test-driven-development

Test-Driven Development

Write the test first. Watch it fail. Write minimal code to pass.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Any code written before its corresponding test must be deleted entirely - no exceptions for "reference" or "adapting" existing work.

Red-Green-Refactor Cycle

RED: Write Failing Test

1. Write ONE minimal failing test demonstrating desired behavior
2. Use clear naming that describes the behavior
3. Test real code, not mocks (mock only external dependencies)
4. Focus on ONE behavior per test

Verify RED (CRITICAL)

Run tests and confirm:

• Test fails (not errors)
• Failure indicates missing feature
• Error message matches expectations

If test passes immediately → DELETE IT (it proves nothing)

GREEN: Make It Pass

1. Implement the simplest code that passes the test
2. Avoid over-engineering
3. Don't add features not requested
4. Keep scope narrow

Verify GREEN

• All tests pass
• No other tests broke

REFACTOR: Clean Up

1. Clean up while ALL tests remain green
2. Remove duplication (DRY)
3. Improve naming
4. Extract helpers if needed

Why Tests-First Matters

Tests written after code pass immediately, proving nothing about their validity. Tests-first:

• Force discovery of edge cases before implementation
• Provide systematic verification impossible with manual testing
• Document expected behavior

Common Rationalizations (ALL REJECTED)

| Excuse | Why It's Wrong | | ------------------------------------ | ---------------------------------------------- | | "Too simple to test" | Simple code still needs verification | | "I'll test after" | Tests-after pass immediately, proving nothing | | "Already manually tested" | Manual testing isn't repeatable or documented | | "Deleting hours of work is wasteful" | Sunk cost fallacy - untested code is liability |

When to Use TDD

Always: New features, bug fixes, refactoring, behavior changes

Exceptions only: Throwaway prototypes, generated code, config files (with permission)

Red Flags Requiring Restart

• Code written before test
• Test passes immediately
• Can't explain why test should fail
• Rationalizing "just this once"

→ DELETE the code and recommence with failing test

When 3+ Test Fixes Fail

STOP. This signals an architectural problem:

1. Return to investigation phase
2. Question whether the underlying pattern is sound
3. Discuss with user before attempting more fixes