Reflector and custom decorators in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this custom decorator usage?

Consider this NestJS custom decorator and its usage in a controller method. What will the Reflector retrieve as metadata value?

NestJS

import { SetMetadata, Reflector } from '@nestjs/common';

const Roles = (...roles: string[]) => SetMetadata('roles', roles);

class SampleController {
  @Roles('admin', 'user')
  getData() {
    return 'data';
  }
}

const reflector = new Reflector();
const metadata = reflector.get('roles', SampleController.prototype.getData);
console.log(metadata);

A null

B undefined

C ['roles']

D ['admin', 'user']

💡 Hint

Think about what SetMetadata stores and how Reflector retrieves it.

The custom decorator Roles uses SetMetadata to attach an array ['admin', 'user'] under the key 'roles'. Reflector.get retrieves this exact array from the method's metadata.

📝 Syntax

intermediate

Which option correctly defines a custom decorator that sets metadata 'permission'?

You want to create a custom decorator named Permission that sets metadata key 'permission' with a string value. Which code snippet is correct?

A export const Permission = (perm: string) => Reflect.defineMetadata('permission', perm);

B export function Permission(perm: string) { return Reflect.metadata('permission', perm); }

C export const Permission = (perm: string) => SetMetadata('permission', perm);

D export function Permission(perm: string) { SetMetadata('permission', perm); }

💡 Hint

Remember that SetMetadata returns a decorator function.

Option C correctly returns the decorator function from SetMetadata. Option C uses Reflect.metadata which is not the NestJS way. Option C calls Reflect.defineMetadata incorrectly without target. Option C calls SetMetadata but does not return a decorator.

🔧 Debug

advanced

Why does Reflector.get return undefined in this code?

Given this code snippet, why does the Reflector return undefined when trying to get 'roles' metadata?

NestJS

import { SetMetadata, Reflector } from '@nestjs/common';

const Roles = (...roles: string[]) => SetMetadata('roles', roles);

class UserController {
  @Roles('admin')
  getUser() {
    return 'user';
  }
}

const reflector = new Reflector();
const metadata = reflector.get('roles', UserController.getUser);
console.log(metadata);

A Because the 'roles' key is misspelled in the decorator.

B Because UserController.getUser is undefined; method should be accessed via prototype.

C Because Reflector requires the class instance, not the method reference.

D Because SetMetadata does not attach metadata to methods, only classes.

💡 Hint

Check how methods are accessed on classes vs prototypes.

Reflector.get expects the target method from the prototype, not the class itself. UserController.getUser is undefined because getUser is an instance method, so accessing it directly on the class fails.

🧠 Conceptual

advanced

What is the main purpose of the Reflector class in NestJS?

Choose the best description of what the Reflector class does in NestJS.

A It reads metadata set by decorators on classes and methods at runtime.

B It automatically applies decorators to controller methods.

C It compiles TypeScript decorators into JavaScript functions.

D It manages dependency injection container instances.

💡 Hint

Think about how metadata is accessed after decorators run.

Reflector is a helper class that reads metadata attached by decorators at runtime. It does not apply decorators, compile code, or manage DI containers.

❓ state_output

expert

What is the output of this custom decorator and Reflector usage with inheritance?

Given these classes and decorators, what will the Reflector return for 'roles' metadata on ChildController.getData?

NestJS

import { SetMetadata, Reflector } from '@nestjs/common';

const Roles = (...roles: string[]) => SetMetadata('roles', roles);

class ParentController {
  @Roles('admin')
  getData() {
    return 'parent data';
  }
}

class ChildController extends ParentController {
  getData() {
    return 'child data';
  }
}

const reflector = new Reflector();
const metadata = reflector.get('roles', ChildController.prototype.getData);
console.log(metadata);

A undefined

B null

C []

D ['admin']

💡 Hint

Consider how metadata is attached and whether it is inherited by overridden methods.

The ChildController overrides getData without a decorator, so no metadata is attached to its method. Reflector.get does not inherit metadata from parent methods, so it returns undefined.