Test Driven Development (TDD)

--- name: test-driven-development description: Use when implementing any feature or bugfix, before writing implementation code --- # 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. ### 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