Test Driven Development

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.

Red/Green/Refactor checklist

• Red: test fails for the right reason (not setup errors).
• Green: smallest change to pass, no extra behavior.
• Refactor: improve structure without changing behavior or tests.

References

• Common anti-patterns: references/testing-anti-patterns.md