Test-Driven Development (TDD)

The Fundamental Rule

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST.

This is non-negotiable. Code written before tests must be deleted and rewritten with tests first.

Why TDD Matters

Testing after implementation provides false confidence:

• Tests pass immediately (proves nothing)
• No verification that tests catch regressions
• Untested edge cases slip through

TDD ensures every line of production code is proven necessary by a failing test.

The RED-GREEN-REFACTOR Cycle