Reflector and custom decorators in NestJS

Introduction

Reflector helps you read metadata set by custom decorators in NestJS. Custom decorators let you add extra info to classes or methods simply.

You want to mark routes or controllers with special info like roles or permissions.

You need to check metadata inside guards or interceptors to control access.

You want to keep your code clean by separating metadata from logic.

You want to reuse metadata reading logic across different parts of your app.

Syntax

NestJS

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

export const MyDecorator = (value: any) => SetMetadata('myKey', value);

// Using Reflector in a guard or interceptor
import { Reflector } from '@nestjs/core';

constructor(private reflector: Reflector) {}

const data = this.reflector.get('myKey', context.getHandler());

SetMetadata creates a decorator that attaches data to routes or classes.

Reflector reads that data later, usually inside guards or interceptors.

Examples

This custom decorator adds a list of roles as metadata.

NestJS

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

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

Reflector reads the 'roles' metadata from the current route handler.

NestJS

import { Reflector } from '@nestjs/core';

constructor(private reflector: Reflector) {}

const roles = this.reflector.get<string[]>('roles', context.getHandler());

Sample Program

This example shows a custom @Roles decorator that sets roles metadata. The RolesGuard uses Reflector to read this metadata and check if the user has any required role before allowing access.

NestJS

import { Controller, Get, UseGuards, SetMetadata, CanActivate, ExecutionContext, Injectable } from '@nestjs/common';
import { Reflector } from '@nestjs/core';

// Custom decorator to set roles metadata
export const Roles = (...roles: string[]) => SetMetadata('roles', roles);

// Guard that uses Reflector to check roles
@Injectable()
class RolesGuard implements CanActivate {
  constructor(private reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    const requiredRoles = this.reflector.get<string[]>('roles', context.getHandler());
    if (!requiredRoles) {
      return true; // no roles required
    }
    // For demo, pretend user roles come from request
    const request = context.switchToHttp().getRequest();
    const userRoles = request.user?.roles || [];
    return requiredRoles.some(role => userRoles.includes(role));
  }
}

@Controller('cats')
export class CatsController {
  @Get()
  @Roles('admin')
  @UseGuards(RolesGuard)
  findAll() {
    return 'This route is restricted to admins';
  }
}

Important Notes

Always inject Reflector via constructor to access metadata.

Metadata keys should be unique strings to avoid conflicts.

Custom decorators keep your code organized and reusable.

Summary

Reflector reads metadata set by custom decorators.

Custom decorators add extra info to routes or classes.

Use them together to control behavior like access or logging.

Practice

1. What is the primary purpose of the Reflector class in NestJS when used with custom decorators?

easy

A. To read metadata set by custom decorators on classes or methods

B. To create new decorators dynamically at runtime

C. To replace the default routing mechanism in controllers

D. To automatically validate request payloads

Solution

1. Step 1: Understand Reflector's role

   The Reflector class is designed to read metadata that custom decorators attach to classes or methods.

2. Step 2: Differentiate from other functions

   It does not create decorators, change routing, or validate payloads; it only reads metadata.

3. Final Answer:

   To read metadata set by custom decorators on classes or methods -> Option A

4. Quick Check:

   Reflector reads metadata = D [OK]

Hint: Reflector always reads metadata, never creates or changes routes [OK]

Common Mistakes:

• Thinking Reflector creates decorators
• Confusing Reflector with validation tools
• Assuming Reflector changes routing behavior

2. Which of the following is the correct way to define a custom decorator that sets metadata with key 'roles' and value passed as argument?

easy

A. export function Roles() { return SetMetadata('roles'); }

B. export function Roles(roles: string[]) { return Reflect.metadata('roles', roles); }

C. export const Roles = (roles: string[]) => Reflector.set('roles', roles);

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

Solution

1. Step 1: Recall SetMetadata usage

   SetMetadata is the NestJS helper to create decorators that attach metadata with a key and value.

2. Step 2: Check each option

   export const Roles = (roles: string[]) => SetMetadata('roles', roles); correctly returns SetMetadata with key 'roles' and the passed roles array. export function Roles(roles: string[]) { return Reflect.metadata('roles', roles); } uses Reflect.metadata which is not standard in NestJS. export const Roles = (roles: string[]) => Reflector.set('roles', roles); incorrectly calls Reflector.set which does not exist. export function Roles() { return SetMetadata('roles'); } calls SetMetadata without value, which is invalid.

