tdd-mastery

<domain_overview> # 🧪 TDD MASTERY: THE IRON LAW > **Philosophy:** If you didn't watch the test fail, you don't know if it tests the right thing. TDD is not optional—it's the foundation of trustworthy code. **TEST-FIRST INTEGRITY MANDATE (CRITICAL):** Never write production code before a test exists and has been seen failing. AI-generated code often attempts to write implementation and tests simultaneously or implementation first. You MUST strictly adhere to the Red-Green-Refactor cycle. Any code submitted without a preceding failing test or that generates tests after the implementation must be rejected as "Legacy Code on Arrival". --- ## 🚨 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. </domain_overview> <core_workflow> ## 🔴 RED-GREEN-REFACTOR CYCLE ### Phase 1: RED - Write Failing Test Write one minimal test showing what should happen. **Good Example:** ```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 Example:** ```typescript test('retry works', async () => { const mock = jest.fn() .mockRejectedValueOnce(new Error()) .mockResolvedValueOnce('success'); await retryOperation(mock); expect(mock).toHaveBeenCalledTimes(2); }); ``` *Vague name, tests mock not code* **Requirements:** - One behavior per test - Clear, descriptive name - Real code (mocks only if unavoidable) ### Phase 2: VERIFY RED - Watch It Fail **MANDATORY. Never skip.** ```bash npm test path/to/test.test.ts # or pytest tests/path/test.py::test_name -v ``` 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. ### Phase 3: 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 Aren't Gonna Need It } ``` *Over-engineered* Don't add features, refactor other code, or "improve" beyond the test. ### Phase 4: 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. ### Phase 5: REFACTOR - Clean Up After green only: - Remove duplication - Improve names - Extract helpers Keep tests green. Don't add behavior. ### Phase 6: COMMIT ```bash git add tests/path/test.ts src/path/file.ts git commit -m "feat: add specific feature with tests" ``` **Repeat** for next behavior. --- </core_workflow> <quality_standards> ## 📋 GOOD TEST QUALITIES | 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 | | **Real behavior** | Tests actual code | Tests mock behavior | --- ## 🚫 COMMON RATIONALIZATIONS (ALL INVALID) | 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. | | "Deleting X hours is wasteful" | Sunk cost fallacy. Keeping unverified code is debt. | | "Keep as reference" | You'll adapt it. That's testing after. Delete means delete. | | "Need to explore first" | Fine. Throw away exploration, start with TDD. | | "Test hard = skip test" | Hard to test = hard to use. Simplify design. | | "TDD will slow me down" | TDD faster than debugging. Pragmatic = test-first. | | "Existing code has no tests" | You're improving it. Add tests for existing code. | --- ## 🚨 RED FLAGS - STOP AND START OVER If you catch yourself: - Writing code before test - Test passes immediately - Can't explain why test failed - Tests added "later" - "Just this once" - "I already manually tested it" - "Keep as reference" - "TDD is dogmatic, I'm being pragmatic" **ALL of these mean: Delete code. Start over with TDD.** --- </quality_standards> <bug_fix_protocol> ## 🐛 BUG FIX WORKFLOW Bug found? Write failing test reproducing it. Follow TDD cycle. **Example:** ``` 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 ``` **Never fix bugs without a test.** --- </bug_fix_protocol> <integration_and_tooling> ## 🔗 RALPH WIGGUM INTEGRATION When Ralph Wiggum is active: 1. **Before ANY implementation:** Write failing test first 2. **Proactive Gate:** Check edge cases BEFORE coding (use TDD to cover them) 3. **Reflection Loop:** After implementation, verify RED-GREEN was followed 4. **Verification Matrix:** Track test coverage for each feature **Ralph Wiggum will REJECT:** - Code without corresponding tests - Tests that were written after code - Tests that pass without implementation --- ## ✅ 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. --- ## 🛠️ TESTING INFRASTRUCTURE ### Stack Detection & Tool Setup Auto-detect project type and setup appropriate tools: | Project Type | Required Tools | |--------------|----------------| | **Frontend (Vite/React)** | `vitest` + `playwright` | | **Fullstack (Next.js)** | `vitest` + `playwright` | | **Backend (Node)** | `vitest` or `jest` | | **Python** | `pytest` + `pytest-cov` | | **Microservices** | `MSW` (Mock Service Worker) | ### Test Coverage Rules For every new function/component, generate: - **1 Happy Path** - Expected successful behavior - **2 Edge Cases** - Boundary conditions, invalid inputs - **1 Error Case** - Expected failure handling ### Contract-First (MSW) **Rule:** Every frontend-backend interaction MUST have an MSW handler. ```typescript // Example MSW handler import { http, HttpResponse } from 'msw' export const handlers = [ http.get('/api/users/:id', ({ params }) => { return HttpResponse.json({ id: params.id, name: 'Test User' }) }) ] ``` **Benefit:** Decouples frontend development from backend availability. ### Ghost Inspector Protocol AI must scan for "Untested Logic Slabs" (>20 lines without coverage) and flag them: ```bash # Check coverage gaps npm run test -- --coverage # Look for files with <80% coverage ``` --- </integration_and_tooling> <reference_and_audit> ## 📖 RELATED SKILLS - **@testing-anti-patterns.md** - Common mock/test mistakes to avoid - **@clean-code** - Code quality standards - **@verification-mastery** - Evidence before completion claims - **@debug-mastery** - When tests reveal bugs --- ## 🏁 FINAL RULE ``` Production code → test exists and failed first Otherwise → not TDD ``` No exceptions without explicit user permission. </reference_and_audit>