# test-driven-development

> Use when implementing any feature or bugfix, before writing implementation code

- Author: botarhythm
- Repository: botarhythm/DegiHara-ChatBot
- Version: 20260206143943
- Stars: 0
- Forks: 0
- Last Updated: 2026-02-06
- Source: https://github.com/botarhythm/DegiHara-ChatBot
- Web: https://mule.run/skillshub/@@botarhythm/DegiHara-ChatBot~test-driven-development:20260206143943

---

---
name: test-driven-development
description: Use when implementing any feature or bugfix, before writing implementation code
---

# 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.

## Red-Green-Refactor

### RED - Write Failing Test
Write one minimal test showing what should happen.

```typescript
test('rejects empty email', async () => {
  const result = await submitForm({ email: '' });
  expect(result.error).toBe('Email required');
});
```

**Requirements:**
- One behavior
- Clear name
- Real code (no mocks unless unavoidable)

### Verify RED - Watch It Fail
**MANDATORY. Never skip.**

```bash
npm test path/to/test.test.ts
```

Confirm:
- Test fails (not errors)
- Failure message is expected
- Fails because feature missing (not typos)

### GREEN - Minimal Code
Write simplest code to pass the test. Don't add features beyond the test.

### Verify GREEN - Watch It Pass
**MANDATORY.**

Confirm:
- Test passes
- Other tests still pass
- Output pristine (no errors, warnings)

### REFACTOR - Clean Up
After green only:
- Remove duplication
- Improve names
- Extract helpers

Keep tests green. Don't add behavior.

## Verification Checklist
Before marking work complete:

- [ ] Every new function/method has a test
- [ ] Watched each test fail before implementing
- [ ] Each test failed for expected reason
- [ ] Wrote minimal code to pass each test
- [ ] All tests pass
- [ ] Output pristine (no errors, warnings)

## Common Rationalizations
| Excuse | Reality |
|--------|---------|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests passing immediately prove nothing. |
| "Need to explore first" | Fine. Throw away exploration, start with TDD. |
| "TDD will slow me down" | TDD faster than debugging. |

## Red Flags - STOP and Start Over
- Code before test
- Test after implementation
- Test passes immediately
- Can't explain why test failed
- "Just this once"

**All of these mean: Delete code. Start over with TDD.**