Test driven development

<skill> <purpose> Write the test first. Watch it fail. Write minimal code to pass. If you didn't watch the test fail, you don't know if it tests the right thing. </purpose> <triggers> <rule>Use when implementing any new feature, before writing implementation code.</rule> <rule>Use when fixing any bug — write a failing test that reproduces it first.</rule> <rule>Use when refactoring or changing behavior in existing code.</rule> </triggers> <warning level="hard"> NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST. Violating the letter of the rules is violating the spirit. Write code before the test? Delete it. Start over. No exceptions without your human partner's permission. </warning> <rules> <rule>Test-first is non-negotiable. RED before GREEN.</rule> <rule>Wrote code before the test? Delete it. Don't keep it as "reference", don't "adapt" it, don't look at it. Delete means delete. Implement fresh from tests.</rule> <rule>Exceptions (ask your human partner first): throwaway prototypes, generated code, configuration files.</rule> </rules> <process> <step>**RED** — Write one minimal failing test showing what should happen. One behavior, clear name, real code (no mocks unless unavoidable).</step> <step>**Verify RED** — Run `npm test path/to/test.test.ts`. Confirm: test *fails* (not errors), failure message is expected, fails because feature is missing (not typos). Test passes immediately? You're testing existing behavior — fix the test.</step> <step>**GREEN** — Write the simplest code to pass the test. Don't add features, refactor other code, or "improve" beyond the test.</step> <step>**Verify GREEN** — Run tests. Confirm: test passes, *all other* tests still pass, output is pristine (no errors, warnings). Test fails? Fix code, not test. Other tests fail? Fix now.</step> <step>**REFACTOR** — After green only: remove duplication, improve names, extract helpers. Keep tests green. Don't add behavior.</step> <step>**Repeat** — Next failing test for next behavior.</step> </process> <flow format="dot"> ```dot digraph tdd_cycle { rankdir=LR; red [label="RED\nFailing test", shape=box, style=filled, fillcolor="#ffcccc"]; green [label="GREEN\nMinimal code", shape=box, style=filled, fillcolor="#ccffcc"]; refactor [label="REFACTOR\nClean up", shape=box, style=filled, fillcolor="#ccccff"]; red -> green -> refactor -> red; } ``` </flow> <example type="good"> **RED** — Bug: empty email accepted ```typescript test('rejects empty email', async () => { const result = await submitForm({ email: '' }); expect(result.error).toBe('Email required'); }); ``` **Verify RED** — `FAIL: expected 'Email required', got undefined`

**GREEN**
```typescript
function submitForm(data: FormData) {
  if (!data.email?.trim()) return { error: 'Email required' };
  // ...
}
```
**Verify GREEN** — `PASS`

**REFACTOR** — Extract validation for multiple fields if needed.

</example> <example type="bad"> Writing production code first, then writing tests that pass immediately. Passing immediately proves nothing — you never saw the test catch the bug. </example> <antipattern> "I'll write tests after to verify it works" — 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. </antipattern> <antipattern> "Keep as reference, write tests first" — You'll adapt it. That's testing after. Delete means delete. </antipattern>

Common Rationalizations

Excuse	Reality
"Too simple to test"	Simple code breaks. Test takes 30 seconds.
"I'll test after"	Tests passing immediately prove nothing.
"Already manually tested"	Ad-hoc != systematic. No record, can't re-run.
"Deleting X hours is wasteful"	Sunk cost fallacy. Keeping unverified code is tech debt.
"Need to explore first"	Fine. Throw away exploration, start with TDD.
"Test hard = design unclear"	Listen to the test. Hard to test = hard to use.
"TDD will slow me down"	TDD is faster than debugging.
"This is different because..."	No. Start over with TDD.

Red Flags — STOP and Start Over

• Code before test
• Test passes immediately
• Can't explain why test failed
• Tests added "later"
• Rationalizing "just this once"
• "It's about spirit not ritual"

All of these mean: Delete code. Start over with TDD.

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.

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 with pristine output
• Tests use real code (mocks only if unavoidable)
• Edge cases and errors covered

Can't check all boxes? You skipped TDD. Start over.

<see file="testing-anti-patterns.md"/> </skill>