Top 30 Node.js Interview Questions And Answers

This tutorial provides frequently asked Node.js interview questions and answers with explanations to help you prepare for the Node JS interview:

JavaScript was popular for browser-side validations and interactive web developments. An open-source, platform-independent JavaScript runtime environment, namely Node.js was introduced to allow JavaScript code to run outside the browser, especially on the server.

Node.js is a single-threaded non-blocking asynchronous runtime environment that offers an advantage over requests by eliminating waits and handling the next request. Node.js is suitable for building RESTful API for backend databases like NoSQL and MongoDB and network and data-intensive applications, like data streaming, real-time data exchange like chat applications.

What Is Node.js

Node.js is an open-source JavaScript runtime environment that executes JavaScript code out of the browser with the help of V8 engine. This feature of Node.js allows developers to use JavaScript in designing command-line tools, run server-side scripts that create dynamic content for the web page before reaching the user’s web browser.

Node.js Interview Questions

Event-driven architecture makes Node.js allow asynchronous input/output processing for other processing such as arithmetic calculations, compiling the code, etc. to continue before completion of the transmission.

Developers can share source code, publish, install, update or uninstall packages of Node.js, using npm a package manager. Netflix, PayPal, SAP, Microsoft, Walmart, Yahoo!, and Amazon Web Services are some of the companies that use Node.js in the development of various back-end API services and traditional websites and applications.

Suggested Reading =>> Node.Js Tutorial

Most Frequently Asked Node JS Interview Questions

Q #1) What is Node.js?

Answer: It is an open-source, platform-independent runtime JavaScript environment build on V8 Chrome JavaScript engine that runs JavaScript code on Server.

It is a lightweight web framework to develop real-time applications like Voice Over Internet Protocol (VoIP), video-conferencing, online gaming, and data-intensive applications like streaming and e-commerce payment gateways, making Node.js part of web development stacks namely MERN, MEVN, and MEAN stack.

  • MERN stands for MongoDB, Express, React, Node
  • MEVN stands for MongoDB, Express. js, VueJS, Node
  • MEAN stands for MongoDB, ExpressJS, AngularJS and Node

Q #2) List the uses of Node.js.

Answer: Node.js is used in applications that involve responses instantaneously in real-time.

These include:

  • Real-time chat, messaging, or online games.
  • Single-page applications like Gmail, Twitter that run on JavaScript with Node.js as backend.
  • IoT (Internet of Things) applications like delivery tracking, predictive maintenance, and security software.
  • Location-based applications such as Global positioning system, Wi-Fi, GeoCouponsAlert mobile applications.
  • Microservices architecture development which fits event-driven I/O model like PayPal -payment gateway application and Netflix – video streaming application.

Q #3) Compare other popularly used frameworks with Node.js.

Answer: Node.js uses asynchronous I/O and single-thread event-driven programming making it unique from other server-side frameworks or technologies such as Ruby on Rails, Java, PHP, and .net. Developers can utilize their JavaScript programming skills in developing server-side applications.

Node.js can improve the performance and scalability of the developed applications, such as REST API data-intensive real-time tracking applications for user statistics and applications that help analyze market trends on commodities, currencies, stokes, futures, options, etc. Developers with JavaScript knowledge can quickly develop node.js APIs for mobile or web integration with ease and flexibility.

Q #4) List functionalities of some of the Node.js core modules.

Answer: Some of the important Node.js core modules are explained below:

  • HTTP module contains classes, events, and methods to create Node.js HTTP server.
  • URL consists of methods for URL parsing and resolution.
  • fs module has classes, events, and methods that work with file I/O.
  • path contains methods to deal with file paths.
  • util is the utility functions useful for programmers that are included in util module.
  • querystring module consists of methods to deal with the query string.

Q #5) What are modules in Node.js? Explain with few examples.

Answer: Simple or complex functionalities are grouped as Modules in Node.js. They are primarily divided into the following three types:

  • Core Modules
  • Local Modules
  • Third-Party Modules

Core module is loaded and used in code as below:

var protocol = require('http');
var myserver = protocol.createServer(function(request, response) {
	//write code here

Local module is inserted in the code as below:

var log_module = require('./Log.js');'Node.js started');

Q #6) Differentiate between Angular and Node.js

Answer: Angular is used by JavaScript developers to develop client-side interactive web applications, whereas Node.js is used to develop fast and scalable network and server-side applications.

