--- name: Test-Driven Development (TDD) description: Write the test first, watch it fail, write minimal code to pass when_to_use: when implementing any feature or bugfix, before writing implementation code version: 3.1.0 languages: all --- # 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. ```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 ```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 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. ```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" 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. ### 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 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. **"TDD is dogmatic, being pragmatic means adapting"** TDD IS pragmatic: - Finds bugs before commit (faster than debugging after) - Prevents regressions (tests catch breaks immediately) - Documents behavior (tests show how to use code) - Enables refactoring (change freely, tests catch breaks) "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. You test what you built, not what's required. You verify remembered edge cases, not discovered ones. Tests-first force edge case discovery before implementing. Tests-after verify you remembered everything (you didn't). 30 minutes of tests after ≠ TDD. You get coverage, lose proof tests work. ## 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** ```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.

When to use