Refactoring Guru
refactoring.guru
A beautifully illustrated guide to design patterns and refactoring techniques — covering all 23 Gang of Four patterns and dozens of refactoring recipes with before/after code examples in multiple languages.
Why it is useful
Makes design patterns actually stick through concrete illustrations and real-world analogies rather than abstract UML diagrams. The refactoring catalog is equally useful — each recipe shows the problem it solves, step-by-step instructions, and the resulting code in your language of choice. Covers Java, Python, C#, PHP, TypeScript, Go, Rust, Swift, and more. Partially free; the full book is paid.
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