Use when implementing any feature or bugfix, before writing implementation code - write the test first, watch it fail, write minimal code to pass; ensures tests actually verify behavior by requiring failure first

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 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 RED Write Failing Test Write one minimal test showing what should happen. <Good Clear name, tests real behavior, one thing </Good <Bad 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. 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 Just enough to pass </Good <Bad Over engineered </Bad Don't add features, refactor other code, or "improve" beyond the test. Verify GREEN Watch It Pass MANDATORY. 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 Verify RED GREEN Verify GREEN 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 No exceptions without your human partner's permission. Testing Anti Patterns Overview Tests must verify real behavior, not mock behavior. Mocks are a means to isolate, not the thing being tested. Core principle: Test what the code does, not what the mocks do. Following strict TDD prevents these anti patterns. The Iron Laws Anti Pattern 1: Testing Mock Behavior The violation: Why this is wrong: You're verifying the mock works, not that the component works Test passes when mock is present, fails when it's not Tells you nothing about real behavior your human partner's correction: "Are we testing the behavior of a mock?" The fix: Gate Function Anti Pattern 2: Test Only Methods in Production The violation: Why this is wrong: Production class polluted with test only code Dangerous if accidentally called in production Violates YAGNI and separation of concerns Confuses object lifecycle with entity lifecycle The fix: Gate Function Anti Pattern 3: Mocking Without Understanding The violation: Why this is wrong: Mocked method had side effect test depended on (writing config) Over mocking to "be safe" breaks actual behavior Test passes for wrong reason or fails mysteriously The fix: Gate Function Anti Pattern 4: Incomplete Mocks The violation: Why this is wrong: Partial mocks hide structural assumptions You only mocked fields you know about Downstream code may depend on fields you didn't include Silent failures Tests pass but integration fails Mock incomplete, real API complete False confidence Test proves nothing about real behavior The Iron Rule: Mock the COMPLETE data structure as it exists in reality, not just fields your immediate test uses. The fix: Gate Function Anti Pattern 5: Integration Tests as Afterthought The violation: Why this is wrong: Testing is part of implementation, not optional follow up TDD would have caught this Can't claim complete without tests The fix: When Mocks Become Too Complex Warning signs: Mock setup longer than test logic Mocking everything to make test pass Mocks missing methods real components have Test breaks when mock changes your human partner's question: "Do we need to be using a mock here?" Consider: Integration tests with real components often simpler than complex mocks TDD Prevents These Anti Patterns Why TDD helps: 1. Write test first → Forces you to think about what you're actually testing 2. Watch it fail → Confirms test tests real behavior, not mocks 3. Minimal implementation → No test only methods creep in 4. Real dependencies → You see what the test actually needs before mocking If you're testing mock behavior, you violated TDD you added mocks without watching test fail against real code first. Quick Reference Anti Pattern Fix Assert on mock elements Test real component or unmock it Test only methods in production Move to test utilities Mock without understanding Understand dependencies first, mock minimally Incomplete mocks Mirror real API completely Tests as afterthought TDD tests first Over complex mocks Consider integration tests Red Flags Assertion checks for mock test IDs Methods only called in test files Mock setup is 50% of test Test fails when you remove mock Can't explain why mock is needed Mocking "just to be safe" The Bottom Line Mocks are tools to isolate, not things to test. If TDD reveals you're testing mock behavior, you've gone wrong. Fix: Test real behavior or question why you're mocking at all.

test-driven-development