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Interview Questions and Node.js

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Top Node js Interview Questions and Answers

Posted in Interview Questions, Node.js
Node js Interview Questions

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Node.js was initially developed by Ryan Dahl and saw its first release in May 2009. It is a JS environment used to execute JavaScript code outside the browser. Based on Chrome’s V8 engine, it represents the JavaScript anywhere and everywhere programming paradigm, unifying the web app development to a single language rather than different languages for server and client-side scripting.

Many corporate such as LinkedIn, IBM, GoDaddy, Groupon, Netflix, and Paypal use Node.js. Its increasing popularity has sparked a demand for Node.js developers.

Node JS Interview Questions and Answers

If you wish to make a career in Node.js, below are the top Node js interview questions and answers which will make you interview-ready.

Question: Explain the basics of Node.js.

Answer: In its elemental sense, Node.js is a JS runtime built using Chrome’s V8 JavaScript engine as a platform. Node.js has gained popularity as it is lightweight and efficient, majorly due to its event-driven and non-blocking I/O model. Built with performance as its primary focus, Node.js is responsible for processing the JavaScript code into the native machine code, which can then be used by your computer to execute the processes. Even though it is based on the V8 engine, which is used in Google Chrome and other chromium-based browsers (Vivaldi, Brave and Opera), it does not run in the browser itself. During development, various features such as file system API, HTTP library and OS utility methods were added to the engine, so that Node.js can be executed as a program on a computer.

Question: How are ‘Child Threads’ handled in Node.js?

Answer: In its core sense, Node.js is a single thread process. Also, it does not expose any child threads and the modes of thread management to the developer. However, child threads may be generated in Node.js in a variety of processes, one of them being asynchronous I/O. Although the child threads spawned through these processes run in the backdrop, they don’t block the main code or execute any application code. But if you require threading support in an application powered by Node.js, multiple tools are available for utilization.

Question: State some differences between Angular JS and Node.js.

Answer: Below are some differences between Angular JS and Node.js

Angular JS Node.js
Angular JS is written in TypeScript Node.js is written in a variety of languages, such as C, C++, and JavaScript
It is ideal while creating highly interactive web pages It is suited for small-scale projects and applications
It is an open-source framework for web application development It is a run-time environment based on multiple platforms
Used to create single-page applications for client-side Use to create server-side networking applications
Helps in splitting an application into model-view-controller (MVC) components Helps in generating queries for databases
Appropriate for the development of real-time applications It is appropriate in situations when something faster and highly scalable is required
Angular itself is a web application framework Node.js has many frameworks, including Express.js, Partial.js and more

Question: State the primary uses of Node.js.


  • Complex single-page applications: Node.js is ideal for creating single-page applications that are complex in nature, such as online drawing tools, mail solutions, and social networking. These types of applications are limited to one-page and the UX is similar to that of a desktop application. Node.js can be used here due to its asynchronous data flow happening at the backend.
  • Real-Time Applications (RTA): We use a hefty number of Real-Time applications in our day-to-day life. For example, Google Docs, Slack, Skype, WhatsApp and many more. Node.js’ event API, WebSockets, and asynchronous data flow can ensure a faultless server operation, which will update the data instantly.
  • Chat Rooms: This may be clubbed under RTA, but since instant messaging and chatting has emerged as one of the top real-time application models, it needs a special focus. If your product is in this realm, you are looking at requirements such as lightweight, high traffic capacity and substantial data flow. All these requirements can be fulfilled using Node.js and some JavaScript framework at the backend. The aforementioned web sockets come in handy in receiving and sending messages in a chat room environment.
  • Browser-Based Games: The above-mentioned chat rooms can also be integrated into browser-based games, where Node.js is a perfect choice. Combining the Node.js technology with HTML5 and even JS tools will help you create real-time browser-based games.
  • Data Streaming Applications: Another type of application where Node.js is perfect is data streaming applications. The key selling point of these applications is that they can process data in the unloading phase. Through this, while some parts can be downloaded upfront to keep the connection and can download the other parts if and when necessary. In this context, Node.js streaming applications deal with both audio and video data.
  • Representational State Transfer (REST) Application Programming Interfaces (APIs): APIs based on REST hold a key position in the construction of modern commercial software architecture due to the wide usage of the HyperText Transfer Protocol (HTTP). The Express.js framework of the Node.js ecosystem can help in building fast and light REST APIs.
  • Server-Side Web Applications: While Node.js and its frameworks can help in creating server-side web applications, it should be noted that CPU-heavy operations should not be expected.
  • Command Line Tools: Node.js’ expansive ecosystem is an advantage when it comes to building a command line tool, and there are a variety of tutorials available online which can help you build your own CLT.

