Definition of legacy code

To me, legacy code is simply code without tests — Michael Feathers

The site maintained by Nicolas Carlo, understandlegacycode.com, boasts a blog post that summarizes the key points of Working Effectively with Legacy Code.

Nicolas himself uses a somewhat different definition of legacy code.

Legacy Code is valuable code you’re afraid to change — Nicolas Carlo

as you may sometimes encounter code without (sufficient) tests, but the code reveals its intention so well, that you're not afraid to modify it.

Practice materials

• Five coding exercises to practice refactoring legacy code

References

• Awesome legacy systems, a curated list of awesome resources and links about tackling legacy systems that gives hope.
• Nicolas Carlo on separating infrastructure (tests) from domain (tests): If you mock, are you even testing?
• TDD and legacy code: creating a snapshot with approval tests
• TDD and legacy code: moving behavior back to where it belongs
• An article about the middle man and train wreck code smells.
• Splitting up a monolith using a team first approach
• What NOT to fix in a Legacy Codebase

Nicolas Carlo shares tips & tricks to work with Legacy Code on his blog: understandlegacycode.com

• The key points of Working Effectively with Legacy Code
• 7 techniques to regain control of your legacy codebase
• Reveal code structure using a word counter
• When is it the best time to refactor?
• Elevate Your Coding: Efficient Practices for Refactoring Katas
• What is Wrong With Boolean Parameters
• Refactoring... but at scale!
• When should you stop refactoring Legacy Code?