nestjs-expert

NestJS Expert

Stack: NestJS + MongoDB/Mongoose + TypeScript strict mode.

Module architecture

Every feature is a self-contained module. No cross-module direct imports — use exported providers.

src/
├── app.module.ts           # Root — imports feature modules only
├── common/                 # Shared guards, pipes, filters, interceptors
│   ├── filters/
│   ├── guards/
│   ├── interceptors/
│   └── pipes/
├── config/                 # ConfigModule setup
└── {feature}/
    ├── {feature}.module.ts
    ├── {feature}.controller.ts
    ├── {feature}.service.ts
    ├── {feature}.repository.ts  # optional, wraps Mongoose model
    ├── dto/
    │   ├── create-{feature}.dto.ts
    │   └── update-{feature}.dto.ts
    ├── schemas/
    │   └── {feature}.schema.ts
    └── {feature}.types.ts

Dependency injection rules

• Inject interfaces, not concrete classes where possible
• Use @Injectable({ scope: Scope.DEFAULT }) (singleton) unless you need request-scoped
• Circular deps = architectural problem — fix with forwardRef only as last resort
• Test with Test.createTestingModule — always mock external services

Controllers

@Controller('resources')
@UseGuards(JwtAuthGuard)
@UseInterceptors(ResponseTransformInterceptor)
export class ResourceController {
  constructor(private readonly resourceService: ResourceService) {}

  @Get()
  async findAll(@Query() query: PaginationQueryDto) {
    return this.resourceService.findAll(query);
  }

  @Post()
  @HttpCode(HttpStatus.CREATED)
  async create(@Body() dto: CreateResourceDto, @CurrentUser() user: UserDocument) {
    return this.resourceService.create(dto, user._id);
  }
}

Rules:

• Controllers are thin — no business logic, no DB calls
• Always type @Body(), @Query(), @Param() with DTOs
• Use @CurrentUser() custom decorator, never @Req()

DTOs + validation

import { IsString, IsEnum, IsOptional, MinLength, MaxLength } from 'class-validator';
import { Transform } from 'class-transformer';

export class CreateResourceDto {
  @IsString()
  @MinLength(1)
  @MaxLength(255)
  name: string;

  @IsEnum(ResourceStatus)
  status: ResourceStatus;

  @IsOptional()
  @IsString()
  @Transform(({ value }) => value?.trim())
  description?: string;
}

Global validation pipe in main.ts:

app.useGlobalPipes(new ValidationPipe({
  whitelist: true,        // strip unknown props
  forbidNonWhitelisted: true,
  transform: true,        // auto-transform primitives
  transformOptions: { enableImplicitConversion: true },
}));

MongoDB / Mongoose

// schema
@Schema({ timestamps: true, versionKey: false })
export class Resource {
  @Prop({ required: true, index: true })
  name: string;

  @Prop({ type: Types.ObjectId, ref: 'User', required: true, index: true })
  userId: Types.ObjectId;

  @Prop({ enum: ResourceStatus, default: ResourceStatus.ACTIVE })
  status: ResourceStatus;
}

export const ResourceSchema = SchemaFactory.createForClass(Resource);
export type ResourceDocument = Resource & Document;

// service
@Injectable()
export class ResourceService {
  constructor(
    @InjectModel(Resource.name) private readonly model: Model<ResourceDocument>,
  ) {}

  async findAll(userId: Types.ObjectId, query: PaginationQueryDto) {
    const { page = 1, limit = 20 } = query;
    return this.model
      .find({ userId, deletedAt: null })
      .sort({ createdAt: -1 })
      .skip((page - 1) * limit)
      .limit(limit)
      .lean()
      .exec();
  }
}

Rules:

• Always .lean() for read queries (plain objects, ~30% faster)
• Always .exec() to get a real Promise
• Use Types.ObjectId not string for references in service layer
• Soft delete: deletedAt: Date | null, never hard delete user data

Auth pattern

// JWT strategy
@Injectable()
export class JwtStrategy extends PassportStrategy(Strategy) {
  constructor(configService: ConfigService) {
    super({
      jwtFromRequest: ExtractJwt.fromAuthHeaderAsBearerToken(),
      secretOrKey: configService.get<string>('JWT_SECRET'),
      ignoreExpiration: false,
    });
  }

  async validate(payload: JwtPayload): Promise<UserDocument> {
    // return value is injected as req.user
    return { _id: payload.sub, email: payload.email };
  }
}

// custom decorator
export const CurrentUser = createParamDecorator(
  (data: unknown, ctx: ExecutionContext) => ctx.switchToHttp().getRequest().user,
);

Guards

@Injectable()
export class ResourceOwnerGuard implements CanActivate {
  constructor(private readonly resourceService: ResourceService) {}

  async canActivate(context: ExecutionContext): Promise<boolean> {
    const { user, params } = context.switchToHttp().getRequest();
    const resource = await this.resourceService.findById(params.id);
    return resource?.userId.equals(user._id) ?? false;
  }
}

Exception filter

@Catch()
export class GlobalExceptionFilter implements ExceptionFilter {
  private readonly logger = new Logger(GlobalExceptionFilter.name);

  catch(exception: unknown, host: ArgumentsHost) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse<Response>();

    if (exception instanceof HttpException) {
      return response.status(exception.getStatus()).json({
        statusCode: exception.getStatus(),
        message: exception.message,
      });
    }

    this.logger.error('Unhandled exception', exception instanceof Error ? exception.stack : exception);
    return response.status(500).json({ statusCode: 500, message: 'Internal server error' });
  }
}

Config

// config/app.config.ts
export default registerAs('app', () => ({
  port: parseInt(process.env.PORT ?? '3000', 10),
  jwtSecret: process.env.JWT_SECRET,
  mongoUri: process.env.MONGO_URI,
}));

// access in service
constructor(private config: ConfigService) {}
const port = this.config.get<number>('app.port');

Never use process.env directly outside config files.

Performance rules

• lean() on all read queries
• Add indexes for every field used in find() filter or sort()
• Compound indexes for multi-field queries: { userId: 1, createdAt: -1 }
• Use select() to project only needed fields on large documents
• Cache with @nestjs/cache-manager for expensive reads

Common mistakes

Wrong	Right
Business logic in controller	Move to service
any type anywhere	Define interface/DTO
console.log	new Logger(ClassName.name)
req.user directly	@CurrentUser() decorator
Hard-coding env vars	ConfigService
.find() without .lean() on reads	Always .lean().exec()
string for ObjectId refs	Types.ObjectId

Related skills

• nestjs-queue-architect — BullMQ async job patterns
• mongodb-migration-expert — schema migrations
• error-handling-expert — global error strategy