Question: What is Event-Driven programming?

Answer: Event-Driven programming approach profoundly uses events to trigger various functions. In this scenario, an event can be anything, such as the pressing of a key or clicking of a mouse button. Whenever an event occurs, a call-back function already registered with the element is executed, following the ‘publish-subscribe’ pattern. Due to this programming approach, Node.js is faster than other comparable technologies.

Question: In the context of Node.js, what is REPL?

Answer: In the context of Node.js, REPL is Read, Eval, Print and Loop. REPL is a computer environment (similar to a Windows console or Linux shell) where any command that is entered is met with a system responded output. The REPL environment is bundled with Node.js by default, and it performs the following tasks:

  • Read: Reads the inputs from the user and converts it into JavaScript data-structure, finally storing it in memory
  • Eval: It receives the data structure and evaluates it
  • Print: It prints the final output
  • Loop: It loops the provided command till CTRL + C is pressed two times

Question: What is a test pyramid in Node.js?

Answer: In Node.js, a test pyramid is a figure which explains the proportion of unit tests, integrations tests, and end-to-end tests are required for the fruitful development of a project. The components of a test pyramid are given below:

  • Unit Tests: They test the individual units of code in isolation. They are fast and you might perform a lot of these tests
  • Integrations Tests: They test the integration among dissimilar units.
  • End-to-End (E2E) Tests: They test the system as a whole, right from the User Interface to the data store, and back.

Question: What is libuv?

Answer: Libuv, a support library of Node.js, is used for asynchronous Input/output. While it was initially developed just for Node.js, it now witnesses practice with other systems such as Luvit, Julia, Pyuv and more. Some of the features of Libuv are:

  • File System Events
  • Child Processes
  • Full-featured event loop backed
  • Asynchronous TCP & UDP sockets

Question: Is Node.js the best platform for CPU-heavy applications?

Answer: Even though Node.js can help in creating server-side applications, CPU-incentive applications are not the strong suit of Node.js. The CPU-heavy operations pave the way for the blockage of incoming requests and push the thread into critical situations.

Question: What is the purpose of the Express JS Package?

Answer: Built on top of Node.js, ExpressJS is a JS framework used to manage the flow of information between the routes and server in server-side apps. Being lightweight, flexible and filled with a variety of relevant features, it is apt for mobile and web application development.

Question: What are the main differences between Node.js vs Javascript?


Node.js JavaScript
Cross-platform open source JS runtime engine.  A high-level scripting language based on the concept of OOPS.
Code can be run outside the browser.  Code can run only in the browser.
Used on server-side. Used on client-side.
No capabilities to add HTML tags. HTML tags can be added. 
Can be run only on Google Chrome's V8 engine. Can be run on any browser.
Written in C++ and JavaScript. An upgraded version of ECMA script written in C++.

Question: What are the major benefits of Node.js?

Answer: Some major benefits of Node.js are:

  • easy to learn and huge community support.
  • easy scalability and high performance.
  • highly extensible with extended support.
  • Full-stack JavaScript.
  • Caching mechanism allows web pages to load faster.
  • Non-blocking I/O systems.

Question: Explain the difference between Asynchronous and Non-blocking?

Answer: Asynchronous or non-synchronous means that for a message sent, we will not receive a response immediately; thus, there is no dependency or order of execution. The server stores information and an acknowledgement is received when the action is performed. This improves performance and efficiency.

Non-blocking operation, as the name suggests, does not stop or block any operations. The difference is that non-blocking operation receives a response immediately with whatever data is available. If data is not available, it returns an error. It is mostly used with I/O.

Question: Explain package.json?

Answer: The npm packages contain a file package.json in the project root folder, which contains the metadata relevant to the project. It gives information to the npm through which npm identifies a project and its dependencies. Apart from this, it contains other metadata like project description, version, license information and configuration data. 

Question: Describe error-first callback in Node.js?

Answer: The error-first callback, also called as errorback gives the error and data. It takes a few arguments, the first one being the error object and others being the data. The error-first callback pattern has many advantages: since it is consistent, leads to more adoption, if there is no reference to the data, there is no need to process it. If there is no error, the callback is called with null as the first argument. 

Question: Explain the purpose of module.exports.

Answer: The purpose is to give instructions to tell Node.js about which part of the code like objects, functions, strings etc. should be exported from a given file so that other files can access it. Suppose we have a module that looks like:

   id: '.',
   exports: {},
   parent: null,
   filename: '/modtest.js',
   loaded: false,
   children: [],

Note that the exports property is empty, now if we apply code to this property, that will become the export of the module. If we require the module in another file, that will be the export property value:

module.exports.stringProperty = "Hello, welcome";
   id: '.',
   exports: { stringProperty: 'Hello, welcome' }

Question: What is the Reactor Pattern in Node.js?

