Agent Skills: Testing Skill

Testing Skill

Stub-Driven Test-Driven Development and layer boundary testing for functional core and effectful edge architecture.

Core Principle: Stub-Driven TDD

Test-Driven Development workflow for the functional core / effectful edge pattern:

1. Stub   → Create minimal interface/function signatures
2. Test   → Write tests against stubs
3. Implement → Make tests pass with real implementation
4. Refactor  → Improve code while keeping tests green

Key insight: Write interface signatures first, test against those, then implement—not the other way around.

See [references/stub-driven-tdd.md] for complete workflow examples.

Layer Boundary Testing

Test at the boundaries between functional core and effectful edge, not internal implementation.

Test here ──────▼──────────────────▼────── Test here
          Effectful Edge    │    Functional Core
              (stub)        │       (unit test)

Where to Test Each Layer

| Layer | Test Type | What to Stub | What to Assert | |-------|-----------|--------------|-----------------| | Entity | Unit | Nothing (pure) | Validation, rules, transforms | | Service | Unit | Repositories | Orchestration logic, error handling | | Router | Integration | Service | Status codes, response format | | Repository | Integration | DB connection | CRUD operations, queries | | Consumer | Integration | Service | Event parsing, service calls |

See [references/boundaries.md] for detailed testing patterns by layer.

Functional Core Testing

Entity Tests (Pure Functions)

Focus: Validation, business rules, data transformations

describe('Order entity', () => {
  describe('validation', () => {
    it('rejects empty items', () => {
      const order = new Order('1', 'C1', [], 'pending', 0);
      expect(order.validate().ok).toBe(false);
    });
  });

  describe('business rules', () => {
    it('prevents cancelling shipped order', () => {
      const order = new Order('1', 'C1', [], 'shipped', 0);
      expect(order.canCancel()).toBe(false);
    });
  });

  describe('transformations', () => {
    it('converts request to entity with calculated total', () => {
      const order = Order.fromRequest({
        customerId: 'C1',
        items: [
          { productId: 'P1', quantity: 2, price: 10 },
          { productId: 'P2', quantity: 1, price: 15 }
        ]
      });
      expect(order.total).toBe(35);
    });
  });
});

Service Tests (Stubbed Dependencies)

Focus: Orchestration logic with stubbed repositories

describe('OrderService.createOrder', () => {
  let service: OrderService;
  let mockRepo: OrderRepository;

  beforeEach(() => {
    mockRepo = {
      save: vi.fn().mockResolvedValue({ id: '123' }),
      findById: vi.fn()
    };
    service = new OrderService(mockRepo);
  });

  it('creates order with valid data', async () => {
    const result = await service.createOrder({
      customerId: 'C1',
      items: [{ productId: 'P1', quantity: 2 }]
    });

    expect(result.ok).toBe(true);
    expect(mockRepo.save).toHaveBeenCalled();
  });

  it('does not save when validation fails', async () => {
    const result = await service.createOrder({
      customerId: 'C1',
      items: [] // Invalid
    });

    expect(result.ok).toBe(false);
    expect(mockRepo.save).not.toHaveBeenCalled();
  });
});

See [references/core-testing.md] for comprehensive Entity and Service examples.

Effectful Edge Testing

Router, Repository, Consumer Integration Tests

Focus: Real HTTP/database/events with stubbed core

// Router: real HTTP, stub service
describe('POST /orders', () => {
  it('returns 201 for valid request', async () => {
    const mockService = {
      createOrder: vi.fn().mockResolvedValue(Ok({ id: '123' }))
    };
    const app = createApp(mockService);

    const response = await request(app)
      .post('/orders')
      .send({ customerId: 'C1', items: [{ productId: 'P1', quantity: 2 }] });

    expect(response.status).toBe(201);
  });
});

// Repository: real test database
describe('OrderRepository.save', () => {
  it('persists order to database', async () => {
    const repo = new OrderRepository(testDb);
    const saved = await repo.save({
      id: '123',
      customer_id: 'C1',
      items: '[]',
      status: 'pending',
      total: 0
    });

    const found = await testDb.orders.findOne({ id: '123' });
    expect(found).toBeDefined();
  });
});

// Consumer: real events, stub service
describe('OrderConsumer', () => {
  it('handles OrderPlaced event', async () => {
    const mockService = {
      processOrder: vi.fn().mockResolvedValue(Ok({}))
    };
    const consumer = new OrderConsumer(mockService);

    await consumer.handle({
      type: 'OrderPlaced',
      data: { orderId: '123' }
    });

    expect(mockService.processOrder).toHaveBeenCalledWith('123');
  });
});

See [references/edge-testing.md] for Router, Repository, Consumer, Producer, and Client patterns.

Test Coverage Guidelines

Aim for strategic coverage, not 100%:

High Coverage (Critical):

• Entity validation and business rules
• Service orchestration logic
• Critical user journeys (integration tests)
• Data transformations with logic

Medium Coverage (Important):

• Error handling paths
• Edge cases in business logic
• API contract validation

Low Coverage (Optional):

• Simple getters/setters
• Framework boilerplate
• Trivial mappings
• Internal utilities

What NOT to Test

Avoid testing implementation details, framework behavior, and trivial code:

• Don't test private methods (test through public API)
• Don't test simple getters/setters (no logic = no test value)
• Don't test framework behavior (Express, database driver already tested)
• Don't test third-party library behavior (lodash, validation libraries)
• Don't test trivial mappings without logic

See [references/anti-patterns.md] for anti-patterns with examples and fixes.

Testing → Implementation Flow

Follow this dependency order:

1. Entity tests    (pure functions, fast)
2. Service tests   (stubbed dependencies, fast)
3. Integration tests (real IO, slower)

This enables TDD: write tests first at lower layers, then implement, then build upward.

Quick Reference

For Entity Testing: See [references/core-testing.md] For Service Testing: See [references/core-testing.md] For Router/Repo/Consumer: See [references/edge-testing.md] For Workflow Examples: See [references/stub-driven-tdd.md] For What NOT to Do: See [references/anti-patterns.md]