Tester Agent

What problem does it solve?

Automates the creation of thorough tests for Next.js 14+ projects, ensuring code quality and full coverage.

Core Features & Use Cases

• Unit/Integration: Vitest + React Testing Library
• E2E: Playwright
• Mocking: vi.mock(), msw (Mock Service Worker) for API mocking
• Database: Test with a real Postgres test DB or in-memory SQLite via Prisma
• Coverage: Istanbul (built into Vitest)

What to Test

React Components

For every React component, generate tests covering:

1. Default render — component mounts without errors with minimal required props
2. All prop variants — test each significant prop combination (e.g., variant="destructive", disabled, loading)
3. User interactions — userEvent.click(), userEvent.type(), form submission
4. Conditional rendering — branches that show/hide elements
5. Error states — component renders error UI when appropriate
6. Accessibility — run axe via @axe-core/react or check for role, aria-label presence
7. Snapshot — only for highly stable presentational components; prefer explicit assertions

Example structure: describe('Button', () => { it('renders with default props', () => { ... }) it('renders loading spinner when isLoading=true', () => { ... }) it('calls onClick handler when clicked', async () => { ... }) it('is disabled when disabled=true', () => { ... }) it('applies destructive variant styles', () => { ... }) it('meets accessibility requirements', async () => { ... }) })

Custom Hooks

For every custom hook, generate tests covering:

1. Initial state — hook returns correct initial values
2. State transitions — calling returned functions updates state correctly
3. Async operations — loading/error/success states
4. Cleanup — subscriptions and timers are cleaned up on unmount
5. Dependencies — hook re-runs when deps change

Use renderHook from React Testing Library.

API Routes (Route Handlers)

For every route.ts in src/app/api/:

1. Success path — valid request returns 200 with expected shape
2. Validation errors — invalid input returns 400 with error details
3. Auth enforcement — unauthenticated requests return 401; unauthorized return 403
4. Not found — missing resource returns 404
5. Method not allowed — wrong HTTP method returns 405
6. Server errors — DB failures return 500 with safe error message (not stack trace)

Server Actions

1. Happy path — correct input produces expected mutation and return value
2. Validation failure — invalid input returns structured error
3. Auth check — unauthenticated call throws or returns auth error
4. Revalidation — revalidatePath or revalidateTag is called correctly

Utility Functions

For every function in lib/ and utils/:

1. Test every branch of conditional logic
2. Test edge cases: empty string, zero, null, undefined, empty array
3. Test error throwing: invalid inputs throw with descriptive messages
4. For pure functions: use property-based testing when applicable

Quick Start

Generate comprehensive Vitest and Playwright tests for a given Next.js 14+ module, ensuring 100% test coverage for new code where applicable.