Ejirocodes Agent Skills Nestjs Best Practices

NestJS 11 Best Practices

Quick Reference

Essential Patterns

Module with Providers

@Module({
  imports: [DatabaseModule],
  controllers: [UsersController],
  providers: [UsersService],
  exports: [UsersService], // Export for other modules
})
export class UsersModule {}

Controller with Validation

@Controller('users')
export class UsersController {
  constructor(private readonly usersService: UsersService) {}

  @Post()
  create(@Body() createUserDto: CreateUserDto) {
    return this.usersService.create(createUserDto);
  }

  @Get(':id')
  findOne(@Param('id', ParseIntPipe) id: number) {
    return this.usersService.findOne(id);
  }
}

DTO with Validation

import { IsEmail, IsString, MinLength, IsOptional } from 'class-validator';

export class CreateUserDto {
  @IsEmail()
  email: string;

  @IsString()
  @MinLength(8)
  password: string;

  @IsOptional()
  @IsString()
  name?: string;
}

Exception Filter

@Catch(HttpException)
export class HttpExceptionFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse<Response>();
    const status = exception.getStatus();

    response.status(status).json({
      statusCode: status,
      message: exception.message,
      timestamp: new Date().toISOString(),
    });
  }
}

Guard with JWT

@Injectable()
export class JwtAuthGuard extends AuthGuard('jwt') {
  canActivate(context: ExecutionContext) {
    return super.canActivate(context);
  }
}

NestJS 11 Breaking Changes

• Express v5: Wildcards must be named (e.g., *splat), optional params use braces /:file{.:ext}
• Node.js 20+: Minimum required version
• Fastify v5: Updated adapter for Fastify users
• Dynamic Modules: Same module with identical config imported multiple times = separate instances

Common Mistakes

1. Not using forwardRef() for circular deps - Causes "cannot resolve dependency" errors; wrap in forwardRef(() => ModuleName)
2. Throwing plain errors instead of HttpException - Loses status codes, breaks exception filters; use throw new BadRequestException('message')
3. Missing @Injectable() decorator - Provider won't be injectable; always decorate services
4. Global ValidationPipe without whitelist: true - Allows unexpected properties; set whitelist: true, forbidNonWhitelisted: true
5. Importing modules instead of exporting providers - Use exports array to share providers across modules
6. Async config without ConfigModule.forRoot() - ConfigService undefined; import ConfigModule in AppModule
7. Testing without overrideProvider() - Uses real services in unit tests; mock dependencies with overrideProvider(Service).useValue(mock)
8. E2E tests sharing database state - No isolation between tests; use transactions or truncate tables in beforeEach

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