# Test Driven Development

> Úsalo al implementar cualquier feature o corrección de bug, antes de escribir código de implementación.

Fuente: https://skillsagentes.com/skills/obra/superpowers/test-driven-development
Markdown: https://skillsagentes.com/skills/obra/superpowers/test-driven-development.md

Repositorio: https://github.com/obra/superpowers
Autor: obra
Licencia: MIT
Actualizado: hace 2 meses
Coste de contexto: 20 tok instalada, 2.3k tok al activarse, 4.3k tok con todos los archivos del bundle
Bundle: 2 archivos, 17 KB
Permisos que pide: ninguno declarado

## Instalación

Un skill son archivos markdown: los mismos archivos valen para cualquier agente y lo único que cambia es el directorio de destino, es decir la bandera `--agent`. Añade `-g` para instalarlo en todos los proyectos de la máquina.

```bash
# Claude Code
npx -y skills add obra/superpowers --skill test-driven-development --agent claude-code
# Cursor
npx -y skills add obra/superpowers --skill test-driven-development --agent cursor
# Codex
npx -y skills add obra/superpowers --skill test-driven-development --agent codex
# Gemini CLI
npx -y skills add obra/superpowers --skill test-driven-development --agent gemini
# Windsurf
npx -y skills add obra/superpowers --skill test-driven-development --agent windsurf
# Cline
npx -y skills add obra/superpowers --skill test-driven-development --agent cline
```

## Qué hace

- Enforce el ciclo Red-Green-Refactor: escribir un test que falle, verificar el fallo, escribir código mínimo para pasarlo, luego refactorizar
- Obliga a borrar cualquier código de producción escrito antes del test correspondiente, sin conservarlo como 'referencia'
- Bloquea marcar el trabajo como completo si no se cumplió la checklist de verificación de TDD

## Cuándo usarla

- Al implementar cualquier feature nueva
- Al corregir un bug
- Al refactorizar código
- Al cambiar el comportamiento de algo

## Cuándo no

- Prototipos descartables (previa consulta con el humano)
- Código generado (previa consulta con el humano)
- Archivos de configuración (previa consulta con el humano)

## Qué la activa

- "Implementa la función de reintentos con TDD"
- "Arregla este bug siguiendo TDD, escribe el test primero"
- "Añade validación de email a este formulario con test-first"

## Antes de instalar

- Necesita en el PATH: npm

## Archivos

- SKILL.md — 9 KB
- writing-good-tests.md — 8 KB

## SKILL.md

Reproducido tal cual desde obra/superpowers bajo MIT. Esta sección es el documento original y está en inglés.

# 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

```dot
digraph tdd_cycle {
    rankdir=LR;
    red [label="RED\nWrite failing test", shape=box, style=filled, fillcolor="#ffcccc"];
    verify_red [label="Verify fails\ncorrectly", shape=diamond];
    green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"];
    verify_green [label="Verify passes\nAll green", shape=diamond];
    refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"];
    next [label="Next", shape=ellipse];

    red -> verify_red;
    verify_red -> green [label="yes"];
    verify_red -> red [label="wrong\nfailure"];
    green -> verify_green;
    verify_green -> refactor [label="yes"];
    verify_green -> green [label="no"];
    refactor -> verify_green [label="stay\ngreen"];
    verify_green -> next;
    next -> red;
}
```

### 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
</Good>

<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
</Bad>

**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)

**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
</Good>

<Bad>
```typescript
async function retryOperation<T>(
  fn: () => Promise<T>,
  options?: {
    maxRetries?: number;
    backoff?: 'linear' | 'exponential';
    onRetry?: (attempt: number) => void;
  }
): Promise<T> {
  // YAGNI
}
```
Over-engineered
</Bad>

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

### 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)

**Test fails?** Fix code, not test.

**Other tests fail?** Fix now.

**"Other tests" means the project's suite, not just your file.** A
green run of the test you wrote is not a green suite. Before you call
the change done, run the project's test command (bare `pytest`,
`npm test`, `cargo test` — whatever the repo uses) even when your task
named only one test file. A scope statement in your task bounds the
deliverable, not your verification. Any failure that run shows —
including one you didn't cause — goes in your report by name; a red
test you watched scroll past and didn't mention is a report falsified
by omission.

