Top 20+ Node.js Interview Questions - (2025)

Introduction

Node.js has become a base of modern web development, powering everything from small APIs to large-scale production level applications. Its event-driven, non-blocking architecture makes it ideal for building fast and scalable backend services, which is why it's widely adopted by companies like Netflix, LinkedIn, and PayPal.

As the demand for skilled backend developers continues to rise, Node.js remains one of the most in-demand technologies in technical interviews. Whether you're a fresher aiming to land your first developer job or an experienced engineer preparing for a senior-level Node.js role, having a strong grasp of Node.js fundamentals is essential.

In this blog, we've covered a comprehensive list of Node.js interview questions - ranging from basic to advanced - along with detailed answers to help you prepare with confidence. From understanding the event loop to designing scalable systems, we'll cover the key concepts that interviewers are most likely to ask in 2025.

Let's started and get you interview ready.

Node.js Interview Questions for Freshers

If you're just starting your journey as a backend developer, interviewers typically test your understanding of core Node.js concepts. These questions focus on the basics - what Node.js is, how it works, and why it's used. Here are some of the most common questions asked in interviews for entry-level roles:

1. What is Node.js?

Node.js is an open-source, cross-platform JavaScript runtime environment that allows you to run JavaScript on the server side. It uses Google Chrome's V8 engine to execute code outside the browser, enabling the development of scalable and high-performance backend applications.

Key features of Node.js?

• Asynchronous: Node.js uses non-blocking I/O operations to handle multiple requests concurrently.
• Single-Threaded but Highly Scalable: Thanks to the event loop, Node.js can handle thousands of connections without creating multiple threads.
• Fast Execution: Powered by the V8 engine, Node.js offers lightning-fast execution of JavaScript code.
• Package Management: The Node Package Manager (npm) provides access to thousands of reusable packages.

2. What is npm in Node.js?

npm (Node Package Manager) is the default package manager for Node.js. It allows developers to install, share, and manage packages or libraries in their projects. It also handles versioning and dependency management.

Common Commands:

• npm init: Initialize a project.
• npm install express: Install a package.
• npm run start: Execute a script.

3. What is the role of the package.json file?

The package.json file is the heart of a Node.js project. It contains metadata about the project, including its name, version, description, dependencies, scripts, and more. This file helps npm know which packages are required for the project and how to run specific commands.

The package.json file stores metadata about the project, including:

• Dependencies (dependencies, devDependencies).
• Scripts (e.g., start, test).
• Version, author, and license info.

Example:

1{  
2"name": "my-app",  
3"scripts": {  
4  "start": "node index.js"  
5},  
6"dependencies": {  
7  "express": "^4.18.2"  
8}  
9}  

4. Is Node.js single-threaded or multi-threaded?

Node.js is single-threaded in nature, but it can handle multiple concurrent operations using an event loop and callbacks. This makes it lightweight and efficient for I/O-heavy tasks, such as API handling or database interactions.

5. How to create a simple HTTP server in Node.js?

Use the built-in http module:

1const http = require('http');  
2const server = http.createServer((req, res) => {  
3res.writeHead(200, { 'Content-Type': 'text/plain' });  
4res.end('Hello World!');  
5});  
6server.listen(3000, () => console.log('Server running on port 3000'));  

6. What is REST API?

REST (Representational State Transfer) is an architectural style for designing networked applications. It uses standard HTTP methods:

• GET: Retrieve data.
• POST: Create data.
• PUT: Update data.
• DELETE: Remove data.

7. Difference between synchronous and asynchronous code in Node.js?

• Synchronous Code: Executes line by line. Each operation waits for the previous one to complete.
• Asynchronous Code: Doesn't block execution. Functions like setTimeout(), fs.readFile(), or API calls run in the background, allowing the next lines of code to execute immediately.

8. What are modules in Node.js?

Modules in Node.js are reusable blocks of code that help structure your application. They can be built-in (like fs, http, or path), third-party (installed via npm), or custom-created by developers. Node.js uses the CommonJS module system (require() and module.exports) to import/export modules.

9. What is the difference between require() and import?

• require(): Part of CommonJS (Node.js default) for importing modules.
• import: Part of ES Modules (used with "type": "module" in package.json).

Node.js Advanced Interview Questions

For senior or experienced backend developers, Node.js interviews often dive into the internal workings, performance, scalability, and architecture-level understanding of applications. Below are some of the most asked Node.js interview questions, along with detailed answers to help you stand out in your next technical interview.

10. What is the Event Loop, and how does it work in Node.js?

The event loop is the core of Node.js's non-blocking architecture. It continuously checks the call stack and the callback queue. When the call stack is empty, it pushes the first callback from the queue to the stack for execution. This allows Node.js to handle thousands of concurrent connections using a single thread efficiently.

11. What is the difference between process.nextTick(), setImmediate(), and setTimeout()?

• process.nextTick(): Executes after the current operation completes but before the event loop continues.

