Note: This post is a translated version of an article originally published on my personal blog. You can read the original Korean post here . What Are Decorators? Decorators are a metaprogramming syntax that lets you attach metadata — or extend behavior — on classes, methods, properties, and parameters without modifying the original code . They're commonly used for: Logging — track when methods are called Validation — enforce constraints on class properties Dependency Injection — wire up services automatically Authorization — guard methods behind permission checks Frameworks like Angular and NestJS lean heavily on decorators for all of the above. Setting Up First, enable decorators in your tsconfig.json : { "compilerOptions" : { "experimentalDecorators" : true } } How Decorators Work A decorator is just a function. When you apply it to a class or method, TypeScript calls that function at definition time with some useful arguments.…