Stage 3 decorators, metadata, NestJS patterns, execution order.

TypeScript Decorators — Class, Method, Property, and Parameter

TypeScript decorators (now Stage 3 standard) attach metadata and transform classes, methods, properties, and parameters at declaration time. Class decorators receive the class constructor. Method decorators receive the class prototype + property descriptor. Decorators compose from bottom to top (inner to outer). NestJS uses decorators extensively: @Controller, @Get, @Injectable, @Body. Reflect.metadata stores and retrieves decorator-attached metadata.

Interactive Pipeline Flow

UnderstandImplementOptimise

📝 Syntax — @decorator

Decorator syntax: @expression. expression must evaluate to a function. @sealed is a decorator; @sealed() is a decorator factory (returns a decorator).

Architecture Diagrams

How NestJS uses decorators to define a controller.

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Key Design Decisions

📝Decorators are compile-time, metadata is runtime

Decorator evaluation happens at class definition time (module load), not at call time. Reflect.metadata stores configuration as runtime metadata on the prototype, read later by frameworks.

📐Decorator order is bottom-to-top for methods

With @A @B methodFn(), B is applied first (closer to the method), then A wraps B. This matters for decorator chains: put authentication before authorization in reading order (they apply inside-out).

🏭Decorator factories enable parameterisation

Most real decorators are factories: @Role('admin') is a function call that returns the decorator. This allows passing configuration like timing, caching TTL, or permission levels.

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