# test-driven-development

> When implementing any feature or bugfix - write the test first, watch it fail, write minimal code to pass; if you didn't watch the test fail, you don't know if it tests the right thing

- Author: jglaspey
- Repository: jglaspey/GiveJasonFeedbackWidget
- Version: 20251219101309
- Stars: 0
- Forks: 0
- Last Updated: 2026-02-07
- Source: https://github.com/jglaspey/GiveJasonFeedbackWidget
- Web: https://mule.run/skillshub/@@jglaspey/GiveJasonFeedbackWidget~test-driven-development:20251219101309

---

---
name: test-driven-development
description: When implementing any feature or bugfix - write the test first, watch it fail, write minimal code to pass; if you didn't watch the test fail, you don't know if it tests the right thing
---

# 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.

**Violating the letter of the rules is violating the spirit of the rules.**

## 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

## When to Use

**Always:**
- New features
- Bug fixes
- Refactoring
- Behavior changes

**Exceptions (ask your partner):**
- Throwaway prototypes
- Generated code
- Configuration files

Thinking "skip TDD just this once"? Stop. That's rationalization.

## Red-Green-Refactor

### RED - Write Failing Test

Write one minimal test showing what should happen.

```typescript
test('retries failed operations 3 times', async () => {
  let attempts = 0;
  const operation = () => {
    attempts++;
    if (attempts < 3) throw new Error('fail');
    return 'success';
  };

  const result = await retryOperation(operation);

  expect(result).toBe('success');
  expect(attempts).toBe(3);
});
```

**Requirements:**
- One behavior per test
- Clear name describing the behavior
- 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)

**Test passes?** You're testing existing behavior. Fix test.

### GREEN - Minimal Code

Write simplest code to pass the test. Don't add features, refactor other code, or "improve" beyond the test.

```typescript
async function retryOperation<T>(fn: () => Promise<T>): Promise<T> {
  for (let i = 0; i < 3; i++) {
    try {
      return await fn();
    } catch (e) {
      if (i === 2) throw e;
    }
  }
  throw new Error('unreachable');
}
```

### Verify GREEN - Watch It Pass

**MANDATORY.**

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

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.

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests passing immediately prove nothing. |
| "Already manually tested" | Ad-hoc ≠ systematic. No record, can't re-run. |
| "TDD will slow me down" | TDD faster than debugging. |
| "Deleting X hours is wasteful" | Sunk cost fallacy. Keeping unverified code is technical debt. |

## Red Flags - STOP and Start Over

- Code before test
- Test passes immediately
- Tests added "later"
- "Just this once"
- "Already manually tested it"
- "Keep as reference"

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

## Example: Bug Fix

**Bug:** Empty email accepted

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

**Verify RED** - see it fail

**GREEN**
```typescript
function submitForm(data: FormData) {
  if (!data.email?.trim()) {
    return { error: 'Email required' };
  }
  // ...
}
```

**Verify GREEN** - all tests pass

## Verification Checklist

Before marking work complete:

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

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

## The Bottom Line

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

No exceptions without your partner's permission.