Test-Driven Development

Iron Law

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Cycle

RED — Write a failing test

Write a test that defines the expected behavior BEFORE writing any production code.

• The test MUST fail before you write production code
• If the test passes without any changes, your test is wrong — rewrite it
• The test should describe behavior, not implementation

GREEN — Write the minimum production code

Write the minimum production code to make the test pass.

• Do NOT write more than necessary
• Do NOT optimize
• Do NOT refactor
• Ugly code that passes is correct at this stage

REFACTOR — Clean up without changing behavior

Improve the code while keeping all tests green.

• All tests must still pass after refactoring
• Improve naming, extract functions, remove duplication
• If any test fails after refactoring, your refactor changed behavior — revert

Red Flags — If You Catch Yourself Thinking:

Thought	Reality
"I'll write the implementation first, then add tests"	That's not TDD. Write the test FIRST.
"This is too simple to test"	If it's too simple to test, it's simple enough to test quickly.
"I'll write all the tests first, then implement"	One test at a time. RED-GREEN-REFACTOR. One cycle.
"The test is basically the same as the implementation"	Then you're testing implementation, not behavior. Rewrite the test.

Rules

• ONE test at a time, ONE cycle at a time
• NEVER skip RED — if the test doesn't fail first, it proves nothing
• NEVER skip REFACTOR — clean code is part of the deliverable
• Commit after each GREEN (passing test + minimal implementation)