Test-Driven Design (tdd)

The distinction that matters

Test-first is about ordering. Test-driven is about the design being driven and defended by tests. Writing the test first is a means; it is not the goal. The goal is four properties of what you produce:

1. A suite so strong a lazy hack cannot pass it. (defensible)
2. Every edge and malformed input explicitly decided, never accidental. (decided)
3. A design that changed when the tests told you it was wrong. (design feedback)
4. A suite that reads as the specification of the design. (spec)

If you wrote tests first but produced none of these, you did test-first, not test-driven. This skill exists to produce these results in a single pass, by attacking your own work with things you actually run — because reasoning about whether your tests are good is unreliable, but running a cheat against them is not.

Stance: classicist