3. Final Answer:

   export const Roles = (roles: string[]) => SetMetadata('roles', roles); -> Option D

4. Quick Check:

   SetMetadata creates decorators with key-value metadata = A [OK]

Hint: Use SetMetadata(key, value) to create custom decorators [OK]

Common Mistakes:

• Using Reflector to set metadata instead of reading
• Omitting the value argument in SetMetadata
• Using Reflect.metadata instead of SetMetadata

3. Given this code snippet, what will be logged when accessing the getData method?

import { Controller, Get, UseGuards, SetMetadata } from '@nestjs/common';
import { Reflector } from '@nestjs/core';

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

@Controller('items')
export class ItemsController {
  @Get()
  @Roles(['admin'])
  getData() {
    return 'secret data';
  }
}

// In a guard:
const reflector = new Reflector();
const roles = reflector.get<string[]>('roles', ItemsController.prototype.getData);
console.log(roles);

medium

A. ['admin']

B. undefined

C. An error because metadata key is missing

D. [] (empty array)

Solution

1. Step 1: Understand metadata retrieval context

   Reflector.get requires the target object and the property key (method name as string) to read metadata.

2. Step 2: Analyze the call

   The code calls reflector.get with ItemsController.prototype.getData as the second argument, which is a function, not the method name string 'getData'. This causes Reflector to fail to find metadata and return undefined.

3. Final Answer:

   undefined -> Option B

4. Quick Check:

   Reflector.get needs method name string, else returns undefined [OK]

Hint: Pass method name as string to Reflector.get to read metadata [OK]

Common Mistakes:

• Passing method function instead of method name string
• Expecting metadata on class prototype instead of method
• Assuming metadata always returns array

4. Identify the error in this custom decorator usage and how to fix it:

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

function Permissions(permissions: string[]) {
  return (target: any, key: string) => {
    Reflect.defineMetadata('permissions', permissions, target, key);
  };
}

class UserController {
  @Permissions(['read'])
  getUser() {
    return 'user';
  }
}

medium

A. The decorator must be applied to the class, not method

B. The decorator is missing the return type annotation

C. The decorator should use SetMetadata instead of Reflect.defineMetadata

D. The decorator function parameters are incorrect; should include descriptor

Solution

1. Step 1: Check decorator implementation

   The decorator manually calls Reflect.defineMetadata, which works but is not recommended in NestJS.

2. Step 2: Use NestJS helper for consistency

   SetMetadata is the standard NestJS helper that wraps Reflect.defineMetadata and ensures compatibility with Reflector and other NestJS internals.

3. Final Answer:

   The decorator should use SetMetadata instead of Reflect.defineMetadata -> Option C

4. Quick Check:

   Use SetMetadata for custom decorators in NestJS [OK]

Hint: Always use SetMetadata helper for custom decorators [OK]

Common Mistakes:

• Using Reflect.defineMetadata directly instead of SetMetadata
• Ignoring NestJS decorator conventions
• Misunderstanding decorator parameters

5. You want to create a guard that allows access only if the user has at least one role from the @Roles decorator metadata. Which approach correctly uses Reflector and the custom decorator to achieve this?

hard

A. Use Reflector.getAllAndOverride('roles', [context.getHandler(), context.getClass()]) to get roles, then check user roles against it

B. Use Reflector.get('roles', context.getHandler()) only, ignoring class-level metadata

C. Manually parse request headers for roles instead of using Reflector

D. Use Reflector.set('roles', roles) inside the guard to assign roles dynamically

Solution

1. Step 1: Understand metadata inheritance

   Roles can be set on method or class level. To get effective roles, use Reflector.getAllAndOverride which checks method first, then class.

2. Step 2: Implement guard logic

   Retrieve roles with getAllAndOverride, then compare with user's roles to allow or deny access.

3. Final Answer:

   Use Reflector.getAllAndOverride('roles', [context.getHandler(), context.getClass()]) to get roles, then check user roles against it -> Option A

4. Quick Check:

   getAllAndOverride merges metadata from method and class [OK]

Hint: Use getAllAndOverride to combine method and class metadata [OK]

Common Mistakes:

• Ignoring class-level metadata roles
• Trying to set metadata inside guard
• Not using Reflector to read metadata