Reflector and custom decorators in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a custom decorator that sets metadata with the key 'role'.

NestJS

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

export const Roles = (role: string) => SetMetadata('[1]', role);

Drag options to blanks, or click blank then click option'

Arole

Baccess

Cpermission

Droles

Attempts:

3 left

💡 Hint

Common Mistakes

Using singular 'role' instead of 'roles' causes metadata retrieval issues.

Using unrelated keys like 'permission' or 'access' won't match expected metadata.

The metadata key used in NestJS examples for roles is usually 'roles'. This key is used later to retrieve the metadata.

2fill in blank

medium

Complete the code to inject Reflector in a guard constructor.

NestJS

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

@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private readonly [1]: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    // guard logic
    return true;
  }
}

Drag options to blanks, or click blank then click option'

AReflector

Breflector

Creflect

DreflectorService

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Reflector' as variable name causes errors.

Using unrelated names like 'reflect' or 'reflectorService' breaks convention.

The injected Reflector instance is usually named 'reflector' in the constructor for clarity and convention.

3fill in blank

hard

Fix the error in retrieving metadata inside the guard using Reflector.

NestJS

const roles = this.reflector.get<string[]>('[1]', context.getHandler());

Drag options to blanks, or click blank then click option'

Aroles

Brole

Cpermission

Daccess

Attempts:

3 left

💡 Hint

Common Mistakes

Using singular 'role' causes the metadata to be undefined.

Using unrelated keys like 'permission' or 'access' returns no metadata.

The metadata key must match the one used in the custom decorator, which is 'roles'.

4fill in blank

hard

Fill both blanks to create a custom decorator and retrieve metadata in a guard.

NestJS

export const Roles = (role: string) => SetMetadata([1], role);

const roles = this.reflector.get<string[]>([2], context.getHandler());

Drag options to blanks, or click blank then click option'

A'roles'

B'role'

C'permissions'

D'access'

Attempts:

3 left

💡 Hint

Common Mistakes

Using different keys in decorator and reflector causes metadata mismatch.

Forgetting quotes around the key causes syntax errors.

Both the decorator and the reflector must use the same metadata key 'roles' to work correctly.

5fill in blank

hard

Fill all three blanks to create a guard that checks roles metadata and returns access decision.

NestJS

canActivate(context: ExecutionContext): boolean {
  const roles = this.reflector.get<string[]>([1], context.getHandler());
  if (!roles) {
    return [2];
  }
  const request = context.switchToHttp().getRequest();
  const user = request.user;
  return roles.some(role => user.roles?.includes([3]));
}

Drag options to blanks, or click blank then click option'

A'roles'

Btrue

Crole

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Returning false when no roles metadata causes unwanted blocking.

Using wrong variable names inside the some() callback causes errors.

Using wrong metadata key causes roles to be undefined.

The guard retrieves 'roles' metadata, returns true if no roles are set, and checks if the user has any required role.

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