Testing — three tiers, not one

Three test tiers — unit, integration, e2e — so CI stays fast and a red test names the broken layer.

Nx·Testing & Contracts·Medium·Since 2026-05-01

nx-testing-three-tiers

Why it matters

Failure modes if this rule is ignored

Stake	If ignored
Slow CI	If everything is e2e, CI takes 40 minutes and nx affected can't trim the test graph.
Unclear failure	One red e2e — is it logic, wiring, or API? Hours split across layers before anyone knows.
API drift	Backend changes a response field — no contract or integration test fails until a browser run, if that path is covered at all.

How to fix

NestJS invites writing everything as e2e with Test.createTestingModule. That's a mistake. Three tiers:

• Unit — service or utility, no NestJS DI, mocks by hand. Fast, focused.
• Integration — module wired up with Test.createTestingModule, real DB (testcontainers), no HTTP. Verifies wiring.
• E2E — full app with supertest. Verifies public contracts.

Examples

Bad

describe('Everything (e2e)', () => {
  it('creates a site end-to-end', async () => {
    await request(app.getHttpServer())
      .post('/sites')
      .send({ name: 'Test', slug: 'test' })
      .expect(201);
  });
});

Good

// 1. Unit — fast, no NestJS
describe('SitesService', () => {
  it('rejects duplicate slugs', async () => {
    const repo: SitesRepository = { findBySlug: async () => ({ id: '1' } as Site), /* ... */ };
    const service = new SitesService(repo);
    await expect(service.create({ slug: 'taken' })).rejects.toThrow(DuplicateSlugError);
  });
});

// 2. Integration — module wired, real DB
describe('SitesModule (integration)', () => {
  let app: TestingModule;
  beforeAll(async () => {
    app = await Test.createTestingModule({
      imports: [SitesModule, TestDbModule],
    }).compile();
  });
  // verifies wiring + SQL
});

// 3. E2E — full app
describe('Sites API (e2e)', () => {
  it('POST /sites creates a site', async () => {
    await request(app.getHttpServer())
      .post('/sites')
      .send({ name: 'Test', slug: 'test' })
      .expect(201);
  });
});

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