### 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 |

When writing or changing any test, read [writing-good-tests.md](writing-good-tests.md) for the rules that keep tests honest:
- Name the production change that would make the test fail — before writing it
- Assert on real behavior, never on mock behavior
- Keep test-only code in test utilities, out of production classes
- Understand a dependency's side effects before mocking it

## Common Rationalizations

| Excuse | Reality |
|--------|---------|
| "Too simple to test" | Simple code breaks. Test takes 30 seconds. |
| "I'll test after" | Tests written after pass immediately — which proves nothing. They may test the wrong thing, test the implementation instead of the behavior, or miss the edge case you forgot. You never watched it fail, so you never proved it can catch the bug. Test-first forces that failure. |
| "Tests after achieve same goals (spirit not ritual)" | Tests-after answer "what does this do?"; tests-first answer "what should this do?" Tests written after are biased by the code you already wrote — you verify the cases you remembered, not the ones you'd have discovered. Coverage without proof the tests work. |
| "Already manually tested" | Manual testing is ad-hoc: no record of what you covered, no way to re-run it when the code changes, easy to forget cases under pressure. "Worked when I tried it" ≠ comprehensive. Automated tests run the same way every time. |
| "Deleting X hours is wasteful" | Sunk cost fallacy — that time is already spent either way. The real choice: rewrite with TDD (high confidence) vs. keep it and bolt tests on after (low confidence, likely bugs). Keeping code you can't trust is the waste. |
| "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 IS the pragmatic path: catches bugs before commit, prevents regressions, lets you refactor without fear. "Pragmatic" shortcuts mean debugging in production — slower, not faster. |
| "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**
```bash
$ npm test
FAIL: expected 'Email required', got undefined
```

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

**Verify GREEN**
```bash
$ npm test
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.

## Dónde encaja

- Categoría: [Testing y QA](https://skillsagentes.com/categorias/testing-qa.md) — Flujos de testing unitario, de integración y end-to-end.
- Creador: [obra](https://skillsagentes.com/creators/obra.md) — 14 skills en el directorio
- [Todas las skills](https://skillsagentes.com/skills.md)
- [Ranking de instalaciones](https://skillsagentes.com/ranking.md)

## Otras skills del mismo repositorio

- [Subagent Driven Development](https://skillsagentes.com/skills/obra/superpowers/subagent-driven-development.md): Úsalo al ejecutar planes de implementación con tareas independientes dentro de la sesión actual.
- [Brainstorming](https://skillsagentes.com/skills/obra/superpowers/brainstorming.md): Debes usarlo antes de cualquier trabajo creativo: crear features, construir componentes, añadir funcionalidad o modificar comportamiento. Explora la intención, requisitos y diseño antes de implementar.
- [Using Superpowers](https://skillsagentes.com/skills/obra/superpowers/using-superpowers.md): Úsala al iniciar cualquier conversación: establece cómo encontrar y usar skills, exigiendo su invocación antes de CUALQUIER respuesta, incluso preguntas aclaratorias.
- [Writing Skills](https://skillsagentes.com/skills/obra/superpowers/writing-skills.md): Úsala al crear nuevas skills, editar skills existentes o verificar que funcionan antes de desplegarlas.
- [Finishing A Development Branch](https://skillsagentes.com/skills/obra/superpowers/finishing-a-development-branch.md): Úsalo cuando la implementación esté completa, todos los tests pasen, y necesites decidir cómo integrar el trabajo.

## Skills relacionadas

- [Receiving Code Review](https://skillsagentes.com/skills/obra/superpowers/receiving-code-review.md): Úsalo al recibir feedback de code review, antes de implementar sugerencias, sobre todo si el feedback parece poco claro o técnicamente cuestionable: exige rigor técnico y verificación, no acuerdo performativo ni implementación ciega.
- [Requesting Code Review](https://skillsagentes.com/skills/obra/superpowers/requesting-code-review.md): Úsalo al completar tareas, implementar features mayores, o antes de mergear, para verificar que el trabajo cumple los requisitos.
- [Systematic Debugging](https://skillsagentes.com/skills/obra/superpowers/systematic-debugging.md): Úsalo ante cualquier bug, fallo de test o comportamiento inesperado, antes de proponer arreglos.
- [Verification Before Completion](https://skillsagentes.com/skills/obra/superpowers/verification-before-completion.md): Úsalo antes de afirmar que un trabajo está completo, corregido o pasando, antes de hacer commit o crear PRs: exige ejecutar comandos de verificación y confirmar la salida antes de cualquier afirmación de éxito.
- [Brainstorming](https://skillsagentes.com/skills/obra/superpowers/brainstorming.md): Debes usarlo antes de cualquier trabajo creativo: crear features, construir componentes, añadir funcionalidad o modificar comportamiento. Explora la intención, requisitos y diseño antes de implementar.

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