Nestjs Testing

NestJS Testing

Best practices for testing NestJS applications, following the official testing docs (https://docs.nestjs.com/fundamentals/testing). Covers isolated unit tests built with Test.createTestingModule and full-stack e2e tests driven through supertest. Each rule pairs an antipattern with the idiomatic Jest pattern.

When to Apply

Reference these guidelines when:

• Writing unit tests for a controller or service with Test.createTestingModule
• Mocking a unit's dependencies (repositories, HTTP clients, other services)
• Writing e2e tests that hit the real HTTP server via supertest
• Bypassing guards/auth or swapping providers in e2e
• Managing test data, fixtures, and teardown between tests
• Reviewing or refactoring an existing NestJS test suite

Rules

• test-createtestingmodule (CRITICAL) — build the unit under test with Test.createTestingModule(...).compile()
• mock-dependencies (CRITICAL) — provide mock providers (useValue) for every dependency of the unit
• unit-test-services (HIGH) — test service logic against mocked repos/clients, never a real DB
• e2e-with-supertest (HIGH) — exercise the whole app over HTTP with createNestApplication + supertest
• override-guards-in-e2e (HIGH) — bypass auth in e2e with overrideGuard / overrideProvider
• isolated-test-data (HIGH) — fresh fixtures per test; reset mocks and tear down the app/DB between tests
• test-behavior-not-implementation (MEDIUM) — assert returned values and HTTP contracts, not internal call shapes

How to Use

Read the individual rule file for the explanation and before/after example:

rules/test-createtestingmodule.md
rules/mock-dependencies.md
rules/unit-test-services.md
rules/e2e-with-supertest.md
rules/override-guards-in-e2e.md
rules/isolated-test-data.md
rules/test-behavior-not-implementation.md

Each rule file contains:

• A short explanation of why it matters, tied to the official docs
• An Incorrect example (the antipattern)
• A Correct example (the idiomatic NestJS + Jest pattern)

All examples target NestJS v10/v11 with the @nestjs/testing package and supertest.