Answer: It is a concept of non-blocking I/O operations in Node.js. Through this pattern, we get the handler (or callback function) for each I/O operation. Each I/O request is submitted to a demultiplexer, that handles concurrency and queues the requests/events. Reactor pattern consists of resources, event notifier/demultiplexer, event loop, event queue, request handler.

Question: Explain LTS releases of Node.js

Answer: LTS or Long-Term Support is applied to release lines that are supported and maintained by Node.js project for an extended period. There are two types of LTS: 

  1. Active, which is actively maintained and upgraded, and 
  2. maintenance line that is nearing its end of the line and is maintained for a short window of time.

Question: What is a URL module?

Answer: The URL module provides APIs to work with URLs:

  • a legacy API which is specific to Node.js.
  • newer API that implements WHATWG URL Standard used by web browsers.

Some example methods are URL.port, URL.password, URL.host, url.toString() under the URL class. For the full documentation, check the official URL module page.

Question: Explain the working mechanism of control flow function?

Answer: Control flow function is the sequence in which statements or functions are executed. Since I/O operations are non-blocking in Node.js, control flow cannot be linear. Therefore, it registers a callback to the event loop and passes the control back to the node, so that the next lines of code can run without interruption. For example:

[code language="javascript"]
fs.readFile('/root/text.txt', func(err, data){
console.log("This is displayed first");

In this, the readFile operation will take some time; however, the next console.log is not blocked. Once the operation completes, data will be displayed.

Question: What are the main differences between spawn() and fork() methods in Node.js?


Spawn Fork
Designed to run system commands. A special instance of spawn() that runs a new instance of V8.
Does not execute any other code within the node process.  Can create multiple workers that run on the same Node codebase.
child_process.spawn(command[, args][, options]) creates a new process with the given command.  Special case of spawn() to create child processes using. child_process.fork(modulePath[, args][, options])
Creates a streaming interface (data buffering in binary format) between parent and child process. Creates a communication (messaging) channel between parent and child process.
More useful for continuous operations like data streaming (read/write), example, streaming images/files from the spawn process to the parent process. More useful for messaging, example, JSON or XML data messaging.

Question: What is the purpose of ExpressJS package in Node.js?

Answer: ExpressJS package or framework is built on top of Node.js to fast-track the development of single-page, multi-page and hybrid server-based applications. Express is said to be the backend part of the MEAN stack.

Question: Highlight the differences between process.nextTick() and setImmediate().


let execseq = function() {  
setImmediate(() => console.log("immediate")); 
process.nextTick(() => console.log("nextTick")); 
console.log("event loop");

This will execute the "event loop", then "nextTick" and then "immediate".

Question: Explain the difference between Node.js vs Ajax?

Answer: Ajax is a client-side technology and is used for updating page content without refreshing the same. Node.js is a server-side JavaScript that is used to develop server software. Further, Node.js is a full-fledged development environment, whereas Ajax is used just to obtain data or run scripts.

Question: Give the reason why Express' app' and 'server' must be kept separate.

Answer: If we keep the app and server functionalities separate, the code can be divided into multiple modules, thereby reducing the dependency between modules. Each module will perform a single task. Also, due to the separation of logic, we can avoid duplicate code.

Question: Explain the Difference between setImmediate() vs setTimeout().

Answer: While the word immediate is slightly misleading, the callback happens only after the I/O events callbacks, when we call setImmediate(). setTimeout() is used to set a delay (in milliseconds) for execution of a one-time callback. If we execute, 

setImmediate(function() {
setTimeout(function() {
}, 0)

We will get the output as 'setTimeOut' and then 'setImmediate'.

Answer: The core modules of Node.js are:

http includes classes, methods and events for creating Node.js http server.
URL contains methods for URL resolution and parsing.
querystring deals with query string.
path contains methods for working with file paths.
fs consists of classes, methods, and events for handling file I/O.
util util module includes utility functions useful for developers.


Question: Add an example of Streams in Node.js?

Answer: Example of reading from stream:

var readStream = fs.createReadStream('data.txt');
readStream.on('data', function(chunk) {
  data += chunk;

Example of writing into stream:

var writeStream = fs.createWriteStream('dataout.txt');

Question: What is crypto in Node.js? How is it used?

Answer: The crypto module in Node.js is used for cryptography, i.e. includes a set of wrappers for open SSL's hash, HMAC, sign, decipher, cipher and verify functions.

