Custom Decorators

NestJS Custom Decorators

When to Use This Skill

Use this skill when:

• Extracting specific properties from request objects
• Creating shortcuts for common parameter patterns
• Setting metadata for guards, interceptors, or filters
• Composing multiple decorators into one
• Implementing role-based access control
• Creating domain-specific abstractions
• Reducing boilerplate in controllers
• Building reusable decorator libraries

What are Custom Decorators?

Decorators are TypeScript/ES2016 features that allow you to add metadata and modify classes, methods, or parameters. NestJS extensively uses decorators and provides utilities to create your own.

Types of Custom Decorators

1. Parameter decorators - Extract data from request/context 2. Method decorators - Add metadata to route handlers 3. Class decorators - Add metadata to controllers 4. Composed decorators - Combine multiple decorators

Built-in Parameter Decorators

NestJS provides these built-in decorators:

@Request(), @Req()          // req
@Response(), @Res()         // res
@Next()                     // next
@Session()                  // req.session
@Param(key?: string)        // req.params / req.params[key]
@Body(key?: string)         // req.body / req.body[key]
@Query(key?: string)        // req.query / req.query[key]
@Headers(key?: string)      // req.headers / req.headers[key]
@Ip()                       // req.ip
@HostParam()                // req.hosts

Creating Parameter Decorators

Basic User Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const User = createParamDecorator(
  (data: unknown, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    return request.user;
  },
);

Usage:

@Controller('users')
export class UsersController {
  @Get('profile')
  getProfile(@User() user: UserEntity) {
    return user;
  }
}

Decorator with Data Parameter

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const User = createParamDecorator(
  (data: string, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    const user = request.user;

    return data ? user?.[data] : user;
  },
);

Usage:

@Get('profile')
getProfile(@User('id') userId: string) {
  return `User ID: ${userId}`;
}

@Get('email')
getEmail(@User('email') email: string) {
  return `Email: ${email}`;
}

Current User ID Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const CurrentUserId = createParamDecorator(
  (data: unknown, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    return request.user?.id;
  },
);

Usage:

@Get('posts')
getUserPosts(@CurrentUserId() userId: string) {
  return this.postsService.findByUser(userId);
}

IP Address Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const IpAddress = createParamDecorator(
  (data: unknown, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    return request.ip || request.connection.remoteAddress;
  },
);

Usage:

@Post('login')
login(@IpAddress() ip: string) {
  return `Login from IP: ${ip}`;
}

Working with Pipes

Custom decorators work seamlessly with pipes:

@Get()
findOne(@User(ValidationPipe) user: UserEntity) {
  return user;
}

@Get()
findOne(@User('id', ParseIntPipe) userId: number) {
  return userId;
}

Setting Metadata with Decorators

Roles Decorator

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

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

Usage:

@Controller('cats')
export class CatsController {
  @Post()
  @Roles('admin')
  create(@Body() createCatDto: CreateCatDto) {
    return 'Only admins can create cats';
  }
}

Public Route Decorator

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

export const IS_PUBLIC_KEY = 'isPublic';
export const Public = () => SetMetadata(IS_PUBLIC_KEY, true);

Usage:

@Controller('auth')
export class AuthController {
  @Public()
  @Post('login')
  login() {
    return 'This route is public';
  }
}

Permissions Decorator

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

export const PERMISSIONS_KEY = 'permissions';
export const RequirePermissions = (...permissions: string[]) =>
  SetMetadata(PERMISSIONS_KEY, permissions);

Usage:

@Controller('posts')
export class PostsController {
  @Get()
  @RequirePermissions('posts:read')
  findAll() {
    return [];
  }

  @Delete(':id')
  @RequirePermissions('posts:delete', 'posts:admin')
  remove(@Param('id') id: string) {
    return `Deleted post ${id}`;
  }
}

Decorator Composition

Combine multiple decorators into a single decorator:

import { applyDecorators } from '@nestjs/common';
import { ApiResponse, ApiOperation } from '@nestjs/swagger';

export function ApiStandardResponse(description: string) {
  return applyDecorators(
    ApiOperation({ summary: description }),
    ApiResponse({ status: 200, description: 'Success' }),
    ApiResponse({ status: 400, description: 'Bad Request' }),
    ApiResponse({ status: 401, description: 'Unauthorized' }),
    ApiResponse({ status: 500, description: 'Internal Server Error' }),
  );
}

Usage:

@Get()
@ApiStandardResponse('Get all cats')
findAll() {
  return [];
}

Auth Decorator Composition

import { applyDecorators, UseGuards } from '@nestjs/common';
import { AuthGuard } from './guards/auth.guard';
import { RolesGuard } from './guards/roles.guard';
import { Roles } from './decorators/roles.decorator';

export function Auth(...roles: string[]) {
  return applyDecorators(
    UseGuards(AuthGuard, RolesGuard),
    Roles(...roles),
  );
}

Usage:

@Controller('admin')
export class AdminController {
  @Get('dashboard')
  @Auth('admin', 'superadmin')
  getDashboard() {
    return 'Admin dashboard';
  }
}

