Top Node.js Interview Questions & Answers (2026)

Node.js is a JavaScript engine used for executing JavaScript code outside the browser. It is normally used to build the backend of the application and is highly scalable.

Node.js makes building scalable network programs easy. Some of its advantages include:

• It is generally fast
• It rarely blocks
• It offers a unified programming language and data type
• Everything is asynchronous 
• It yields great concurrency

JavaScript is a scripting language whereas Node.js is an engine that provides the runtime environment to run JavaScript code.

JavaScript: It is a light-weighted programming language (“scripting language”) used to develop interactive web pages. It can insert dynamic text into the HTML elements. JavaScript is also known as the browser’s language.

Node.js: It is used to run JavaScript programs outside the browser and it mostly runs server-side code. It cannot be used to run HTML tags.

• It provides an easy way to build scalable network programs
• Generally fast
• Great concurrency
• Asynchronous everything
• Almost never blocks

It can be used for the following purposes.

• Web applications ( especially real-time web apps )
• Network applications
• Distributed systems
• General purpose applications

Yes,  Node.js is single-threaded for async processing. By doing async processing on a single-thread under typical web loads, more performance and scalability can be achieved instead of the typical thread-based implementation.

It is use for.....

1. Real-time chats
2. Internet of Things
3. Complex SPAs (Single-Page Applications)
4. Real-time collaboration tools
5. Streaming applications
6. Microservices architecture

When it comes to Node, there is a special package handler called NPM- Node Package Manager, it is in charge of handling all of Node.js’ packages and modules. NPM (Node Package Manager) manages the majority of functions. If you want to find online repositories for node.js packages and node module/s then use the search.nodejs.org function. Also, if you want to install Node.js packages and maintain their versions and dependencies, you can do it easily using a command-line utility.

const http = require('http');

// Create a server object

http.createServer(function (req, res) {

    res.write('Welcome to Node Js'); 

    res.end();

}).listen(3000);

• Import the HTTP module
• Use createServer function with a callback function using request and response as parameters.
• Type “Welcome to Node Js." 
• Set the server to listen to port 3000 and assign an IP address

Mid-level developers with Node.js experience should know that the package.json file contains metadata for a specific project. They’ll be able to explain that this file is in the Node.js application or module’s root directory.

Ideal responses will also specify what the metadata contains, including the project’s version, name, and dependency details. Look out for answers that mention that the file gives information to the NPM package manager to identify the project.

A callback in Node.js is a function that is called after a given task. Thus it helps in preventing any blocking.

Callback function is used in node.js to deal with multiple requests made to the server. Like if you have a large file which is going to take a long time for a server to read and if you don’t want a server to get engage in reading that large file while dealing with other requests, call back function is used. Call back function allows the server to deal with pending request first and call a function when it is finished. 

There are mainly two commonly used libraries in Node.js:

1) Express JS: Express is a flexible Node.js application framework that provides many features to develop web and mobile applications.
2) Mongoose: Mongoose is a Node.js application framework used to connect an application to a database.

The buffer class in Node.js is defined to handle and store the raw binary data that is similar to an array of integers. Each buffer corresponds to a raw memory allocation outside of V8. Unlike arrays, they are not resizable.

The two types of API functions in Node.js are:

• Asynchronous, non-blocking functions
• Synchronous, blocking functions

A generic piece of code which runs in between several asynchronous function calls is known as control flow function.

Step:

• Control the order of execution
• Collect data
• Limit concurrency
• Call the next step in program