Test-Driven Development

Write the test first, then write the smallest change that makes it pass, then refactor safely.

Core loop

1. Red: write a failing test that expresses the desired behavior.
2. Green: implement the minimum change to pass.
3. Refactor: improve structure while keeping tests green.

Quick Start

• Start from the public interface (API, function, UI behavior), not private helpers.
• Prefer “behavioral” test names (what), not “implementation” names (how).
• When fixing bugs: reproduce with a test first.

Guardrails

• Don’t over-mock: prefer integration at boundaries; mock only slow/flaky externals.
• Keep tests deterministic: control time, randomness, and network.
• If a test is hard to write, your design likely needs an interface seam.