Node.js Interview Questions and Answers for Beginners and Professionals
If you’re a developer working with JavaScript, chances are you’ve already heard of Node.js, the powerful runtime that lets you run JavaScript on the server side.
Whether you are preparing for your next interview or brushing up your backend skills, this post covers the most commonly asked Node.js interview questions with simple explanations and examples.
What is Node.js?
Node.js is an open-source, cross-platform runtime environment that allows you to run JavaScript code outside a browser typically on the server side.
It’s built on Google’s V8 engine, which makes it fast and efficient.
Example:
// hello.js
console.log("Hello from Node.js!");
Run it with:
node hello.js
What makes Node.js different from traditional server technologies?
Node.js uses an event-driven, non-blocking I/O model, which means it can handle thousands of concurrent requests without creating multiple threads.
In simple terms: Instead of waiting for one task to finish before starting another, Node.js continues executing other tasks making it perfect for real-time applications.
What is NPM and why is it important?
NPM (Node Package Manager) is the default package manager for Node.js. It helps developers install, share, and manage dependencies easily.
Example:
npm install express
This command installs Express.js, one of the most popular Node.js frameworks.
What is a Module in Node.js?
Modules are reusable blocks of code that help organize your application. Node.js provides three types of modules:
- Core Modules: Built-in (e.g.,
fs,http,path) - Local Modules: Custom modules you create
- Third-party Modules: Installed via NPM
Example:
const fs = require('fs');
fs.writeFileSync('demo.txt', 'Hello DebugBoat!');
5. How does the Event Loop work in Node.js?
The Event Loop is what allows Node.js to perform non-blocking operations.
It continuously checks the call stack and the callback queue, executing tasks asynchronously.
Real-life analogy:
Imagine a chef (event loop) who keeps cooking while waiting for orders (callbacks) to arrive — never idle, always efficient!
6. What is Middleware in Node.js?
Middleware functions are commonly used in frameworks like Express.js.
They sit between a request and response — processing data, handling authentication, or logging.
Example:
app.use((req, res, next) => {
console.log('Request received at:', Date.now());
next();
});
7. How do you handle errors in Node.js?
You can handle errors using:
try...catchblocks- Error-first callbacks
- Express.js middleware for centralized error handling
Example:
try {
const data = fs.readFileSync('file.txt');
} catch (err) {
console.error('Error reading file:', err);
}
8. What are Streams in Node.js?
Streams allow reading and writing data piece by piece making them ideal for large files.
Types of Streams:
- Readable
- Writable
- Duplex
- Transform
Example:
const fs = require('fs');
const stream = fs.createReadStream('largefile.txt');
stream.on('data', chunk => console.log('Reading...', chunk.length));
9. What is Express.js?
Express.js is a lightweight Node.js framework that simplifies building web applications and APIs.
Example:
const express = require('express');
const app = express();
app.get('/', (req, res) => {
res.send('Hello, Express!');
});
app.listen(3000, () => console.log('Server running on port 3000'));
10. How is Node.js used in real-world applications?
Node.js powers many modern applications such as:
- Real-time chat apps (Slack, Discord)
- Streaming services (Netflix)
- APIs and microservices (Uber, PayPal)
Its scalability, speed, and simplicity make it a favorite for developers building fast, modern web backends.
Conclusion
Node.js continues to dominate backend development due to its speed, scalability, and JavaScript-based simplicity.
Mastering these questions will not only help you in interviews but also strengthen your real-world understanding of backend programming.
Stay tuned on debugboat.com for more interview focused guides on .NET, React, Azure, SQL, and DevOps.