test-driven-development

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

RED - Write Failing Test

Write one minimal test showing what should happen. Clear name, tests real behavior, one thing.

Verify RED - Watch It Fail

MANDATORY. Never skip.

Run the test. Confirm:

• Test fails (not errors)
• Failure message is expected
• Fails because feature is missing (not typos)

Test passes? You are testing existing behavior. Fix the test. Test errors? Fix the error, re-run until it fails correctly.

GREEN - Minimal Code

Write the simplest code to pass the test. Do not add features, refactor other code, or improve beyond the test.

Verify GREEN - Watch It Pass

MANDATORY.

Run the test. Confirm:

• Test passes
• Other tests still pass
• Output is clean (no errors, warnings)

Test fails? Fix code, not the test. Other tests fail? Fix now.

REFACTOR - Clean Up

After green only:

• Remove duplication
• Improve names
• Extract helpers

Keep tests green. Do not add behavior.

Repeat

Write the next failing test for the 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

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.
"Already manually tested"	Manual testing has no record, no replay, no edge-case coverage.
"Need to explore first"	Fine. Throw away the exploration. Start TDD fresh.
"Test hard = design unclear"	Listen to the test. Hard to test means hard to use.

Red Flags - STOP and Start Over

• Code before test
• Test after implementation
• Test passes immediately without implementation
• Can't explain why the test failed
• "Just this once"

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

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 is clean (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.

Final Rule

Production code → test exists and failed first
Otherwise → not TDD

No exceptions without your human partner's permission.