• setImmediate(): Executes in the check phase of the event loop, after I/O events.

• setTimeout(): Executes after a minimum delay, scheduled in the timers phase.

They all queue callbacks differently within the event loop, and understanding the phases is key to performance-sensitive coding.

12. What is clustering in Node.js and why is it used?

Clustering allows you to take advantage of multi-core systems by spawning multiple Node.js processes (workers) that share the same server port. This improves the performance and scalability of applications, especially under heavy load, as each worker can handle a portion of the traffic.

13. What is the purpose of the Buffer class in Node.js?

Buffers are used to handle raw binary data, especially when dealing with streams or file operations. Since JavaScript doesn't natively support binary, the Buffer class allows developers to manipulate data at the byte level efficiently.

14. What is middleware in Express.js?

Middleware functions are used to modify requests or responses in Express.js. They can:

• Execute code.
• Modify req and res objects.
• End the request-response cycle.

Example:

1app.use((req, res, next) => {  
2console.log('Request URL:', req.url);  
3next(); // Pass control to the next middleware  
4});

15. How to connect Node.js to MongoDB?

Use Mongoose (an ODM library):

1const mongoose = require('mongoose');  
2mongoose.connect('mongodb://localhost:27017/mydb')  
3.then(() => console.log('Connected!'))  
4.catch(err => console.error('Connection failed:', err));  

Most Asked Node.js Interview Questions

Node.js interview questions are becoming more practical, real-world, and performance-focused. Interviewers are testing developers not only on theory but also on their ability to write clean, efficient, and secure backend code. Below are some of the most asked Node.js questions, along with well-explained answers.

16. How would you implement JWT authentication in Node.js?

Use the jsonwebtoken package to sign and verify tokens. A typical flow:

• User logs in - server creates a JWT - sends it to the client.
• Client sends JWT in headers - server verifies it on each request.

1const jwt = require('jsonwebtoken');
2const token = jwt.sign({ userId }, 'secretKey', { expiresIn: '1h' });

17. What is a passport in Node.js?

In Node.js, Passport is a popular authentication middleware used to authenticate requests in web applications. It is designed to work with any Express.js based application and provides a simple and consistent way to handle various forms of authentication, such as:

• Local authentication (username and password)
• OAuth (Google, Facebook, Twitter, etc.)
• JWT (JSON Web Token) authentication
• Session-based authentication
• Other third-party authentication methods like GitHub, LinkedIn, etc.

18. What do you know about ESLint?

ESLint is a widely-used static code analysis tool for identifying and fixing problems in JavaScript (and sometimes other languages) code. It helps developers maintain consistent coding styles and avoid common errors by enforcing a set of rules across the project.

19. What is the difference between Node.js and Ajax?

Node.js and AJAX are two different technologies, often used together in modern web applications, but they serve very different purposes. Here is a simple difference between them:

• Node.js is a runtime environment that allows you to run JavaScript on the server-side. It is built on Chrome's V8 JavaScript engine and provides an event-driven, non-blocking I/O model, making it highly efficient and suitable for building scalable network applications, such as web servers.
• AJAX is a technique for creating asynchronous requests in web browsers using JavaScript. It allows web pages to fetch data from a server and update parts of a web page dynamically without reloading the entire page.

20. What is a stream in Node.js and why is it useful?

A stream is a special type of object in Node.js that lets you process data in chunks, rather than loading it all at once into memory. This is especially useful for:

• Reading large files (e.g., videos, logs)
• Uploading or downloading content
• Real-time data processing

Frequently Asked Questions

• Q: Is Node.js still relevant in 2025?

  A: Absolutely. Node.js remains one of the most popular backend technologies in 2025 due to its speed, scalability, and large ecosystem. It's widely used in startups, enterprises, and real-time applications like chats and collaborative tools.

• Q: What is the average salary of a Node.js developer in 2025?

  A: While salaries vary by country and experience, on average, a Node.js developer earns between $60,000 to $120,000+ annually. Senior developers or full-stack Node.js engineers can earn significantly more.

• Q: Is Node.js suitable for large-scale applications?

  A: Yes, Node.js is used in large-scale applications by companies like Netflix, PayPal, and LinkedIn. Its event-driven, non-blocking nature and scalability through clustering make it suitable for high-traffic systems.

• Q: Do I need to know Express.js to crack Node.js interviews?

  A: Yes. Express.js is the most widely used framework in the Node.js ecosystem. Understanding routing, middleware, error handling, and request/response flows is essential.

Conclusion

In the modern age, the of Node.js development continues to evolve, with new features, tools, and practices improving how developers build scalable, efficient, and high-performance applications. Whether you're preparing and looking for your first Node.js interview questions or you're a seasoned developer looking to advanced Node.js interview questions, understanding the key concepts and questions in this blog will give you a competitive edge.

By mastering the core features of Node.js, such as the event loop, asynchronous programming, and Express.js middleware, you'll be well-prepared to tackle the real-world challenges that arise in any Node.js-based project.

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