test-driven-development

Test-Driven Development (TDD)

Overview

Write the test first. Watch it fail. Write minimal code to pass. Refactor. Repeat.

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.

The Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Wrote 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.

When to Use

Always:

• New features
• Bug fixes
• Refactoring
• Behavior changes

Exceptions (confirm with human partner):

• Throwaway prototypes
• Generated code (migrations, scaffolds)
• Configuration files
• Static assets

Thinking "skip TDD just this once"? Stop. That's rationalization.

When NOT to Use

• Writing documentation (use writing-documentation)
• Configuration changes with no behavioral impact
• Evaluating approaches (use brainstorming first, then TDD the chosen approach)
• CSS-only styling changes (visual verification, not testable behavior)

Anti-Shortcut Rules

YOU CANNOT:
- Write production code first "to understand the problem" — the test IS how you understand it
- Write multiple tests before any implementation — one test, one implementation cycle
- Skip the RED phase — if you didn't see it fail, you don't know what it tests
- Accept a test that passes without implementation — the test is wrong
- Modify a test to make it pass — the implementation must satisfy the test
- Refactor while tests are RED — GREEN first, then refactor
- Write tests that test implementation details — test BEHAVIOR
- Skip the REFACTOR phase — "it works" is not the end, "it's clean" is

Common Rationalizations

Excuse	Reality
"I'll write tests after"	You won't. And if you do, they'll test your implementation, not requirements
"It's too simple to test"	If it's too simple to test, it's too simple to get wrong. So write the test
"Tests slow me down"	Tests slow you down NOW. Bugs slow you down MORE later
"I need to see the code first"	TDD IS how you see the code — one test at a time
"The framework handles this"	Does it? Prove it with a test
"It works, I checked manually"	Manual checking doesn't prevent regressions
"I'll just write a few tests at the end"	Post-hoc tests verify implementation, not behavior. They're testing the code you wrote, not the code you need
"This test is too hard to write"	If it's too hard to test, the design is wrong. Simplify the interface