What is backend testing?
Backend testing proves your service behaves correctly without manually poking it every time. Unit tests check single functions, integration tests check the service against a real database, and contract tests check that an API still matches what its consumers expect.
Why it matters
Tests are what let you change code without fear. A backend without them gets slower to work on over time, because every change risks breaking something unseen. Teams expect tests, and the ability to write good ones — fast, deterministic, meaningful — is a core professional skill.
What to learn
- The testing pyramid: many unit, fewer integration, few end-to-end
- A test runner like Vitest or Jest
- Testing API routes against a real test database
- Test data setup and teardown, and isolation between tests
- Mocking external services, and when not to
- Contract tests so an API change does not silently break clients
- Running tests in CI on every push
Common pitfall
Mocking the database in every test. Mocks that simulate your database test your assumptions, not reality — they pass while the real query is wrong. Use a real (disposable) database for integration tests so you catch the schema and query bugs that mocks hide.
Resources
Primary (free):
- Vitest — Documentation · docs
- Testing Library — Guiding principles · docs
- Martin Fowler — Test pyramid · article
Practice
Write tests for one API endpoint at two levels: a unit test for its validation logic, and an integration test that runs the route against a real test database and checks the stored result. Wire both into CI. Done when a broken query fails the integration test, not just the unit test.
Outcomes
- Apply the testing pyramid to a service.
- Write integration tests against a real test database.
- Mock external services without mocking your own database.
- Add a contract test that catches breaking API changes.