Here is an example of using cipher for encryption:

const crypto = require('crypto');  
const cipher = crypto.createCipher('usrnm', 'pwdd'); 
var encryptd = cipher.update('Welcome to hackr', 'utf8', 'hex'); 
encryptd += cipher.final('hex'); 

Let’s use decipher to decrypt the above to see if we get the same text back:

const crypto = require('crypto');  
const decipher = crypto.createDecipher('usrnm', 'pwdd'); 
var encryptd = '<enter the previous output-encrypted code here>'; 
var decryptd = decipher.update(encryptd, 'hex', 'utf8'); 
decryptd += decipher.final('utf8'); 

Question: Explain the use of the DNS module in Node.js?

Answer: The DNS module is used for resolving a name. It is provided by the operating system and is used for the actual DNS lookup too. With this module, it is not required to memorize the IP addresses as the DNS servers convert domain/subdomain into IP addresses.

Question: Explain the security mechanism of Node.js?

Answer: Some mechanisms are:

  • Authorization codes: We can use authorization codes to secure Node.js. That way, any third party that wants to access Node.js goes through the GET request of the resource provider's network. 
  • Certified Modules: Certification modules are like filters that scan the libraries of Node.js to identify if any third-party code is present or not. Any hacking can be detected using certifications. 
  • Curated Screening register: This is a quality control system where all the packages (code and software) are checked to ensure their safety. This scan helps to eliminate unverified or unreliable libraries getting into your application.
  • Regular updates: Downloading the newest version of Node.js will prevent potential hackers and attacks. 

Question: Explain various types of API functions in Node.js.

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

  • Asynchronous/Non-blocking: These requests do not wait for the server to respond. They continue to process the next request, and once the response is received, they receive the same.
  • Synchronous/Blocking: These are requests that block any other requests. Once the request is completed, only then the next one is taken up.

Question: What are LTS versions of Node.js?

Answer: Long Term Support or LTS version/releases of Node.js are the releases which receive all the critical fixes, performance step ups and security updates. These versions receive support for at least 1.5 years and have a focus on security and stability of the application.

Question: Explain the working of assert in Node.js

Answer: Assert is used to write tests in Node.js. The feedback is provided only if any of the test cases that are running fails. To test invariants, the module gives you a set of assertion tests. It is used internally by Node.js, but if you use require (‘assert’) code, you will be able to use it in other applications as well.

Question: What is callback hell?

Answer: In Node.js, callback hell is also known as the Pyramid of Doom. Caused by intensively nested, unreadable and unmanageable callbacks, making the code harder to read and debug. It is caused due to improper execution of the asynchronous logic.

Question: What is stub in Node.js?

Answer: Stubs are programs or functions used to stimulate component behavior. Stubs provide the answers to the functions during test cases.

Question: What is Event Loop?

Answer: All the asynchronous callbacks are handled by an event loop in Node.js. It is the foundation of the non-blocking input/output in Node.js, making it one of the most vital features of the environment. Due to the nature of Node.js being event-driven, a listener can be attached to an event for the callback to be executed by the former when the event occurs. Node.js executes the event loop and then moves on to the rest of the code, without having to wait for the output. Once the whole operation culminates, it receives the output and the callback function is executed. Once it receives the response, the functions are executed one by one.

Question: What is stream in Node.js? What are its types?

Answer: In Node.js, streams are the collection of data similar to strings and arrays. Moreover, streams are objects through which you can read source data or write destination data continuously. These streams are particularly helpful for reading and processing large amounts of information. There are four types of streams in Node.js, which are:

  • Readable: Used to read large amount of data from source
  • Writeable: Used to write data to destination
  • Duplex: Used for both read and write
  • Transform: A duplex stream used for data modification

Question: List and explain the timing features of Node.js.

Answer: A timer module containing multiple functions for the execution of the code after a specific time period is provided by Node.js. Some of the functions provided in this module are:

  • process.nextTick: This function schedules a callback function which is required to be invoked in the next iteration of the event loop
  • setTimeout/clearTimeout: This function schedules code execution after the assigned amount of time (in milliseconds)
  • setImmediate/clearImmediate: This functions executes code at the conclusion of the existing event loop cycle
  • setInterval/clearInterval: This function is used to execute a block of code a number of times

Question: Highlight the differences between process.nextTick() and setImmediate().

Answer: Both process.nextTick() and setImmediate() are functions of the Timers module, but the difference lies in their execution.

