Test-Driven Development (TDD)

Overview

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

Core principle: If you didn't watch the test fail, you don't know if it tests the right thing.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (ask your human partner):

• Throwaway prototypes
• Generated code
• Configuration files

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Write code before the test? Delete it. Start over.

No exceptions:

• Don't keep it as "reference"
• Don't "adapt" it while writing tests
• Delete means delete

Red-Green-Refactor Cycle

┌─────────────────────────────────────────────────────────────┐
│                                                             │
│   ┌───────┐      ┌───────┐      ┌──────────┐               │
│   │  RED  │ ───► │ GREEN │ ───► │ REFACTOR │ ───► (repeat) │
│   └───────┘      └───────┘      └──────────┘               │
│                                                             │
│   Write         Write          Clean up                    │
│   failing       minimal        (stay green)                │
│   test          code                                       │
│                                                             │
└─────────────────────────────────────────────────────────────┘

RED - Write Failing Test

Write one minimal test showing what should happen.

• Test describes behavior, not implementation
• Test should fail for the right reason (feature missing, not typo)
• Run the test and confirm it fails

GREEN - Write Minimal Code

Write the minimum code to make the test pass.

• Don't add features the test doesn't require
• Don't optimize
• Just make it pass

REFACTOR - Clean Up

Improve code quality while keeping tests green.

• Remove duplication
• Improve naming
• Run tests after each change

Pre-Completion Checklist

Before marking work complete:

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for expected reason (feature missing, not typo)
• Wrote minimal code to pass each test
• All tests pass
• Output pristine (no errors, warnings)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

When Stuck

Debugging Integration

Bug found? Write failing test reproducing it. Follow TDD cycle. Test proves fix and prevents regression.

Never fix bugs without a test.

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.