Following are the list of some of the differences between them:

An open source web application based on TypeScript – superset of JavaScript, used in front end developmentJavaScript runtime environment build using C, C++, JavaScript, used in developing server side JavaScript applications
With Angular single page, client side web applications are designed.Node.js is used in designing event driven I/O models like Real Time, data intensive applications.
Angular utilizes objects and directives Node.js are based on asynchronous, event driven non blocking I/O applications
It supports mobile browserNode.js is used in designing Android application with AndroidJS

Q #7) Explain Event-driven programming.

Answer: Programming based on user interactions on the graphical user interface is called Events; like clicking the submit button, selecting an option from the radio button, typing a text into the text field, uploading the image file.

Event handler or listener that can also be a callback is a method that is called. It accepts user input as parameters and performs some tasks on user and browser actions like page load, HTML page popup, etc.

Event-driven programming

Q #8) Explain Event Loop in Node.js.

Answer: Event Loop is a mechanism that lets Node.js continue performing I/O operations to the system kernel without any interruption. Event loop is initialized by Node.js that processes input script making async API calls, call process.nextTick() or schedule timers followed by event loop processing.

Event loop process timers, pending callbacks, remain idle, prepare for poll for incoming data and connections, checks for next request, and close callback till next request.

Q #9) List and explain phases of the Event loop in their order of operation.

Answer: Event loop in Node.js follows various phases in the order listed below:

Event Loop PhasesDescription
TimersThis is the first phase where scheduled callbacks by functions setTimeOut() and setInterval() for delayed code execution.
Pending callbacksNext phase - pending callbacks allows I/O callbacks execution that are delayed to next loop iteration.
Idle, prepareFollowed by idle, prepare phase that are internally applied
PollNew I/O events are retrieved, callbacks related to I/O, scheduled by timers and setImmediate() function, node gets block at appropriate points,
CheckCheck invokes setImmediate() callbacks, at end of event loop.
Close callbacksClose callbacks like socket.on() runs between each event loop run. Node.js verifies any timers or asynchronous I/O presence, and shuts these callbacks if no events phase is available.

Q #10) Explain the working of Node.js.

Answer: Node.js is a runtime environment that has V8 – JavaScript engine, NPM package manager for Node.js, and Libuv libraries.

V8 is a JavaScript engine that converts browser JavaScript and Node.js code into machine code, making JavaScript run everywhere. NPM is a package manager- a package repository that contains library files containing features that can be included in Node.js.

Real-time applications installed on mobile that displays real-time information on request such as Market share prices, or availability of flights, movie tickets, etc. need data (current status). This data can be retrieved from any server using a single thread of Node.js using non-blocking I/O, wherein waiting for information is avoided, can attend multiple requests simultaneously.

Working of Node.js

Node.js asynchronous feature is utilized when the response from another server returns with the required information, wherein callback function is executed. Hence Node.js is applicable for I/O intensive applications.

Further, to deal with multiple requests, Node.js uses a concept called Libuv – libraries built in C language. These libraries use a system kernel that uses multiple threads for these requests making Node.js run fast.

Q #11) What is Node.js Libuv library and its uses?

Answer: Libuv is asynchronous input/output that comes as libraries with Node.js installation.

Various features Libuv has are listed below:

  • Asynchronous
    • TCP & UDP sockets
    • DNS resolution
    • File and file system
    • Thread pool
    • Signal handling,
  • Backed with the full-featured event loop
  • Child processes
  • File System Events

Q #12) Describe NPM and its functionality in Node.js

Answer: Node Package Manager (NPM) is an online repository of JavaScript libraries that has more than 350,000 packages that can be utilized to build efficient applications and Node.js projects with ease.

It is a command-line utility for accessing an online repository that facilitates inversion and dependency management and package installation. Various features can be searched from this repository here.

Once found you can install it on your client machine by running the command – npm install feature_file. You can use the feature by using require(‘./feature_file’) in JavaScript code.

