Questions
3 of 7
1What is a decorator in TypeScript and how does NestJS use decorators?
2How do you create a custom parameter decorator in NestJS?
3What is Reflector and how does it work with SetMetadata in NestJS?
4What are the different types of decorators and their execution order in TypeScript?
5How do you use a custom param decorator together with a pipe for automatic validation in NestJS?
6How do you create a custom parameter decorator in NestJS?
7How do you compose multiple decorators into a single reusable decorator in NestJS?
NestJSNestJS
Basics
Modules
Decorators
Controllers
Dependency Injection
Providers
Architecture
Request Lifecycle
Middleware
Guards
Interceptors
Pipes
Filters
Exception Handling
API Design
GraphQL
Database and ORM
Microservices
Authentication and Authorization
03 / 07

What is Reflector and how does it work with SetMetadata in NestJS?

SetMetadata(key, value) attaches metadata to a class or method using Reflect.defineMetadata. Reflector is a helper injected into guards and interceptors to read that metadata using getAllAndOverride or getAllAndMerge. This is the standard pattern for RBAC guards and feature flags.

SetMetadata + Reflector pattern for RBAC
Reflector methods:
  1. 1

    get(key, target) — reads metadata from a single target.

  2. 2

    getAllAndOverride(key, targets) — returns first defined value across targets; method overrides class.

  3. 3

    getAllAndMerge(key, targets) — merges arrays/objects from all targets.