Reflector and custom decorators in NestJS - Mini Project: Build & Apply

Using Reflector and Custom Decorators in NestJS

📖 Scenario: You are building a NestJS application that needs to handle user roles for access control. You want to create a custom decorator to mark which roles can access certain routes, and then use the Reflector service to read these roles inside a guard.

🎯 Goal: Build a custom @Roles() decorator that stores roles metadata on route handlers, and use the Reflector service inside a guard to read this metadata and decide access.

📋 What You'll Learn

Create a custom decorator called Roles that accepts an array of strings representing roles

Use SetMetadata to attach the roles metadata with the key 'roles'

Create a guard class called RolesGuard that injects Reflector

Inside the guard, use Reflector.getAllAndOverride to get the roles metadata for the current route handler

Return true from the guard if no roles are set, or if the user has at least one required role

💡 Why This Matters

🌍 Real World

Role-based access control is common in web applications to restrict certain routes to users with specific permissions or roles.

💼 Career

Understanding how to create custom decorators and use Reflector in NestJS is essential for building secure, maintainable backend applications.

1

Create the custom Roles decorator

Create a custom decorator called Roles that uses SetMetadata to attach an array of roles with the key 'roles'. The decorator should accept a parameter called roles which is an array of strings.

NestJS

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

// Create the Roles decorator here
// Your code here

Hint

Use SetMetadata from @nestjs/common to create the decorator.

2

Create the RolesGuard class and inject Reflector

Create a class called RolesGuard that implements CanActivate. Inject Reflector via the constructor and store it in a private readonly property called reflector.

NestJS

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

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

@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private readonly reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    // Your code here
  }
}

Hint

Use @Injectable() and implement CanActivate. Inject Reflector in the constructor.

3

Use Reflector.getAllAndOverride to get roles metadata inside canActivate

Inside the canActivate method of RolesGuard, use this.reflector.getAllAndOverride with the key 'roles' and pass context.getHandler() and context.getClass() as arguments. Store the result in a variable called requiredRoles.

NestJS

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

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

@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private readonly reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    // Get roles metadata here
    // Your code here
  }
}

Hint

Use getAllAndOverride with the key 'roles' and pass an array with context.getHandler() and context.getClass().

4

Complete canActivate to check user roles and return access decision

Complete the canActivate method to return true if requiredRoles is undefined. Otherwise, get the user roles from context.switchToHttp().getRequest().user.roles and return true if the user has at least one role in requiredRoles. Return false otherwise.

NestJS

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

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

@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private readonly reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    const requiredRoles = this.reflector.getAllAndOverride('roles', [
      context.getHandler(),
      context.getClass(),
    ]);
    // Complete the access check below
    // Your code here
  }
}

Hint

Check if requiredRoles is undefined, then allow access. Otherwise, get user.roles from the request and check if any role matches.

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