Questions
1 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
01 / 07

What is a decorator in TypeScript and how does NestJS use decorators?

A decorator is a function that wraps a class, method, property, or parameter and can attach metadata or modify behavior. NestJS uses decorators extensively — @Module(), @Injectable(), @Controller(), @Get(), @Body() are all decorators — enabled by experimentalDecorators: true in tsconfig.

Decorators are a stage-3 JavaScript proposal and an experimental TypeScript feature. They allow you to annotate and modify classes and their members at design time. NestJS is built entirely around decorators as its primary API.

Decorator types used in NestJS
How NestJS uses decorators:
  1. 1

    @Module() — defines module metadata (imports, providers, exports, controllers).

  2. 2

    @Injectable() — marks a class as a provider eligible for DI.

  3. 3

    @Controller() — marks a class as a request handler with a route prefix.

  4. 4

    HTTP decorators (@Get, @Post, etc.) — map methods to HTTP verbs and routes.

  5. 5

    Parameter decorators (@Body, @Param, @Query) — extract data from requests.