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