lcbp3/.agents/skills/nestjs-best-practices/rules/api-use-dto-serialization.md

---
title: Use DTOs and Serialization for API Responses
impact: MEDIUM
impactDescription: Response DTOs prevent accidental data exposure and ensure consistency
tags: api, dto, serialization, class-transformer
---

## Use DTOs and Serialization for API Responses

Never return entity objects directly from controllers. Use response DTOs with class-transformer's `@Exclude()` and `@Expose()` decorators to control exactly what data is sent to clients. This prevents accidental exposure of sensitive fields and provides a stable API contract.

**Incorrect (returning entities directly or manual spreading):**

```typescript
// Return entities directly
@Controller('users')
export class UsersController {
  @Get(':id')
  async findOne(@Param('id') id: string): Promise<User> {
    return this.usersService.findById(id);
    // Returns: { id, email, passwordHash, ssn, internalNotes, ... }
    // Exposes sensitive data!
  }
}

// Manual object spreading (error-prone)
@Get(':id')
async findOne(@Param('id') id: string) {
  const user = await this.usersService.findById(id);
  return {
    id: user.id,
    email: user.email,
    name: user.name,
    // Easy to forget to exclude sensitive fields
    // Hard to maintain across endpoints
  };
}
```

**Correct (use class-transformer with @Exclude and response DTOs):**

```typescript
// Enable class-transformer globally
async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  app.useGlobalInterceptors(new ClassSerializerInterceptor(app.get(Reflector)));
  await app.listen(3000);
}

// Entity with serialization control
@Entity()
export class User {
  @PrimaryGeneratedColumn('uuid')
  id: string;

  @Column()
  email: string;

  @Column()
  name: string;

  @Column()
  @Exclude() // Never include in responses
  passwordHash: string;

  @Column({ nullable: true })
  @Exclude()
  ssn: string;

  @Column({ default: false })
  @Exclude({ toPlainOnly: true }) // Exclude from response, allow in requests
  isAdmin: boolean;

  @CreateDateColumn()
  createdAt: Date;

  @Column()
  @Exclude()
  internalNotes: string;
}

// Now returning entity is safe
@Controller('users')
export class UsersController {
  @Get(':id')
  async findOne(@Param('id') id: string): Promise<User> {
    return this.usersService.findById(id);
    // Returns: { id, email, name, createdAt }
    // Sensitive fields excluded automatically
  }
}

// For different response shapes, use explicit DTOs
export class UserResponseDto {
  @Expose()
  id: string;

  @Expose()
  email: string;

  @Expose()
  name: string;

  @Expose()
  @Transform(({ obj }) => obj.posts?.length || 0)
  postCount: number;

  constructor(partial: Partial<User>) {
    Object.assign(this, partial);
  }
}

export class UserDetailResponseDto extends UserResponseDto {
  @Expose()
  createdAt: Date;

  @Expose()
  @Type(() => PostResponseDto)
  posts: PostResponseDto[];
}

// Controller with explicit DTOs
@Controller('users')
export class UsersController {
  @Get()
  @SerializeOptions({ type: UserResponseDto })
  async findAll(): Promise<UserResponseDto[]> {
    const users = await this.usersService.findAll();
    return users.map(u => plainToInstance(UserResponseDto, u));
  }

  @Get(':id')
  async findOne(@Param('id') id: string): Promise<UserDetailResponseDto> {
    const user = await this.usersService.findByIdWithPosts(id);
    return plainToInstance(UserDetailResponseDto, user, {
      excludeExtraneousValues: true,
    });
  }
}

// Groups for conditional serialization
export class UserDto {
  @Expose()
  id: string;

  @Expose()
  name: string;

  @Expose({ groups: ['admin'] })
  email: string;

  @Expose({ groups: ['admin'] })
  createdAt: Date;

  @Expose({ groups: ['admin', 'owner'] })
  settings: UserSettings;
}

@Controller('users')
export class UsersController {
  @Get()
  @SerializeOptions({ groups: ['public'] })
  async findAllPublic(): Promise<UserDto[]> {
    // Returns: { id, name }
  }

  @Get('admin')
  @UseGuards(AdminGuard)
  @SerializeOptions({ groups: ['admin'] })
  async findAllAdmin(): Promise<UserDto[]> {
    // Returns: { id, name, email, createdAt }
  }

  @Get('me')
  @SerializeOptions({ groups: ['owner'] })
  async getProfile(@CurrentUser() user: User): Promise<UserDto> {
    // Returns: { id, name, settings }
  }
}
```

Reference: [NestJS Serialization](https://docs.nestjs.com/techniques/serialization)