Test-Driven Development

Purpose

Use tests to drive design. TDD's value is not primarily the tests it produces — it is the pressure it puts on the design, because code that is hard to test is hard to use.

When to Use

• Implementing logic with clear inputs and outputs.
• Fixing a bug (the failing test comes first, always).
• Working in a codebase where you do not fully understand the existing behavior.
• Designing an API you will have to live with.

Capabilities

• The red-green-refactor cycle.
• Test selection: choosing the next test that moves the design forward.
• Outside-in (from the API) and inside-out (from the core) approaches.
• Using tests to characterize existing behavior before changing it.