Integration vs Unit Test Balance: The Testing Trophy

Teams either write only unit tests (fast but miss integration issues) or only E2E tests (slow and brittle). Neither extreme provides the best coverage-to-maintenance ratio.

Follow the Testing Trophy model: prioritize integration tests backed by unit and static analysis.

Testing Trophy (ordered by value for most web apps):

1. Static analysis (TypeScript, ESLint) — free type safety
2. Unit tests — isolated complex logic, algorithms, transformations
3. Integration tests — feature workflows, HTTP endpoints, DB operations (MOST)
4. E2E tests — critical user paths only (login, checkout, payment)

// Integration test example (most valuable tier)
test('POST /users creates a user and sends welcome email', async () => {
  const response = await request(app)
    .post('/users')
    .send({ name: 'Alice', email: 'alice@example.com' });

  expect(response.status).toBe(201);
  expect(response.body.id).toBeDefined();
  expect(emailService.send).toHaveBeenCalledWith(
    expect.objectContaining({ to: 'alice@example.com' })
  );

  const savedUser = await db.getUser(response.body.id);
  expect(savedUser.name).toBe('Alice');
});

Unit tests are fast but test code in isolation — they miss integration failures. E2E tests cover everything but are slow and fragile. Integration tests provide the best value: realistic scenarios at reasonable speed.