jrollin 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 the user):
Use this checklist to decide — if uncertain, default to TDD:
| Question | Yes → exception applies |
|---|---|
| Is the code intended to be fully discarded after the session (not merged)? | Throwaway prototype |
| Was the code produced entirely by a code-generation tool, not hand-written? | Generated code |
| Does the file contain only values and no logic (env vars, JSON config, Dockerfile)? | Configuration file |
If none of the above apply, follow TDD. "It feels too simple" is not an exception.
For files with no logic (configuration, JSON, env vars, Dockerfiles, lockfiles): proceed without a test and note the exception briefly. No need to ask.
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
RED (write test) → VERIFY FAIL → GREEN (minimal code) → VERIFY PASS → REFACTOR → repeat
RED - Write Failing Test
Pre-coding checklist (complete before writing a single line of production code):
- No production code for this behavior exists yet — if it does, delete it first
- This is a behavioral change, not a config file / generated file / throwaway prototype
- I can describe the desired API (inputs, outputs, error cases) without looking at an implementation
- I know which test file to create or extend
Write one minimal test showing what should happen.
Requirements:
- One behavior
- Clear name describing expected outcome
- Real code (no mocks unless unavoidable)
See language-specific examples in references/ directory.
Verify RED - Watch It Fail
MANDATORY. Never skip.
Run the project's test command targeting the specific test.
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 the stupidest code that makes the test green. Not "simple" — minimal.
- One test expects
true? Returntrue. Notemail.includes('@'). Not a regex. - One test expects
3foradd(1, 2)? Returna + b. Not input validation. - Any logic that handles untested cases = code without a failing test = violates the Iron Law.
Don't add features, handle edge cases, refactor other code, or "improve" beyond the test. Future tests will drive the real implementation.
Verify GREEN - Watch It Pass
MANDATORY. Run tests again.
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 |
Red Flags - STOP and Start Over
If you catch yourself doing or saying any of these, delete code and restart with TDD.
| Red Flag | Reality |
|---|---|
| Code before test | Delete it. No exceptions. |
| Test passes immediately | You're testing existing behavior, not new behavior. |
| "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. |
| "This is different because..." | It's not. Follow the cycle. |
Language References
For concrete TDD examples (red/green/refactor walkthroughs, test runners, good/bad patterns), see:
references/typescript.md— Jest/Vitest, async patternsreferences/rust.md— cargo test, Result/Option patterns
Pick the reference matching the project's language.
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 the user. |
| 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. |
Testing Anti-Patterns
When adding mocks or test utilities, read testing-anti-patterns.md to avoid common pitfalls:
- Testing mock behavior instead of real behavior
- Adding test-only methods to production classes
- Mocking without understanding dependencies
Final Rule
Production code → test exists and failed first
Otherwise → not TDD
No exceptions without the user's permission.