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 When writing or changing any test, read [writing good tests.md](writing good tests.md) for the rules that keep tests honest: Name the production change that would make the test fail — before writing it Assert on real behavior, never on mock behavior Keep test only code in test utilities, out of production classes Understand a dependency's side effects before mocking it Common Rationalizations Excuse Reality "Too simple to test" Simple code breaks. Test takes 30 seconds. "I'll test after" Tests written after pass immediately — which proves nothing. They may test the wrong thing, test the implementation instead of the behavior, or miss the edge case you forgot. You never watched it fail, so you never proved it can catch the bug. Test first forces that failure. "Tests after achieve same goals (spirit not ritual)" Tests after answer "what does this do?"; tests first answer "what should this do?" Tests written after are biased by the code you already wrote — you verify the cases you remembered, not the ones you'd have discovered. Coverage without proof the tests work. "Already manually tested" Manual testing is ad hoc: no record of what you covered, no way to re run it when the code changes, easy to forget cases under pressure. "Worked when I tried it" ≠ comprehensive. Automated tests run the same way every time. "Deleting X hours is wasteful" Sunk cost fallacy — that time is already spent either way. The real choice: rewrite with TDD (high confidence) vs. keep it and bolt tests on after (low confidence, likely bugs). Keeping code you can't trust is the waste. "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 IS the pragmatic path: catches bugs before commit, prevents regressions, lets you refactor without fear. "Pragmatic" shortcuts mean debugging in production — slower, not faster. "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.

test-driven-development