Top NodeJS Interview Questions with Answers

Some of the most frequently asked NodeJS interview questions with answers for beginners and professionals are given below.

NodeJs developers are the current hot and most required developers in the market. Due to the large demand, there is also significant competition in various NodeJS interviews. For helping you in this process, we have listed out some of the best NodeJS Interview questions with their answers.

In this article, we have listed out all the important NodeJs interview questions with answers ranging from NodeJS basic to advanced topics. All you need is to start preparing! Good Luck.

NodeJS Interview Questions

What is NodeJS?

Node.js is an open-source, a cross-platform run-time environment built on Chrome’s V8 JavaScript engine. It uses an event-driven, non-blocking I/O model which makes it lightweight and efficient.

It is used to build scalable programs, especially web applications that are computationally simple but are frequently accessed. You can use Node.js in developing I/O intensive web applications like video streaming sites.

What are the applications of NodeJS?

NodeJS can be used in making web applications like video streaming sites. You can also use it for developing: Real-time web applications, Network applications, General-purpose applications, and Distributed systems.

With NodeJS, you can speed up your backend development. And this is the reason why so many startups choose NodeJS for their backend development.

What are the features of NodeJS?

NodeJS comes with many features like it is comparatively faster than the other server-side programming language. It never blocks and asynchronous in nature. It yields great concurrency and can achieve high output via a single-threaded event loop and non-blocking I/O.

NodeJS is single-threaded. What do you mean by this?

Node.js is a single-threaded language which in background uses multiple threads to execute asynchronous code. It is a highly scalable system that utilizes JavaScript as its scripting language. It uses asynchronous, event-driven I/O instead of separate processes or threads.

Node.js is non-blocking which means that all functions ( callbacks ) are delegated to the event loop and they are ( or can be ) executed by different threads. That is handled by Node.js run-time.

What is Event-driven programming?

Event-Driven Programming is a logical pattern that we can choose to confine our programming within to avoid issues of complexity and collision.

In event-driven programming, we build our application based on events and respond accordingly. When an event occurs, like click or keypress, we are running a callback function that is registered to the element for that event.

What is an error-first callback in NodeJS?

Error-first callbacks are used to pass errors and data as well. You have to pass the error as the first parameter, and it has to be checked to see if something went wrong. Additional arguments are used to pass data. Here is an example of the same

fs.readFile(filePath, function(err, data) {  
  if (err) {
    // handle the error, the return is important here
    // so execution stops here
    return console.log(err)
  // use the data object

What is the event loop in NodeJS work? And How does it work?

Event loops are one of the most important parts of the NodeJS programming language as it handles all async callbacks. Node.js is a single-threaded, event-driven language. This means that we can attach listeners to events, and when a said event fires, the listener executes the callback we provided.

Whenever we are call setTimeout, http.get, and fs.readFile, Node.js runs this operation and keep running the other code without waiting for the output of the called function. When the operation is finished, it receives the output and runs our callback function.

So all the callback functions are queued in a loop and will run one-by-one when the response has been received. This is all about the event loop in NodeJS.

What is the difference between NodeJS and Ajax?

Node.js and Ajax (Asynchronous JavaScript and XML) are the advanced implementations of JavaScript. They all serve entirely different purposes.

Talking about the NodeJS, The runtime environment of Node.js interprets JavaScript, which is very easy and simple to understand and code. Due to this reason, it is extensively used for developing client-server applications.

Whereas, Ajax is primarily designed for dynamically updating a particular section of a page’s content, without having to update the entire page. It allows you to retrieve data asynchronously from the server without interfering with the display and behavior of the existing page.

What is EventEmitter in NodeJS?

In NodeJS, all the objects that emit events are instances of the EventEmitter class. These objects expose an eventemitter.on() function that allows one or more functions to be attached to named events emitted by the object.

When the EventEmitter object emits an event, all of the functions attached to that specific event are called synchronously. Here is an example of the event emitter in NodeJS.

// get the reference of EventEmitter class of events module
var events = require('events');

//create an object of EventEmitter class by using above reference
var em = new events.EventEmitter();

//Subscribe for FirstEvent
em.on('FirstEvent', function (data) {
    console.log('First subscriber: ' + data);

// Raising FirstEvent
em.emit('FirstEvent', 'This is my first Node.js event emitter example.');

What is a Callback function in NodeJS?

As we know, NodeJS is an asynchronous programming language and that’s why it doesn’t wait around for things like file I/O to finish. To overcome this, Node.js uses callbacks. A callback is a function called at the completion of a given task; this prevents any blocking and allows other code to be run in the meantime.

When the callback function is finished executing, calls the main code cycle. Callbacks give us an interface with which to say, “and when you’re done doing that, do all this.” This allows us to have as many IO operations as our OS can handle happening at the same time.

Here is a simple example of callbacks in NodeJS

var fs = require("fs");  
fs.readFile('input.txt', function (err, data) {  
    if (err) return console.error(err);  
console.log("Program Ended");  

Note: More Questions are being constantly added on a daily basis.

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