API Documentation Decorator

import { applyDecorators, Type } from '@nestjs/common';
import {
  ApiOkResponse,
  ApiCreatedResponse,
  ApiBadRequestResponse,
  ApiNotFoundResponse,
  ApiTags,
} from '@nestjs/swagger';

export const ApiController = (tag: string) => {
  return applyDecorators(ApiTags(tag));
};

export const ApiCreate = <TModel extends Type<any>>(model: TModel) => {
  return applyDecorators(
    ApiCreatedResponse({ type: model }),
    ApiBadRequestResponse({ description: 'Invalid input' }),
  );
};

export const ApiFindOne = <TModel extends Type<any>>(model: TModel) => {
  return applyDecorators(
    ApiOkResponse({ type: model }),
    ApiNotFoundResponse({ description: 'Not found' }),
  );
};

Usage:

@Controller('cats')
@ApiController('cats')
export class CatsController {
  @Post()
  @ApiCreate(Cat)
  create(@Body() createCatDto: CreateCatDto) {
    return this.catsService.create(createCatDto);
  }

  @Get(':id')
  @ApiFindOne(Cat)
  findOne(@Param('id') id: string) {
    return this.catsService.findOne(id);
  }
}

Request Headers Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const Headers = createParamDecorator(
  (data: string, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    return data ? request.headers[data.toLowerCase()] : request.headers;
  },
);

Usage:

@Get()
findAll(@Headers('authorization') auth: string) {
  return `Auth header: ${auth}`;
}

Cookies Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const Cookies = createParamDecorator(
  (data: string, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    return data ? request.cookies?.[data] : request.cookies;
  },
);

Usage:

@Get()
findAll(@Cookies('sessionId') sessionId: string) {
  return `Session ID: ${sessionId}`;
}

Protocol Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const Protocol = createParamDecorator(
  (data: unknown, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    return request.protocol;
  },
);

Usage:

@Get()
findAll(@Protocol() protocol: string) {
  return `Protocol: ${protocol}`;
}

Hostname Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const Hostname = createParamDecorator(
  (data: unknown, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    return request.hostname;
  },
);

Usage:

@Get()
findAll(@Hostname() hostname: string) {
  return `Hostname: ${hostname}`;
}

Request Context Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export interface RequestContext {
  user: any;
  ip: string;
  userAgent: string;
  timestamp: Date;
}

export const ReqContext = createParamDecorator(
  (data: unknown, ctx: ExecutionContext): RequestContext => {
    const request = ctx.switchToHttp().getRequest();
    return {
      user: request.user,
      ip: request.ip,
      userAgent: request.headers['user-agent'],
      timestamp: new Date(),
    };
  },
);

Usage:

@Get()
findAll(@ReqContext() context: RequestContext) {
  console.log('Request from:', context.ip, 'at', context.timestamp);
  return context.user;
}

Pagination Decorator

import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export interface PaginationParams {
  page: number;
  limit: number;
  offset: number;
}

export const Pagination = createParamDecorator(
  (data: unknown, ctx: ExecutionContext): PaginationParams => {
    const request = ctx.switchToHttp().getRequest();
    const page = parseInt(request.query.page) || 1;
    const limit = parseInt(request.query.limit) || 10;

    return {
      page,
      limit,
      offset: (page - 1) * limit,
    };
  },
);

Usage:

@Get()
findAll(@Pagination() pagination: PaginationParams) {
  return this.catsService.findAll(pagination.limit, pagination.offset);
}

Combining Parameter Decorators

@Get('profile')
getProfile(
  @User('id') userId: string,
  @IpAddress() ip: string,
  @Headers('user-agent') userAgent: string,
) {
  return {
    userId,
    ip,
    userAgent,
  };
}

Timeout Decorator

import { applyDecorators, UseInterceptors } from '@nestjs/common';
import { TimeoutInterceptor } from './interceptors/timeout.interceptor';

export function Timeout(milliseconds: number) {
  return applyDecorators(UseInterceptors(new TimeoutInterceptor(milliseconds)));
}

Usage:

@Get()
@Timeout(5000)
findAll() {
  return this.catsService.findAll();
}

Cache Decorator

import { applyDecorators, UseInterceptors } from '@nestjs/common';
import { CacheInterceptor, CacheTTL } from '@nestjs/cache-manager';

export function Cache(ttl?: number) {
  const decorators = [UseInterceptors(CacheInterceptor)];

  if (ttl) {
    decorators.push(CacheTTL(ttl));
  }

  return applyDecorators(...decorators);
}

Usage:

@Get()
@Cache(60000) // Cache for 60 seconds
findAll() {
  return this.catsService.findAll();
}

Complete Example

// decorators/user.decorator.ts
import { createParamDecorator, ExecutionContext } from '@nestjs/common';

export const User = createParamDecorator(
  (data: string, ctx: ExecutionContext) => {
    const request = ctx.switchToHttp().getRequest();
    const user = request.user;
    return data ? user?.[data] : user;
  },
);

// decorators/roles.decorator.ts
import { SetMetadata } from '@nestjs/common';

