---
name: test-driven-development
description: "Internal skill. Use cc10x-router for all development tasks."
allowed-tools: Read, Grep, Glob, Bash, Write, Edit
---

# 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.

## Test Process Discipline (CRITICAL)

**Problem:** Test runners (Vitest, Jest) default to watch mode, leaving processes hanging indefinitely.

**Mandatory Rules:**
1. **Always use run mode** — Never invoke watch mode:
   - Vitest: `npx vitest run` (NOT `npx vitest`)
   - Jest: `CI=true npx jest` or `npx jest --watchAll=false`
   - npm scripts: `CI=true npm test` or `npm test -- --run`
2. **Prefer CI=true prefix** for all test commands: `CI=true npm test`
3. **After TDD cycle complete**, verify no orphaned processes:
   `pgrep -f "vitest|jest" || echo "Clean"`
4. **Kill if found**: `pkill -f "vitest" 2>/dev/null || true`

## Red-Green-Refactor

```
    ┌─────────┐       ┌─────────┐       ┌───────────┐
    │   RED   │──────>│  GREEN  │──────>│ REFACTOR  │
    │ (Fail)  │       │ (Pass)  │       │ (Clean)   │
    └─────────┘       └─────────┘       └───────────┘
         ^                                    │
         │                                    │
         └────────────────────────────────────┘
                    Next Feature
```

### 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.**

```bash
CI=true 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

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

Don't add features, refactor other code, or "improve" beyond the test. Don't hard-code test values - implement general logic that works for ALL inputs.

### Verify GREEN - Watch It Pass

**MANDATORY.**

```bash
CI=true 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.

### 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 |

## Factory Pattern for Tests (Reference Pattern)

Create `getMockX(overrides?: Partial<X>)` functions for reusable test data:

```typescript
interface User {
  id: string;
  name: string;
  email: string;
  role: 'admin' | 'user';
}

const getMockUser = (overrides?: Partial<User>): User => ({
  id: '123',
  name: 'John Doe',
  email: 'john@example.com',
  role: 'user',
  ...overrides,
});

// Usage - override only what matters for the test
it('shows admin badge for admin users', () => {
  const user = getMockUser({ role: 'admin' });
  render(<UserCard user={user} />);
  expect(screen.getByText('Admin')).toBeTruthy();
});
```

**Benefits:**
- Sensible defaults - less boilerplate per test
- Override specific properties - focus on what test cares about
- Type-safe - catches missing properties
- DRY - change mock in one place

## Mocking External Dependencies (When Unavoidable)

**Rule:** Prefer real code. Mock only when:
- External API (network calls)
- Database (test isolation)
- Time-dependent logic
- Third-party services

### Common Mock Patterns

**Supabase:**
```typescript
jest.mock('@/lib/supabase', () => ({
  supabase: {
    from: jest.fn(() => ({
      select: jest.fn(() => ({
        eq: jest.fn(() => Promise.resolve({ data: mockData, error: null }))
      }))
    }))
  }
}))
```

**Fetch/API:**
```typescript
global.fetch = jest.fn(() =>
  Promise.resolve({ ok: true, json: () => Promise.resolve(mockResponse) })
) as jest.Mock
```

**Redis:**
```typescript
jest.mock('@/lib/redis', () => ({
  get: jest.fn(() => Promise.resolve(cachedValue)),
  set: jest.fn(() => Promise.resolve('OK'))
}))
```

**Environment Variables:**
```typescript
beforeEach(() => {
  process.env.API_KEY = 'test-key'
})
afterEach(() => {
  delete process.env.API_KEY
})
```

**Time:**
```typescript
jest.useFakeTimers()
// In test:
jest.advanceTimersByTime(1000)
```

**Mock quality check:** If mock setup > test code, reconsider design.

## 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

Test-first forces you to see the test fail, proving it actually tests something.

**"I already manually tested all the edge cases"**

Manual testing is ad-hoc. You think you tested everything but:
- No record of what you tested
- Can't re-run when code changes
- Easy to forget cases under pressure
- "It worked when I tried it" ≠ comprehensive

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 now:
- Delete and rewrite with TDD (X more hours, high confidence)
- Keep it and add tests after (30 min, low confidence, likely bugs)

The "waste" is keeping code you can't trust. Working code without real tests is technical debt.

## Red Flags - STOP and Start Over

If you catch yourself:

- 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.**

## Rationalization Prevention

| 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. |

## 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
- [ ] No hanging test processes (pgrep -f "vitest|jest" returns empty)

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

## Coverage Threshold (Project Default)

Target: **80%+ code coverage** across:
- Branches: 80%
- Functions: 80%
- Lines: 80%
- Statements: 80%

**Verify with:** `npm run test:coverage` or equivalent.

**Below threshold?** Add missing tests before claiming completion.

## Test Smells (Anti-Patterns)

| Smell | Bad Example | Why It's Bad | Fix |
|-------|-------------|--------------|-----|
| **Testing implementation** | `expect(component.state.count).toBe(5)` | Breaks when internals change | Test user-visible behavior |
| **Dependent tests** | Test B relies on Test A's state | Flaky, order-dependent | Each test sets up own data |
| **Mocking everything** | Every dependency mocked | Tests mock, not code | Use real code where feasible |
| **Giant setup** | 50 lines of setup per test | Hard to understand | Extract factories |
| **Magic numbers** | `expect(result).toBe(42)` | Meaning unclear | Use named constants |
| **Test name lies** | `test('works')` passes but doesn't test 'works' | Misleading | Name describes actual behavior |
| **No assertions** | `test('loads', () => { loadData() })` | Tests nothing | Always assert outcomes |
| **Commented tests** | `// test('edge case'...` | Dead code, skipped coverage | Delete or uncomment |

**If you spot these in your tests:** Fix before claiming TDD cycle complete.

## 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. |

## Output Format

```markdown
## TDD Cycle

### Requirements
[What functionality is being built]

### RED Phase
- Test: [test name]
- Command: `npm test -- --grep "test name"`
- Result: exit 1 (FAIL as expected)
- Failure reason: [function not defined / expected X got Y]

### GREEN Phase
- Implementation: [summary]
- File: [path:line]
- Command: `npm test -- --grep "test name"`
- Result: exit 0 (PASS)

### REFACTOR Phase
- Changes: [what was improved]
- Command: `npm test`
- Result: exit 0 (all tests pass)
```

## Final Rule

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

No exceptions without your human partner's permission.