Q #13) Explain REPL with context to Node.js

Answer: REPL is an environment of a computer wherein the system responds with an output to the entered command. Some of the tasks are:

  • Read: As the name indicates, user input is read and converted into JavaScript data structure and then stored in memory.
  • Eval: Receives and evaluates data structure.
  • Print: Final output is printed
  • Loop: Command is looped until CTRL + C is pressed twice.

Q #14) Explain Error first callback in Node.js.

Answer: Error-first Callback is a function in Node.js used to pass errors and data. The callback is function asynchronous in nature, called after the Ajax request is completed. The error-first Callback function has the first argument as an error object and the second argument is response data that returns on successful response without any error.

Below is the syntax of the error-first callback function:

Error First Callback

Q #15) Differentiate spawn() and fork() methods in Node.js.

Answer: Both spawn() and fork() are commands to create a child process. The difference between these commands are explained below:

  • spawn() is the command to create a child process in Node.js and can be used to pass arguments and execute that command. As a result, Child Process instance is created when the spawn function is executed. This implements EventEmitter API, register handlers for events directly on the child object.
  • fork() in spawn() function variant for node process spawning, when fork() is used as a channel of communication with child process to exchange messages between forked and parent processes via EventEmitter module interface.

Q #16) What are differences between process.nextTick() and setImmediate() function?

Answer: Both functions control the order of code execution in the event loop. Callback handlers are scheduled in the event queue in both of these functions.

  • setImmediate(): Scheduled callbacks are executed in Check handlers phase of event loop when setImmediate() function is applied. Recursive calls to setImmediate() will not block the event loop, the call is executed on the next event loop iteration.
  • process.nextTick():  Scheduled callbacks are processed at starting of the event loop and between each phase of the event loop when process.nextTick() function is applied. Callbacks to process.nextTick() are resolved before continuation of the event loop, blocking event loop if this function is called recursively.

Q #17) What is the difference between setImmediate() and setTimeout()?

Answer: setImmediate(), setTimeout() and process.nextTick() are Node.js functions that control order of executing code in event loop.

  • setTimeout() is executed after process.nextTick() i.e. after current code executed and before any I/O events. setTimeout() arranges script run after minimum threshold of milliseconds has completed.
  • setImmediate() is executed after setTimeout(), with callback placed in check queue of next cycle of event loop. This command is processed on the check handler phase of the event loop. setImmediate() executes script after current poll phase completes.

Q #18) What are streams in Node.js?

Answer: Streams are methods used to handle reading/writing files, network communication, or end-to-end information exchange. These methods are used to process large data by reading data piece by piece, process its content, especially for data files larger than free memory space.

Streams help to read and write data as: 

  • Readable: Data can be read from the stream using fs.createReadStream() command.
  • Writable: Data can be written into the stream using fs.createWriteStream() command.
  • Duplex: Readable and writable streams are duplex and use net.Socket command.
  • Transform: Duplex streams converting read and written data as zlib.createDeflate().

Q #19) How Crypto is used in Node.js?

Answer: Crypto module of Node.js contains OpenSSL’s hash, HMAC, cipher, decipher, sign and verify functions. An algorithm created by the crypto module is used for data encryption and decryption, for storing passwords in the database in encrypted form.

Q #20) How DNS module is used in Node.js?

Answer: DNS module is used for functionalities, such as Domain Name System (DNS) lookup and operating system name resolution. DNS is like a website address. For example, is converted into IP Address (

Q #21) How assert is used in Node.js?

Answer: Node.js Assert modules are used for function assertion. Assert function works for verifying invariants, returns nothing in output if the condition is true else assertion error is displayed by console.

Q #22) Describe Timer() module methods in Node.js

Answer: Timer module has various methods like setTimeout(), setImmediate(), setInterval().

These are explained below:

  • setTimeout() helps schedule code execution after specified interval in milliseconds.


setTimeout(callback, delay_in_ms, args)
  • setInterval() helps calling a function at specified interval, once after desired period.


  • setImmediate() execute code at end of current event loop.


setImmediate(callback, args)

Q #23) What is the difference between fs.readFile() and fs.createReadStream() in Node.js?

