superpowers-test-driven-development

name	superpowers-test-driven-development
description	Writing or fixing any code? Load first — write the failing test before the code.

Test-Driven Development (TDD)

Overview

Test first. Watch it fail. Write minimal code to pass.

Core principle: Didn't watch the test fail? You don't know it tests the right thing.

Break the letter of the rules = break the spirit.

When to Use

Always: new features, bug fixes, refactors, behavior changes.

Exceptions (ask your human partner): throwaway prototypes, generated code, config files.

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

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Code before the test? Delete it. Start over. Don't keep it as "reference", don't "adapt" it, don't look at it. Delete means delete. Implement fresh from tests.

Red-Green-Refactor

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

One minimal test. Shows 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); });

Clear name, 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

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

Verify RED — watch it fail

MANDATORY. Never skip. Run via bash:

npm test path/to/test.test.ts

Confirm: fails (not errors), failure message is the expected one, fails because feature missing (not a typo).

Passes? Testing existing behavior — fix the test. Errors? Fix, rerun till it fails right.

GREEN — minimal code

Simplest code that passes.

```typescript async function retryOperation(fn: () => Promise): Promise { 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 ```typescript async function retryOperation( fn: () => Promise, options?: { maxRetries?: number; backoff?: 'linear' | 'exponential'; onRetry?: (attempt: number) => void; } ): Promise { // YAGNI } ``` Over-engineered

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

Verify GREEN — watch it pass

MANDATORY.

npm test path/to/test.test.ts

Confirm: passes, other tests still pass, output pristine (no errors/warnings).

Fails? Fix code, not test. Other tests fail? Fix now.

REFACTOR — clean up

After green only: kill duplication, improve names, extract helpers. Stay green. Don't add behavior.

Repeat

Next failing test, 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

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. Keeping unverified code is tech 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 the 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 it.

Red Flags — STOP and Start Over

• Code before test
• Test passes immediately, or you can't explain why it failed
• "I'll test after" / "tests after achieve the same purpose"
• "Keep as reference" / "adapt existing code"
• "Already spent X hours, deleting is wasteful"
• "TDD is dogmatic, I'm being pragmatic" / "this is different because…"

All mean: delete code, start over with TDD.

Example: Bug Fix

Bug: empty email accepted.

RED

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

Verify RED

$ npm test
FAIL: expected 'Email required', got undefined

GREEN

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

Verify GREEN

$ npm test
PASS

REFACTOR — extract validation for more fields if needed.

Verification Checklist

• Every new function/method has a test
• Watched each test fail before implementing
• Each test failed for the expected reason (feature missing, not a typo)
• Wrote minimal code to pass
• All tests pass, output pristine
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

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

When Stuck

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

Debugging Integration

Bug? Write a failing test reproducing it. Follow the cycle — the test proves the fix and blocks regression. Never fix bugs without a test. (See superpowers-systematic-debugging.)

Testing Anti-Patterns

Adding mocks/test utils? Consult the Testing Anti-Patterns reference (auto-included below): testing mock not real behavior; test-only methods in prod classes; mocking without understanding deps.

Final Rule

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

No exceptions without your human partner's permission.