TDD

Test-driven development inverts the conventional code-then-test workflow. The developer first writes a small test that defines expected behavior, watches it fail (red), writes the minimum code to make it pass (green), then restructures the code for clarity without changing behavior (refactor). This red-green-refactor cycle typically lasts minutes, producing code that is covered by tests from the very first line.

The practice was popularized by Kent Beck in his 2002 book Test-Driven Development: By Example. Beyond catching bugs early, TDD shapes design: writing testable code naturally pushes toward smaller functions, explicit dependencies, and clearer interfaces. Tests also serve as living documentation of intended behavior.

TDD applies across testing levels — unit tests, integration tests, and acceptance tests — though the cycle is fastest and most common at the unit level. Frameworks like Specs2 for Scala and Gatling for performance testing bring the same test-first mindset to specialized domains.