working-with-legacy-code

# Working Effectively with Legacy Code A field manual for changing code that has no tests, distilled from Michael C. Feathers' *Working Effectively with Legacy Code*. Use it to get untestable classes into a harness, pin down current behavior with characterization tests, and make changes one safe, verifiable step at a time — without resorting to a rewrite. ## Core Principle **Legacy code is simply code without tests.** Not old code, not ugly code — untested code: without tests you cannot know whether a change preserves behavior, so every edit is a gamble. The craft is breaking dependencies just enough to get tests in place before changing anything — cover and modify, never edit and pray. ## Scoring **Goal: 10/10.** Rate changes to untested code 0-10 against the principles below. Report the current score and the specific steps needed to reach 10/10. - **9-10:** Change points covered by characterization tests before any edit; behavior changes and refactoring shipped as separate verified steps; dependencies broken with the least invasive technique - **7-8:** Tests at most change points, but occasional mixed refactor-plus-behavior commits or heavier dependency surgery than needed - **5-6:** Some characterization tests, yet key paths still changed on faith; sprouted code accumulating with no payback plan - **3-4:** Edit-and-pray with manual verification; tests written after the change, asserting whatever the new code happens to do - **0-2:** Untested edits straight into tangl...