Michael Feathers' must-read for anyone inheriting messy systems, refactoring safely, or adding tests. Learn how to work with code you didn't write and make it better without breaking it.

Working Effectively with Legacy Code is the book I read when I inherited a codebase that had no tests and couldn't be changed safely. Feathers' techniques—sprouting methods, characterization tests, dependency breaking—gave me a systematic approach to adding tests to untested code. The chapter on dependency breaking showed me how to extract dependencies so I could test code in isolation, and the section on characterization tests helped me understand what legacy code actually does before I tried to change it. The book is practical: every technique includes code examples showing the before and after, and Feathers explains when each technique works and when it doesn't. My only critique is that some examples are Java-focused, but the principles apply to any language. I've used this book to add tests to three different legacy systems, and each time it gave me the confidence to make changes without breaking things. Essential reading for anyone working with code they didn't write.

Michael Feathers' guide for inheriting messy systems, refactoring safely, or adding tests. Sprouting methods, characterization tests, and dependency breaking.

Working Effectively with Legacy Code: Refactoring Guide

Working Effectively with Legacy Code

Any fool can write code that a computer can understand. Good programmers write code that humans can understand.

Fowler and Beck’s second edition of Refactoring is still the book I hand to engineers when we’re about to touch legacy code. The catalog of refactorings—Extract Function, Introduce Parameter Object, Replace Conditional with Polymorphism—gives you named moves with mechanics, motivations, and before/after examples you can translate to any language. Over the last year we’ve used it to modernize a decade-old Java service and a pile of TypeScript reducers; the step-by-step recipes paired with tests kept us shipping safely instead of rewriting from scratch. The newer chapters address lambdas, streams, and microservices, and the online catalog makes it easy to search for a particular smell mid-PR. If you refactor as part of your day job (and you should), this belongs on your desk.

Martin Fowler's systematic guide to refactoring code. Learn named refactoring moves with step-by-step mechanics to improve code design safely and effectively.

Refactoring: Improving the Design of Existing Code

Refactoring: Improving the Design of Existing Code (2nd Edition)

Hand-picked legacy code tools and resources we actually use in production. Each item tested by developers, for developers.

Legacy Code Products

Working Effectively with Legacy Code

Refactoring: Improving the Design of Existing Code (2nd Edition)