---
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.

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

## When to Use

**Always:**

- New features
- Bug fixes
- Refactoring
- Behavior changes

**Exceptions (ask your human partner):**

- Throwaway prototypes
- Generated code
- Configuration files

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

## 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
- Don't look at it
- Delete means delete

Implement fresh from tests. Period.

## Red-Green-Refactor

```
RED: Write failing test → Verify it fails correctly → GREEN: Minimal code → Verify all green → REFACTOR: Clean up → Repeat
```

### RED — Write Failing Test

Write one minimal test showing what should happen.

**Good:**

```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);
});
```

Clear name, tests real behavior, one thing.

**Bad:**

```typescript
test('retry works', async () => {
  const mock = jest
    .fn()
    .mockRejectedValueOnce(new Error())
    .mockRejectedValueOnce(new Error())
    .mockResolvedValueOnce('success');
  await retryOperation(mock);
  expect(mock).toHaveBeenCalledTimes(3);
});
```

Vague name, tests mock not code.

**Requirements:**

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

### Verify RED — Watch It Fail

**MANDATORY. Never skip.**

Confirm:

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

**Test passes?** You're testing existing behavior. Fix test.
**Test errors?** Fix error, re-run until it fails correctly.

### GREEN — Minimal Code

Write simplest code to pass the test.

**Good:**

```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');
}
```

Just enough to pass.

**Bad:** Over-engineered with options, backoff strategies, YAGNI features.

Don't add features, refactor other code, or "improve" 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.

### Repeat

Next failing test for next feature.

## Good Tests

| Quality          | Good                                | Bad                                                 |
| ---------------- | ----------------------------------- | --------------------------------------------------- |
| **Minimal**      | One thing. "and" in name? Split it. | `test('validates email and domain and whitespace')` |
| **Clear**        | Name describes behavior             | `test('test1')`                                     |
| **Shows intent** | Demonstrates desired API            | Obscures what code should do                        |

## Why Order Matters

**"I'll write tests after to verify it works"**
Tests written after code pass immediately. Passing immediately proves nothing — might test wrong thing, might test implementation not behavior, might miss edge cases you forgot, you never saw it catch the bug.

**"I already manually tested all the edge cases"**
Manual testing is ad-hoc. No record, can't re-run, easy to forget cases under pressure. Automated tests are systematic — they run the same way every time.

**"Deleting X hours of work is wasteful"**
Sunk cost fallacy. The time is already gone. Your choice: delete and rewrite with TDD (X more hours, high confidence) OR keep it and add tests after (30 min, low confidence, likely bugs).

**"TDD is dogmatic, being pragmatic means adapting"**
TDD IS pragmatic: finds bugs before commit, prevents regressions, documents behavior, enables refactoring. "Pragmatic" shortcuts = debugging in production = slower.

**"Tests after achieve the same goals — it's spirit not ritual"**
No. Tests-after answer "What does this do?" Tests-first answer "What should this do?" Tests-after are biased by your implementation. Tests-first force edge case discovery before implementing.

## Common Rationalizations

| Excuse                                 | Reality                                                                 |
| -------------------------------------- | ----------------------------------------------------------------------- |
| "Too simple to test"                   | Simple code breaks. Test takes 30 seconds.                              |
| "I'll test after"                      | Tests passing immediately prove nothing.                                |
| "Tests after achieve same goals"       | Tests-after = "what does this do?" Tests-first = "what should this do?" |
| "Already manually tested"              | Ad-hoc ≠ systematic. No record, can't re-run.                           |
| "Deleting X hours is wasteful"         | Sunk cost fallacy. Keeping unverified code is technical debt.           |
| "Keep as reference, write tests first" | You'll adapt it. That's testing after. Delete means delete.             |
| "Need to explore first"                | Fine. Throw away exploration, start with TDD.                           |
| "Test hard = design unclear"           | Listen to test. Hard to test = hard to use.                             |
| "TDD will slow me down"                | TDD faster than debugging. Pragmatic = test-first.                      |
| "Manual test faster"                   | Manual doesn't prove edge cases. You'll re-test every change.           |
| "Existing code has no tests"           | You're improving it. Add tests for existing code.                       |

## Red Flags — STOP and Start Over

- Code before test
- Test after implementation
- Test passes immediately
- Can't explain why test failed
- Tests added "later"
- Rationalizing "just this once"
- "I already manually tested it"
- "Tests after achieve the same purpose"
- "It's about spirit not ritual"
- "Keep as reference" or "adapt existing code"
- "Already spent X hours, deleting is wasteful"
- "TDD is dogmatic, I'm being pragmatic"
- "This is different because..."

**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:** `FAIL: expected 'Email required', got undefined`

**GREEN**

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

**Verify GREEN:** `PASS`

**REFACTOR:** Extract validation for multiple fields if needed.

## 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 (feature missing, not typo)
- [ ] Wrote minimal code to pass each test
- [ ] All tests pass
- [ ] Output pristine (no errors, warnings)
- [ ] Tests use real code (mocks only if unavoidable)
- [ ] Edge cases and errors covered

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

## When Stuck

| Problem                | Solution                                                             |
| ---------------------- | -------------------------------------------------------------------- |
| Don't know how to test | Write wished-for API. Write assertion first. Ask your human partner. |
| Test too complicated   | Design too complicated. Simplify interface.                          |
| Must mock everything   | Code too coupled. Use dependency injection.                          |
| Test setup huge        | Extract helpers. Still complex? Simplify design.                     |

## 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.
