MODE_Backend_TDD

Backend TDD Mode

Purpose

Transform backend development from waterfall (code → tests) to iterative TDD (test → code → refactor), where tests drive design decisions and Clean Architecture emerges from test requirements.

Prerequisites: This skill assumes familiarity with nestjs-architect for DDD, Clean Architecture, and NestJS integration patterns.

Activation Triggers

• Flags: --modo-backend-tdd, --tdd-clean-architecture, --strict-quality-mode
• Verbal: "Start with unit tests", "Follow TDD rigorously", "Tests first"
• Context: PRD/TechSpec requires coverage ≥80%, refactoring critical services, MVP with mandatory quality gates

Behavioral Changes

1. Tests First - Draft test specifications BEFORE any production code; never implement without failing test
2. Dependency Inversion - Define interfaces/contracts before implementations; inject via DI
3. Continuous Refinement - After each Red-Green cycle, eliminate coupling/duplication
4. Visible Metrics - Communicate coverage, complexity, execution time as part of decisions

Core Process: Red-Green-Refactor (7 Steps)

Execute iteratively for each use case:

Step 1: Confirm Context

• Validate task objective, acceptance criteria, quality indicators (coverage ≥80%, complexity <10, execution <5s)

Step 2: Model Contracts

• Define interfaces (repositories, services) before implementations
• Follow aggregates and repositories patterns

Step 3: Write Tests (RED)

• Create .spec.ts files in __tests__ directories per testing patterns
• Cover happy path + critical edge cases
• Use Arrange-Act-Assert pattern

Step 4: Execute and Validate Failure (RED)

• Run npm run test and confirm ALL new tests FAIL
• If tests pass without implementation, review test validity

Step 5: Implement Minimum Code (GREEN)

• Write minimal code to make tests pass
• Follow architecture principles for layer separation (domain/application/infrastructure)
• Apply TypeScript strictness (no any, strict mode enabled)
• Run npm run test and confirm GREEN

Step 6: Refactor (REFACTOR)

• Eliminate duplication and coupling while keeping tests green
• Verify Clean Architecture compliance (dependencies point inward)
• Run npm run test continuously during refactoring

Step 7: Register Metrics

• Execute npm run test:cov for coverage report
• Document: coverage %, complexity, execution time
• Evaluate if more scenarios need coverage. If yes, return to Step 2 for next use case.

Expected Outcomes

Validate TDD application by verifying:

1. Test files created BEFORE production code (verify timestamps/commits)
2. Initial RED state documented (console logs showing failures)
3. Implementation organized per architecture structure (core/domain, core/application, core/infra)
4. Interfaces and contracts explicit (repositories.interface.ts, use-cases.interface.ts)
5. Metrics documented: coverage ≥80%, complexity <10, execution <5s

Quality Standards

Mandatory criteria:

1. Tests First - NO production code without failing test; commits show tests → implementation
2. Segregated Layers - Follow architecture structure (domain isolated, unidirectional dependencies)
3. Testable Interfaces - All external dependencies via interfaces with fakes/stubs per repositories
4. Objective Metrics - Run npm run test:cov, verify thresholds, document results
5. Documentation - Architectural decisions and metrics in tasks.md or dev-log/

Example: Transformation

Standard (Waterfall):

// 1. Implement service directly with Prisma
@Injectable()
export class PedidosService {
  constructor(private prisma: PrismaClient) {} // Tight coupling
  async listarPedidos() {
    return this.prisma.pedido.findMany();
  }
}
// 2. Write tests afterward

TDD Mode:

// 1. Define interface (contract)
export interface IPedidosRepository {
  findAll(): Promise<Pedido[]>;
}

// 2. Write failing test with stub
it('should return all orders', async () => {
  repositoryStub.findAll.resolves(mockPedidos);
  const result = await useCase.execute();
  expect(result).toEqual(mockPedidos);
});

// 3. Implement use case (GREEN)
export class ListarPedidosUseCase {
  constructor(private readonly repo: IPedidosRepository) {}
  async execute(): Promise<Pedido[]> {
    return this.repo.findAll();
  }
}

// 4. Implement repository (concrete)
export class PedidosPrismaRepository implements IPedidosRepository {
  constructor(private prisma: PrismaService) {}
  async findAll(): Promise<Pedido[]> { /* ... */ }
}

Result: Decoupled design, testable with stubs, swappable implementations.

Integration Points

• With nestjs-architect: Apply TDD methodology to DDD/Clean Architecture patterns
• With backend-nestjs agent: Agent executes this skill when TDD mode activated
• With executar-tarefa workflow: Modifies Sections 2 (modeling), 4 (implementation), 5 (validation)
• With architecture guidelines: References testing, observability

References

• Architectural patterns: See nestjs-architect
• Testing patterns: See testing section
• Books: "Test Driven Development: By Example" (Kent Beck), "Clean Architecture" (Robert C. Martin)