1. Basic NestJS Questions

1. What is NestJS?

NestJS is a progressive Node.js framework for building scalable and maintainable backend applications using TypeScript. It is built on top of Express.js (or Fastify) and follows an Angular-inspired modular architecture.

2. What are the main features of NestJS?

• TypeScript support

• Modular architecture

• Built-in Dependency Injection

• Support for Express.js & Fastify

• Middleware and Guards

• WebSockets and GraphQL support

3. What is the entry point of a NestJS application?

The entry point is the main.ts file. It bootstraps the application using NestFactory.create().

import { NestFactory } from '@nestjs/core';
import { AppModule } from './app.module';

async function bootstrap() {
  const app = await NestFactory.create(AppModule);
  await app.listen(3000);
}
bootstrap();

4. What are modules in NestJS?

Modules in NestJS are used to organize the application into logical units. Each module is a class decorated with @Module().

Example:

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

@Module({
  imports: [],
  controllers: [],
  providers: [],
})
export class AppModule {}

5. How do you create a new module in NestJS?

Run the following CLI command:

nest g module <module-name>

2. Controllers & Routes

6. What are controllers in NestJS?

Controllers handle incoming HTTP requests and return responses. They are decorated with @Controller().

7. How do you define a simple controller in NestJS?

import { Controller, Get } from '@nestjs/common';

@Controller('users')
export class UserController {
  @Get()
  findAll() {
    return 'This action returns all users';
  }
}

8. How do you define dynamic routes in NestJS?

@Get(':id')
findOne(@Param('id') id: string) {
  return `User with ID ${id}`;
}

9. What are route parameters and how do you access them?

Route parameters are dynamic values in a route. You access them using @Param().

Example:

@Get(':id')
getUser(@Param('id') id: string) {
  return `User ID: ${id}`;
}

10. What is the purpose of the @Body() decorator?

@Body() extracts the request body data.

Example:

@Post()
createUser(@Body() userDto: CreateUserDto) {
  return userDto;
}

3. Services & Dependency Injection

11. What are services in NestJS?

Services handle business logic and are used for code reusability. They are decorated with @Injectable().

12. How do you create a service in NestJS?

Run:

nest g service <service-name>

13. How do you inject a service into a controller?

@Controller('users')
export class UserController {
  constructor(private readonly userService: UserService) {}

  @Get()
  getUsers() {
    return this.userService.findAll();
  }
}

14. What is Dependency Injection in NestJS?

It allows NestJS to automatically provide required dependencies, improving modularity and testability.

15. What is the @Injectable() decorator?

It marks a class as a provider that can be injected into other components.

4. Middleware, Guards, and Interceptors

16. What is middleware in NestJS?

Middleware is a function that runs before the request reaches the controller.

17. How do you create middleware in NestJS?

@Injectable()
export class LoggerMiddleware implements NestMiddleware {
  use(req: Request, res: Response, next: NextFunction) {
    console.log(`Request...`);
    next();
  }
}

18. What are guards in NestJS?

Guards are used for authentication & authorization.

Example:

@Injectable()
export class AuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    return true; // or false based on logic
  }
}

19. How do you apply guards to a route?

@UseGuards(AuthGuard)
@Get('protected')
getProtectedData() {
  return 'This is protected data';
}

20. What are interceptors in NestJS?

Interceptors modify requests or responses before reaching the client.

Example:

@Injectable()
export class LoggingInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<any> {
    console.log('Before request...');
    return next.handle();
  }
}

5. Database & ORM Integration

21. What are the supported ORMs in NestJS?

• TypeORM

• Sequelize

• Prisma

• Mongoose

22. How do you configure TypeORM in NestJS?

@Module({
  imports: [
    TypeOrmModule.forRoot({
      type: 'postgres',
      host: 'localhost',
      port: 5432,
      username: 'user',
      password: 'password',
      database: 'test',
      entities: [User],
      synchronize: true,
    }),
  ],
})
export class AppModule {}

23. What is the @Entity() decorator in TypeORM?

It marks a class as a database entity.

@Entity()
export class User {
  @PrimaryGeneratedColumn()
  id: number;

  @Column()
  name: string;
}

24. How do you create a repository in NestJS?

@Injectable()
export class UserService {
  constructor(
    @InjectRepository(User)
    private userRepository: Repository<User>,
  ) {}

  findAll() {
    return this.userRepository.find();
  }
}

25. How do you integrate MongoDB with NestJS?

@Module({
  imports: [MongooseModule.forRoot('mongodb://localhost/nest')],
})
export class AppModule {}

6. Testing in NestJS

26. What is Jest and why is it used in NestJS?

Jest is a testing framework used for writing unit and integration tests.

27. How do you write a simple test in NestJS?

describe('UserService', () => {
  it('should return users', async () => {
    expect(await userService.findAll()).toEqual([]);
  });
});

28. How do you mock a service in Jest?

jest.mock('./user.service');

7. Advanced Topics

29. What is GraphQL and how does NestJS support it?

GraphQL is a query language for APIs. NestJS provides @nestjs/graphql package.

