NestJS Testing Skill

Use this skill when the user asks to add, fix, review, or improve tests for a NestJS codebase.

This skill mirrors the global test_agent role and always classifies tests into:

• •unit
• •integration
• •e2e

Delegation Rule

• •Default behavior: delegate implementation and test authoring work to the global test_agent.
• •Treat test_agent as the execution engine for test tasks, and keep this skill focused on NestJS-specific constraints and quality bar.
• •Only skip delegation when the user explicitly asks to avoid sub-agents.

Scope and Defaults

• •Target framework: NestJS 11 + Jest + @nestjs/testing + Supertest.
• •In this repo, backend tests live under backend/.
• •Default behavior: add or update tests first, avoid changing production code unless the user explicitly requests implementation changes.

Repo-Specific Commands

Run commands from backend/:

pnpm test
pnpm test -- src/path/to/file.spec.ts
pnpm test:watch
pnpm test:cov
pnpm test:e2e
pnpm test:e2e -- test/path/to/file.e2e-spec.ts

Test Type Classifier

Choose the smallest test type that can prove the behavior:

1. •Unit test

• •Single class/function behavior.
• •Mock collaborators (useValue, useFactory, jest.fn()).
• •No network/DB side effects.

1. •Integration test

• •Multiple providers/modules wired with Nest DI.
• •Real module composition with selected fakes at boundaries.
• •Useful for service + repository + mapper flows.

1. •E2E test

• •Boot INestApplication and hit HTTP endpoints with Supertest.
• •Validate status, payload shape, and core auth/validation behavior.

Workflow

1. •Identify behavior under test

• •Convert request into explicit assertions (input, output, side effects, errors).

1. •Map boundaries

• •Service tests: mock repositories/external APIs.
• •Controller tests: mock service layer.
• •E2E tests: use app module and keep assertions user-facing.

1. •Build deterministic fixture setup

• •Use Test.createTestingModule(...).
• •Keep one focused expectation per test case when possible.
• •Use stable data builders to avoid duplicated literals.

1. •Implement and run targeted tests first

• •Run only changed spec files, then broader suite.

1. •Harden coverage for failure paths

• •Add negative cases: invalid input, not found, unauthorized, dependency error.

NestJS Patterns

Unit: Service

• •Build module with service + mocked providers.
• •Assert calls to dependencies and returned values.
• •Prefer explicit token-based provider mocks for repos/adapters.

Unit: Controller

• •Instantiate controller via testing module.
• •Mock service contract only.
• •Verify request DTO mapping and response mapping expectations.

Integration

• •Import module under test when practical.
• •Replace only external boundaries (DB, Redis, HTTP clients, queues).
• •Assert behavior through public methods/events, not private internals.

E2E

• •Create app from AppModule (or feature test module).
• •await app.init() in setup and await app.close() in teardown.
• •Use Supertest for endpoint behavior and contract assertions.

Quality Bar

• •Tests are deterministic and order-independent.
• •No hidden shared state across test cases.
• •Mocks are minimal and reflect real contracts.
• •Assertions verify behavior, not implementation details.
• •Include success path + at least one failure path for changed logic.

Review Mode (when user asks for review)

Prioritize findings in this order:

1. •Behavioral gaps (missing assertions / false positives)
2. •Incorrect boundaries (over-mocking or under-mocking)
3. •Flaky patterns (timers, async race, global state)
4. •Maintainability (duplication, unreadable fixtures)

Report findings with file and line references, then recommend minimal fixes.