Question 1

What is TestingModule in Nest?

Accepted Answer

A DI sandbox (@nestjs/testing) to create modules with overrides for unit/e2e-style provider tests. overrideProvider/useValue mocks dependencies. compile() builds the Nest context without listening HTTP unless e2e.

Question 2

Unit test a service with mocked repo — pattern?

Accepted Answer

TestingModule with service + mock repo provider token; assert interactions and results. Don't boot the whole app for unit tests. Keep tests fast and focused.

Question 3

e2e testing Nest HTTP apps — approach?

Accepted Answer

supertest against a full AppModule (or test module), real or testcontainers DB, and cleanup between tests. Prefer deterministic seeds. Test auth flows. Separate from unit suite in CI timing.

Question 4

How do you test guards/pipes/interceptors in isolation?

Accepted Answer

Unit-test canActivate/transform/intercept with mocked ExecutionContext/CallHandler. Also cover via e2e for integration confidence. Mock Reflector metadata carefully.

Question 5

Scenario: flaky e2e due to module singletons holding state.

Accepted Answer

Reset modules between tests, avoid shared mutable singletons, or use request-scoped carefully. Close app after suite. Isolate DB. Watch open handles (Jest).

Question 6

Lead: testing strategy pyramid for a large Nest codebase?

Accepted Answer

Many unit tests of domain/services, contract tests for providers, fewer e2e; CI gates on critical journeys. Share test utilities. Ban excessive e2e-only coverage. Measure flake rate.

Testing NestJS Interview Questions

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