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

// decorators/public.decorator.ts
import { SetMetadata } from '@nestjs/common';

export const IS_PUBLIC_KEY = 'isPublic';
export const Public = () => SetMetadata(IS_PUBLIC_KEY, true);

// decorators/auth.decorator.ts
import { applyDecorators, UseGuards } from '@nestjs/common';
import { AuthGuard } from '../guards/auth.guard';
import { RolesGuard } from '../guards/roles.guard';
import { Roles } from './roles.decorator';

export function Auth(...roles: string[]) {
  return applyDecorators(
    UseGuards(AuthGuard, RolesGuard),
    Roles(...roles),
  );
}

// decorators/api-response.decorator.ts
import { applyDecorators, Type } from '@nestjs/common';
import {
  ApiOkResponse,
  ApiCreatedResponse,
  ApiBadRequestResponse,
  ApiUnauthorizedResponse,
  ApiNotFoundResponse,
} from '@nestjs/swagger';

export function ApiStandardResponses() {
  return applyDecorators(
    ApiBadRequestResponse({ description: 'Bad Request' }),
    ApiUnauthorizedResponse({ description: 'Unauthorized' }),
  );
}

export function ApiCreateResponse<TModel extends Type<any>>(model: TModel) {
  return applyDecorators(
    ApiStandardResponses(),
    ApiCreatedResponse({ type: model }),
  );
}

export function ApiFindOneResponse<TModel extends Type<any>>(model: TModel) {
  return applyDecorators(
    ApiStandardResponses(),
    ApiOkResponse({ type: model }),
    ApiNotFoundResponse({ description: 'Not Found' }),
  );
}

// Usage in controller
import { Controller, Get, Post, Body, Param } from '@nestjs/common';
import { ApiTags } from '@nestjs/swagger';
import { User } from './decorators/user.decorator';
import { Public } from './decorators/public.decorator';
import { Auth } from './decorators/auth.decorator';
import { ApiCreateResponse, ApiFindOneResponse } from './decorators/api-response.decorator';
import { Cat } from './entities/cat.entity';
import { CreateCatDto } from './dto/create-cat.dto';

@Controller('cats')
@ApiTags('cats')
export class CatsController {
  @Get()
  @Public()
  findAll() {
    return this.catsService.findAll();
  }

  @Get(':id')
  @ApiFindOneResponse(Cat)
  findOne(@Param('id') id: string) {
    return this.catsService.findOne(id);
  }

  @Post()
  @Auth('admin')
  @ApiCreateResponse(Cat)
  create(
    @Body() createCatDto: CreateCatDto,
    @User('id') userId: string,
  ) {
    return this.catsService.create(createCatDto, userId);
  }

  @Get('my-cats')
  @Auth('user')
  getMyCats(@User('id') userId: string) {
    return this.catsService.findByUser(userId);
  }
}

Best Practices

1. Keep decorators simple - Each decorator should have a single purpose 2. Use descriptive names - Make decorator purpose clear from the name 3. Document decorators - Provide JSDoc comments explaining usage 4. Type decorators properly - Use TypeScript generics for type safety 5. Compose when appropriate - Combine related decorators 6. Extract reusable logic - Create decorators for common patterns 7. Use metadata constants - Export metadata keys for consistency 8. Validate decorator inputs - Check parameters before setting metadata 9. Consider backwards compatibility - Don't break existing decorator usage 10. Test decorators - Unit test custom decorator behavior

Decorator Naming Conventions

• Parameter decorators: Nouns (User, IpAddress, Pagination)
• Metadata decorators: Verbs or adjectives (Roles, Public, RequirePermissions)
• Composed decorators: Descriptive combinations (Auth, ApiStandardResponse)

TypeScript Decorator Types

// Parameter decorator
function paramDecorator(
  target: Object,
  propertyKey: string | symbol,
  parameterIndex: number
): void {}

// Method decorator
function methodDecorator(
  target: any,
  propertyKey: string,
  descriptor: PropertyDescriptor
): PropertyDescriptor {}

// Class decorator
function classDecorator<T extends { new(...args: any[]): {} }>(
  constructor: T
): T {}

// Property decorator
function propertyDecorator(
  target: Object,
  propertyKey: string | symbol
): void {}

Advanced Patterns

Conditional Decorator

export function ConditionalAuth(condition: boolean) {
  if (condition) {
    return Auth('admin');
  }
  return () => {};
}

Dynamic Metadata

export function DynamicRoles(getRoles: () => string[]) {
  const roles = getRoles();
  return Roles(...roles);
}

Common Use Cases

1. Authentication - Extract user from request 2. Authorization - Set roles and permissions 3. API Documentation - Compose Swagger decorators 4. Request Context - Extract IP, user agent, headers 5. Pagination - Parse page and limit parameters 6. Logging - Add metadata for logging interceptors 7. Caching - Control cache behavior 8. Rate Limiting - Set rate limit metadata

Custom decorators are powerful tools for creating clean, reusable, and maintainable NestJS applications. They help reduce boilerplate and create domain-specific abstractions that make your code more expressive and easier to understand.