  • The process.nextTick() function waits for the execution of action till the next pass around in the event loop or when the event loop culminates, then only the callback function is invoked.
  • The setImmediate() function is used for callback method execution on the next cycle of the event loop, which returns it to the event loop for the execution of the input/output operations.

Question: Explain readFile and createReadStream in Node.js.

Answer: Both readFile and createReadStream are ways to read and execute files provided by the Node.js.

The readFile process is fully buffered which returns response(s) only if the complete file is entered into the buffer and can be read. This process is highly memory intensive and can become slow in case the file size is large.

The createReadStream process is partially buffered, treating the entire process as a series of events. In this process, the whole files are split into chunks that are processed and then sent as a response individually in succession. Unlike readFile, createReadStream is effective when it comes to reading and processing large files.

Question: Does Node.js provide a Debugger?

Answer: A built-in TCP protocol and the debugging client is provided by Node.js. If you wish to debug your file, you can use the following argument before the name of your JS file which you wish to debug.

node debug [script.js | -e “script” | :]

Question: Describe the exit codes in Node.js.

Answer: Exit codes in Node.js are a specific group of codes that finish off processes, which can include global objects as well. Some of the exit codes in Node.js are:

  • Internal JavaScript Evaluation Failure
  • Fatal Error
  • Internal Exception handler Run-time failure
  • Unused
  • Uncaught fatal exception

Question: Why is NODE_ENV used?

Answer: When any Node.js project is in the stage of production, Node.js promotes the principle to use NODE_ENV variable to flag it. When the NODE-ENV is set to production, your application will perform at a speed 2 to 3 times faster than usual. The variable also improves judgment during the development phase of projects.

Question: What is Event Emitter in Node.js?

Answer: Node.js has an EventEmitter class which holds all the objects which can emit events. These objects hold a function called eventEmitter.on() using which multiple functions can be attached to the event emitted by the object. Whenever an object from the EventEmitter class throws an event, all the attached functions to the vent are called upon synchronously.

Question: What is Punycode?

Answer: Punycode can be defined as an encoding syntax in Node.js which is helpful for converting the Unicode string of characters into ASCII. This is done as the hostnames can only comprehend ASCII codes and not Unicode. While it was bundled up within the default package in recent versions, you can use it in the previous version using the following code:

punycode = require(‘punycode’);

Question: Explain the concept of JIT and highlight its relation with Node.js.

Answer: A JIT or Just-in-time compiler sends bytecode (consisting of interpretable instructions) to the processor by converting it into instruction. Once you are finished with the writing part of a program, the source language statements are compiled into bytecode by the compiler, rather than the code that carries the data which is similar to the destination hardware platform processor.

Node.js employs JIT compilation which improves the speed of code execution to a great extent. It takes the source code and converts it into machine code in runtime. Through this, functions that are called regularly are compiled to machine code, increasing the overall speed of code execution.

Question: Why is the buffer class used in Node.js?

Answer: In Node.js, the buffer class stores the raw data, in a manner similar to that of an array of integers. However, it communicates to a raw memory dump, allocated outside the V8 heap. The Buffer class is a global class and can be accessed in an application without having to import the buffer module. It is majorly used as pure JavaScript code is not attuned with binary data.

Question: What is the difference between fork () and spawn () methods in Node.js?

Answer: In Node.js, spawn () launches a new process with the available set of commands. This doesn’t generate a new V8 instance only a single copy of the node module is active on the processor. This method can be used when your child process returns a large amount of data to the node.

On the other hand, fork () is a particular case of spawn () which generates a new V8 engines instance. Through this method, multiple workers run on a single node code base for multiple tasks.

Question: State the steps to write an Express JS application.

Answer: To set up an ExpressJs application, you need to follow the following steps:

  • Create a folder with the project name
  • Create a file named package.json inside the folder
  • Run the ‘npm install’ command on the command prompt to install the libraries present in the package file\
  • Create a file named server.js
  • Create the ‘router’ file inside the package consisting of a folder named as index.js
  • The application is created inside the package containing the index.html file

With this, we come to an end of the top Node.js interview questions. Do you think we missed an important question or have any doubts that need to be cleared? Let us know in the comments below. You can also check our best Node.js tutorials, curated by community members.

Follow this udemy course and start brushing up your node.js skills: The Complete Node.js Developer Course (3rd Edition).

Follow this top-notch programming book to crack any programming interview: Cracking the Coding Interview: 189 Programming Questions and Solution.

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Sagar Bhatia

Sagar Bhatia

Sagar is an engineering graduate and a technology lover and has been writing across various disciplines for over 5 years now. An avid gamer himself, he wishes to create a venture revolving around the e-sports domain in India. View all posts by the Author

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