Answer: Node.js uses two ways of reading a file and sending them for execution.

Using file system module, fs.readFile loads entire file you want to manage into the memory, reads entire file before it is sent to clientfs.CreateReadStream reads entire file in chunks of pieces that we specify
fs.readFile() is useful making advantage for large content and low size disksSending data file with fs.createReadStream() becomes faster.
fs.readFile() is memory intensive for high content data filesfs.createReadStream() is effective in memory efficient way of handling data intensive processing of large data files.

Q #24) Use of URL module in Node.js

Answer: Uniform Resource Identifier (URL) is composed of various portions such as protocol, host and port, filepath, and filename. URL module of Node.js help split web address (URL) into sections that user can understand.

var myurl = require('url');
var web_addr ='http://localhost:8080/support/pages/page_one.html?year=2021&month=may';
var str = myurl.parse(web_addr, true);

console.log(; //returns hostname i.e. 'localhost:8080'
console.log(str.pathname); // returns filepath - i.e. 'support/pages/page_one.html'
console.log(; // returns '?year=2021&month=may'

var datapoint = str.query; // returns an object => { year : 2021, month : 'may' }
console.log(datapoint.month); //returns 'may'

Q #25) Explain package.json file of Node.js

Answer: package.json is a JSON file present at the root directory of Node.js and contains metadata about projects like description, version, distribution, license, configuration related to end-user of project, and npm. This file identifies the project and handles the dependencies, provides information about project metadata values to npm.

Q #26) Explain EventEmitter in Node.js

Answer: Custom events can be handled using EventEmitter class from the Events module. In the case of an e-commerce application, when your payment is successful or declined, the user should receive an email informing the status, so interaction with payment gateway i.e. the result should be sent as an email so callback is added to the email event.

There are many more operations that can interact with each other when the payment request is carried out. With event emitter, these events are handled using the following code:

eventEmitter.on('SendMail', listener);

Q #27) List some of the Node.js libraries often used?

Answer: These are as follows:

  • Express: It is a web framework for node.js
  • It is for event-based real-time communication
  • Cors: Node.js pack to provide connect/express middleware
  • Passport: It is used to authenticate requests with strategies extensible plugins
  • Axios: HTTP client who is Promise based for node.js and browser
  • Multer: Handles multipart/form-data and file uploads
  • Morgan: A Node.js middleware that is an HTTP request logger
  • HTTP: errors – It helps to generate HTTP errors for Connect, Koa, and Express

Q #28) What is the most suitable database used along with Node.js?

Answer: Cassandra, CouchDB, MySQL, MongoDB, Neo4j, Oracle, PostgreSQL, and ElasticSearch are some of the databases used along with Node.js. We can install the connection drivers for these Databases with the command npm install driver_name.

However, MongoDB is more suitable for back-end management with Node.js.

Q #29) Explain reactor design pattern of Node.js

Answer: Nonblocking I/O feature is due to reactor pattern, before I/O request is generated, the handler is submitted to a demultiplexer that handles concurrency, collecting requests as events, and executes these events in queues, thereby avoids I/O blocking.

Reactor pattern consists of following:

  • Resources – Multiple applications with I/O operations shares this resource.
  • Event Notifier (Synchronous) – It pushes new events in the event loop.
  • Event Loop – New events occur with the event handler with the event loop.
  • Request Handler – Handlers are used for events registered on resources provided by the application.

Q #30) Explain Buffer class in Node.js

Answer: Buffer class in Node.js provides a way to manage binary data streams.

Unlike Unicode, JavaScript does not support binary data. To process TCP streams or file systems, it is important to handle octet streams. Buffer class in Node.js offers instances that store raw data and allocate raw memory outside V8 heap.

Also Read =>> Advanced JavaScript Interview Questions


Runtime JavaScript environment such as Node.js has been the most popular choice for developing backend web services using RESTful API to communicate with databases like MongoDB or NoSQL that runs on the server.

Node.js is also used to develop applications that are designed for storing and manipulating scalable data-intensive e-commerce and network applications that are event-driven asynchronous in nature.