@Resolver()
export class UserResolver {
  @Query(() => String)
  sayHello() {
    return 'Hello, GraphQL!';
  }
}

30. What are microservices in NestJS?

NestJS supports Microservice Architecture using Redis, MQTT, Kafka, and gRPC.

8. Real-Time Features in NestJS

31. How does NestJS support WebSockets?

NestJS provides built-in WebSocket support using @nestjs/websockets.

Example:

@WebSocketGateway()
export class ChatGateway {
  @SubscribeMessage('message')
  handleMessage(@MessageBody() data: string): string {
    return `Received: ${data}`;
  }
}

32. What is the @SubscribeMessage() decorator?

It listens for WebSocket events and handles real-time messages.

9. Security in NestJS

33. How do you implement authentication in NestJS?

You can use JWT-based authentication with @nestjs/passport and passport-jwt.

Example:

import { Injectable } from '@nestjs/common';
import { PassportStrategy } from '@nestjs/passport';
import { Strategy, ExtractJwt } from 'passport-jwt';

@Injectable()
export class JwtStrategy extends PassportStrategy(Strategy) {
  constructor() {
    super({
      jwtFromRequest: ExtractJwt.fromAuthHeaderAsBearerToken(),
      secretOrKey: 'your_secret_key',
    });
  }

  async validate(payload: any) {
    return { userId: payload.sub, username: payload.username };
  }
}

34. How do you secure API routes using JWT?

Use the AuthGuard with @UseGuards().

@UseGuards(AuthGuard('jwt'))
@Get('profile')
getProfile(@Request() req) {
  return req.user;
}

35. What is CSRF and how do you prevent it in NestJS?

Cross-Site Request Forgery (CSRF) is an attack where malicious requests are made on behalf of an authenticated user.
✅ Use CSRF tokens with csurf middleware.
✅ Implement SameSite cookies.

app.use(csurf());

36. How do you handle rate-limiting in NestJS?

Use @nestjs/throttler to limit API requests and prevent DDoS attacks.

@Throttle(5, 60) // 5 requests per 60 seconds
@Get('data')
getData() {
  return 'Rate-limited data';
}

37. How do you encrypt passwords in NestJS?

Use bcrypt to hash passwords before storing them.

const salt = await bcrypt.genSalt();
const hashedPassword = await bcrypt.hash(password, salt);

10. Logging & Performance Optimization

38. How do you implement logging in NestJS?

Use the built-in Logger service.

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

const logger = new Logger('AppLogger');
logger.log('Application started');

39. How do you enable CORS in NestJS?

const app = await NestFactory.create(AppModule);
app.enableCors();

40. How do you optimize NestJS performance?

✅ Use Fastify instead of Express
✅ Enable caching with cache-manager
✅ Optimize database queries
✅ Use compression middleware

import * as compression from 'compression';
app.use(compression());

11. Microservices & Event-Driven Architecture

41. How do you create a microservice in NestJS?

Use @nestjs/microservices and NestFactory.createMicroservice().

const app = await NestFactory.createMicroservice(AppModule, {
  transport: Transport.TCP,
  options: { port: 3001 },
});
await app.listen();

42. What are message patterns in microservices?

They define how services communicate.

@MessagePattern({ cmd: 'sum' })
sumNumbers(@Payload() data: number[]) {
  return data.reduce((a, b) => a + b, 0);
}

43. What transport layers does NestJS microservices support?

✅ TCP
✅ Redis
✅ Kafka
✅ gRPC
✅ MQTT

12. Deployment & Configuration Management

44. How do you deploy a NestJS application?

✅ Use Docker for containerization.
✅ Deploy using Heroku, AWS, or DigitalOcean.

FROM node:18
WORKDIR /app
COPY . .
RUN npm install
CMD ["npm", "start"]

45. How do you configure environment variables in NestJS?

Use @nestjs/config to load .env variables.

ConfigModule.forRoot({
  isGlobal: true,
});

46. How do you set up a CI/CD pipeline for a NestJS project?

✅ Use GitHub Actions or Jenkins.
✅ Automate testing and deployment.

Example GitHub Action:

name: Deploy
on: push
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - run: npm install
      - run: npm test

13. Miscellaneous & Best Practices

47. How do you implement caching in NestJS?

Use cache-manager for in-memory caching.

import { CacheModule } from '@nestjs/cache-manager';

@Module({
  imports: [CacheModule.register()],
})

48. How do you implement database transactions in NestJS?

With TypeORM:

await this.connection.transaction(async manager => {
  await manager.save(user);
  await manager.save(order);
});

49. How do you handle errors globally in NestJS?

Use exception filters.

@Catch(HttpException)
export class HttpErrorFilter implements ExceptionFilter {
  catch(exception: HttpException, host: ArgumentsHost) {
    const response = host.switchToHttp().getResponse();
    response.status(exception.getStatus()).json({
      message: exception.message,
    });
  }
}

50. What are the best practices for writing NestJS applications?

✅ Use modules to organize your code.
✅ Follow SOLID principles for maintainability.
✅ Use DTOs for data validation.
✅ Implement logging and monitoring.
✅ Write unit and integration tests.

Final Thoughts 💡

NestJS is a powerful framework for building scalable backend applications. These 50 questions cover everything from basic concepts to advanced architecture, security, and microservices.