOT proceed to Phase 2 until:
- [ ] Tech stack versions are identified
- [ ] Existing patterns and conventions are documented
- [ ] Test infrastructure is mapped
- [ ] State management approach is identified

---

## Phase 2: Development

**Goal:** Implement with strict TypeScript, atomic design, and TDD.

### Actions

1. Design component architecture following atomic design
2. Implement with TypeScript strict mode
3. Write tests alongside implementation (TDD when appropriate)
4. Optimize for performance (bundle size, rendering, loading)
5. Ensure accessibility compliance

### Component Architecture Decision Table (Atomic Design)

| Level | Description | Business Logic | Example |
|-------|------------|---------------|---------|
| **Atoms** | Smallest building blocks | None | Button, Input, Icon, Badge |
| **Molecules** | Composed of atoms | Minimal | FormField, SearchBar, Card |
| **Organisms** | Complex with business logic | Yes | DataTable, NavigationBar, CommentThread |
| **Templates** | Page structure without data | Layout only | DashboardLayout, AuthLayout |
| **Pages** | Templates connected to data | Data fetching | UsersPage, SettingsPage |

### Atom Example

```typescript
interface ButtonProps extends React.ButtonHTMLAttributes<HTMLButtonElement> {
  variant?: 'primary' | 'secondary' | 'ghost' | 'danger';
  size?: 'sm' | 'md' | 'lg';
  isLoading?: boolean;
}

export function Button({ variant = 'primary', size = 'md', isLoading, children, ...props }: ButtonProps) {
  return (
    <button className={cn(buttonVariants({ variant, size }))} disabled={isLoading || props.disabled} {...props}>
      {isLoading ? <Spinner size={size} /> : children}
    </button>
  );
}
```

### State Management Decision Table

| State Type | Solution | When to Use |
|------------|----------|-------------|
| Server state | React Query / TanStack Query | API data, caching, sync |
| Form state | React Hook Form + Zod | Form validation, submission |
| Global UI state | Zustand | Theme, sidebar open, modals |
| Local UI state | useState / useReducer | Component-specific state |
| URL state | nuqs / useSearchParams | Filters, pagination, tabs |
| Complex local | useReducer | Multiple related state transitions |
| Shared context | React Context | Theme, locale, auth (infrequent updates) |

### SSR / SSG / ISR Decision Table (Next.js App Router)

| Pattern | Use When | Cache Strategy |
|---------|----------|---------------|
| Static (SSG) | Content rarely changes | Build time |
| ISR | Content changes periodically | Revalidate interval |
| SSR | Content changes per request | No cache |
| Client | User-specific, interactive | Browser |

### Server vs Client Component Decision

| Need | Component Type |
|------|---------------|
| Direct data fetching | Server (default) |
| Event handlers (onClick, onChange) | Client (`'use client'`) |
| useState / useReducer | Client |
| useEffect / useLayoutEffect | Client |
| Browser APIs (window, localStorage) | Client |
| Third-party libs using client features | Client |
| No interactivity needed | Server (default) |

### STOP — Do NOT proceed to Phase 3 until:
- [ ] Components follow atomic design hierarchy
- [ ] TypeScript strict mode is enabled, no `any` types
- [ ] Tests are written for all components
- [ ] Accessibility is verified (axe-core)

---

## Phase 3: Handoff

**Goal:** Verify quality gates and prepare for review.

### Actions

1. Verify test coverage meets >85% threshold
2. Run full lint and type check
3. Document complex components with JSDoc/TSDoc
4. Create Storybook stories for UI components
5. Performance audit (Lighthouse, bundle analysis)

### Performance Checklist

- [ ] Bundle size < 200KB gzipped (initial load)
- [ ] Largest Contentful Paint < 2.5s
- [ ] First Input Delay < 100ms
- [ ] Cumulative Layout Shift < 0.1
- [ ] Images: next/image with proper sizing and formats
- [ ] Fonts: next/font with display swap
- [ ] No layout thrashing (batch DOM reads/writes)
- [ ] Virtualization for lists > 100 items

### Coverage Thresholds

```json
{
  "coverageThreshold": {
    "global": {
      "branches": 85,
      "functions": 85,
      "lines": 85,
      "statements": 85
    }
  }
}
```

### STOP — Handoff complete when:
- [ ] Test coverage >85% verified
- [ ] Lint and type check pass with zero errors
- [ ] Performance audit completed
- [ ] Complex components documented

---

## Testing Requirements

### Unit Tests (Vitest + React Testing Library)

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

  it('shows loading state', () => {
    render(<Button isLoading>Click me</Button>);
    expect(screen.getByRole('button')).toBeDisabled();
  });

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

### Integration Tests

- Component compositions (form submission flow)
- Data fetching with MSW (Mock Service Worker)
- Routing and navigation
- Error boundaries and fallbacks

### E2E Tests (Playwright)

```typescript
test('user can complete checkout', async ({ page }) => {
  await page.goto('/products');
  await page.getByRole('button', { name: 'Add to cart' }).first().click();
  await page.getByRole('link', { name: 'Cart' }).click();
  await expect(page.getByText('1 item')).toBeVisible();
  await page.getByRole('button', { name: 'Checkout' }).click();
});
```

---

## React Query Patterns

```typescript
function useUsers(filters: UserFilters) {
  return useQuery({
    queryKey: ['users', filters],
    queryFn: () => fetchUsers(filters),
    staleTime: 5 * 60 * 1000,
    placeholderData: keepPreviousData,
  });
}

function useUpdateUser() {
  const queryClient = useQueryClient();
  return useMutation({
    mutationFn: updateUser,
    onMutate: async (newUser) => {
      await queryClient.cancelQueries({ queryKey: ['users'] });
      const previous = queryClient.getQueryData(['users']);
      queryClient.setQueryData(['users'], (old) =>
        old.map(u => u.id === newUser.id ? { ...u, ...newUser } : u)
      );
      return { previous };
    },
    onError: (err, newUser, context) => {
      queryClient.setQueryData(['users'], context.previous);
    },
    onSettled: () => {
      queryClient.invalidateQueries({ queryKey: ['users'] });
    },
  });
}
```

---

## Memoization Decision Table

| Technique | Use When | Do NOT Use When |
|-----------|----------|----------------|
| `useMemo` | Expensive computation, referential equality for deps | Simple calculations, primitive values |
| `useCallback` | Functions passed to memoized children | Functions not passed as props |
| `React.memo` | Component re-renders often with same props | Props change on every render |
| None | Default — do not memoize | Always profi