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Unit Testing

What it is

A unit test exercises one unit — usually a function or a class — in isolation, with every collaborator replaced by a stand-in the test controls. It is the only one of the three testing skills that runs entirely inside your process: no database, no network, no browser. That is the boundary. If a test crosses a real boundary, it is an integration test; if it drives the whole system the way a user would, it is an end-to-end test.

Why it matters

Unit tests are the fastest feedback a developer gets, and speed is what makes them worth writing. A suite that runs in seconds is run on every save, so a mistake is found in the minute it was made rather than in a deploy. Because nothing external is involved, a failure points at one unit rather than at a system, which turns debugging into reading. This is also the safety net that makes refactoring possible: a behaviour-preserving change can only be verified quickly when the behaviour is pinned by tests that do not need infrastructure.

In this track

  1. What makes a good unit test (01-cs-fundamentals/10-unit-testing/01-what-makes-a-good-unit-test) — behaviour over implementation, one reason to fail, and a name that states the expectation.
  2. Test doubles and test data (01-cs-fundamentals/10-unit-testing/02-test-doubles-and-test-data) — stubs, fakes and spies, and building input data that keeps tests readable.

Prerequisites

None.

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