# Asynchronous Programming (Node.js / NestJS)

> Callbacks, Promises, async/await, error handling, Promise.all, Promise.race

- 25 interview questions
- Junior
- [Interview Questions: Node.js / NestJS](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions.md)

## 1. In NestJS, how to make a controller method asynchronous?

**Answer**

Add the async keyword before the method and it will automatically return a Promise. NestJS automatically handles Promise resolution and sends the result to the client. You can also directly return a Promise without async if needed. NestJS also supports RxJS Observables. Using async/await makes the code more readable for asynchronous operations (DB, external APIs, etc.).

## 2. What is the main problem with callbacks in JavaScript?

**Answer**

Callback hell (pyramid of doom) occurs when multiple asynchronous operations are nested, creating code that is difficult to read and maintain. Each indentation level adds complexity. Promises and async/await were introduced to solve this problem by allowing more linear asynchronous code writing.

## 3. What is a Promise in JavaScript?

**Answer**

A Promise is an object representing the eventual completion (or failure) of an asynchronous operation and its resulting value. It can be in three states: pending, fulfilled (resolved successfully), or rejected (failed with an error). Promises allow chaining asynchronous operations with .then() and .catch().

## 22 more questions available

- What are the three possible states of a Promise?
- How to handle errors with Promises?

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## Other Node.js / NestJS interview topics

- [Node.js Fundamentals](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/nodejs-fundamentals.md): 20 questions, Junior
- [Node.js Core APIs](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/nodejs-core-apis.md): 25 questions, Junior
- [Express.js Basics](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/express-basics.md): 20 questions, Junior
- [NestJS Fundamentals](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/nestjs-fundamentals.md): 23 questions, Junior
- [REST API Design](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/rest-api-design.md): 20 questions, Junior
- [Validation & DTO](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/validation-dto.md): 20 questions, Junior
- [API Documentation & Contracts](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/api-documentation.md): 20 questions, Junior
- [Error Handling](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/error-handling.md): 20 questions, Junior
- [Unit Testing](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/testing-unit.md): 20 questions, Junior
- [Task Scheduling](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/scheduling-cron.md): 15 questions, Junior
- [NestJS Modules & DI](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/nestjs-modules-di.md): 20 questions, Mid-Level
- [Configuration & Environment Management](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/configuration-environment.md): 20 questions, Mid-Level
- [JWT Authentication](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/authentication-jwt.md): 25 questions, Mid-Level
- [Authorization & RBAC](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/authorization-rbac.md): 20 questions, Mid-Level
- [Database with TypeORM](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/database-typeorm.md): 30 questions, Mid-Level
- [Prisma ORM](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/prisma-orm.md): 25 questions, Mid-Level
- [Middleware & Interceptors](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/middleware-interceptors.md): 20 questions, Mid-Level
- [File Upload](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/file-upload.md): 15 questions, Mid-Level
- [WebSockets](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/websockets.md): 20 questions, Mid-Level
- [GraphQL with NestJS](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/graphql-basics.md): 25 questions, Mid-Level
- [End-to-End Testing](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/testing-e2e.md): 20 questions, Mid-Level
- [Caching with Redis](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/caching-redis.md): 20 questions, Mid-Level
- [Queues with Bull](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/queues-bull.md): 20 questions, Mid-Level
- [DevOps, Logging & CI/CD](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/logging-monitoring.md): 25 questions, Mid-Level
- [Docker & Containerization](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/docker-containerization.md): 25 questions, Mid-Level
- [Microservices](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/microservices.md): 30 questions, Senior
- [Security Best Practices](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/security-best-practices.md): 25 questions, Senior
- [Performance & Cloud Deployment](https://sharpskill.dev/en/technologies/node-nestjs/interview-questions/performance-optimization.md): 30 questions, Senior

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