Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Implement just enough to pass. Refactor while staying green. If you didn’t see the test fail, you don’t know whether it tests the right behavior.

Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Wrote code before the test? Delete it. Start from tests.

When to Use

• Every feature, bug fix, refactor, or behavior change.
• Exceptions only with explicit human approval (throwaway prototypes, generated code, etc.).