Refactoring: Improving the Design of Existing Code (Addison-Wesley Signature Series (Fowler)): Amazon.co.uk: Fowler, Martin: 9780134757599: Books

Most code starts decent and slowly rots. Features pile up, abstractions leak, and suddenly you're debugging spaghetti that made perfect sense six months ago. Fowler's book is about the systematic practice of fixing this without breaking things - refactoring is changing code structure while keeping behavior identical. It's the difference between hacking fixes into a mess and actually improving the underlying design.

The book's real value is the catalog of specific refactorings with clear recipes. Extract Method when functions get too long. Replace Conditional with Polymorphism when switch statements sprawl. Move Field when data lives in the wrong place. Each refactoring has a name, motivation, mechanics, and examples. This shared vocabulary matters - when you can say "we should Extract Class here" instead of vaguely gesturing at messy code, you can actually coordinate design improvements across a team.

Fowler emphasizes test-driven refactoring. You refactor in tiny steps, running tests after each change. This sounds tedious but it's actually how you refactor safely in production code. Make one small change, verify nothing broke, commit. Then the next change. Big rewrites fail because you change too much at once and lose your way back. Small refactorings with continuous verification let you improve design without the fear that drives teams to just live with bad code.

The book also teaches you to spot code smells - patterns that indicate design problems. Long functions, duplicated code, primitive obsession, feature envy. These aren't bugs, they're warning signs that your design is fighting you. Learning to recognize smells is how senior engineers look at unfamiliar code and immediately see where the problems are. The examples use JavaScript but the concepts work in any language because design problems look similar everywhere.