Refactoring

Refactoring is the third step of TDD: after GREEN, assess whether refactoring adds value. A refactor improves the structure of working code; it never changes observable behaviour.

When to Refactor

• Assess after every GREEN: once per behaviour, not once after a batch
• Refactor only if it improves the code; if the code is already clean, move on
• Establish a reversible baseline with a known test result before refactoring (principle-capture-the-baseline)
• Pin the behaviour the refactor must preserve with a passing test first
• Refactor only code that green tests already cover; characterize anything else first or leave it alone, however trivial it looks
• Refactor in small reversible steps; never refactor code you cannot restore
• Keep external APIs unchanged and keep every test result unchanged

Reversible Baseline