Node.js interview questions and answers

Designed by Ryan Dahl back in 2009, Node.js is a server-side platform built on the Javascript Engine of Google Chrome. It is an open-source, cross-platform runtime environment that developers can use for developing networking and server-side applications. Node.js apps are written in JavaScript. Also, you can run these apps in the Node.js runtime on OSX, Microsoft, and Linux. This runtime allows developers to execute the JavaScript code on any machine outside the browser.

There are several areas where Node.js is highly helpful by being the best and perfect technology partner, such as the data streaming applications, single page applications, data-intensive real-time applications, I/O bound applications, and JSON APIs-based applications.

Thanks to its efficiencies, several industry-leading firms are using Node.js, such as GoDaddy, PayPal, Yahoo, Wikipins, General Electric, and Uber are some of the brands to name a few.

The platform has rich libraries of several JavaScript modules that simplify web application development to a great extent. You can use Node.js for free under the MIT license.

It is one of the few reasons most enterprises demand a certified Node.js developer to manage their server side. The experience in Node.js brings in several career opportunities. If you are planning to go to the Node.js interview, you should know about the top Node.js interview questions and answers which you should prepare. By reading this guide, you will get a competitive edge to help you ace your interview.

Benefits And Features

• All APIs of the Node.js library are non-blocking. In other words, it is a based service that does not wait for the API to return the data. The server will shift to the next API after calling it, and the event mechanism of Node.js helps the server respond better to the previous API call.

• Node.js is made on Google Chrome’s V8 JavaScript engine. Moreover, the platform consists of a library that is quick in code execution.

• The applications built with Node.js never buffer any data; these apps simply output the data chunks.

1 . What is Node.js? Where can you use it?

Node.js is an open-source, cross-platform JavaScript runtime environment and library to run web applications outside the client’s browser. It also helps to create server-side web applications.

Node.js is perfect for data-intensive applications as it uses an asynchronous, event-driven model. You can use  I/O intensive 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.

2 . Why use Node.js?

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

3 . Explain the difference between frontend and backend development?

4 . How do you create a simple server in Node.js that returns Hello World?

var http = require("http");
http.createServer(function (request, response) {
  response.writeHead(200, {'Content-Type': 'text/plain'});
  response.end('Hello World\n');
}).listen(3000);

5 . What is the difference between Node.js and JavaScript?

6 . Why is Node.js Single-threaded?

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

7 . If Node.js is single-threaded, then what about the concurrency?

The Multi-Threaded Request/Response Stateless Model is not followed by the Node JS Platform. Also, it adheres to the Single-Threaded Event Loop Model. The Node JS Processing paradigm is also heavily influenced by the JavaScript Event-based model and also the JavaScript callback system. As a result, Node.js can easily manage more concurrent client requests. The event loop is the processing model’s beating heart in Node.js.

8 . Explain callback.

Programmer calls a callback function after a given task. It allows other code to be run in the meantime and also prevents any blocking.  Being an asynchronous platform, Node.js heavily relies on callback.You must prefer to write all APIs of Node to support callbacks.

9 . What are the advantages of using promises instead of callbacks?

• The control flow of asynchronous logic is more specified and structured.
• The coupling is low.
• We’ve built-in error handling.
• Improved readability.

10 . How would you define the term I/O?

• The term I/O describes any program, operation, or device that transfers data to or from a medium and to or from another medium
• Every transfer is an output from one medium and an input into another. The medium can be a physical device, network, or files within a system

Uses

Node.js has following applications:

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

11 . What is NPM?

NPM stands for Node Package Manager, responsible for managing all the packages and modules for Node.js.

Node Package Manager provides two main functionalities:

• Provides online repositories for node.js packages/modules, which are searchable on search.nodejs.org
• Provides command-line utility to install Node.js packages and also manages Node.js versions and dependencies 

12 . What are the modules in Node.js?

Modules are like JavaScript libraries to include a set of functions. To include a module in a application, use the require() function with the parentheses containing the module’s name.

Node.js has many modules to provide the basic functionality needed for a web application. Some of them include:

13 . What are the security implementations?

Following are the important implementations for security:

• Error handling protocols
• Authentication pipelines

14 . What is the purpose of the module. Exports?

In Node.js, a module encapsulates all related codes into a single unit of code that can be parsed by moving all relevant functions into a single file. You may export a module with the module and export the function, which lets it be imported into another file with a needed keyword.

15 . Why is Node.js preferred over other backend technologies like Java and PHP?

Some of the reasons why you should prefer it :

• It is very fast
• Node Package Manager has over 50,000 bundles available at the developer’s disposal
• Perfect for data-intensive, real-time web applications, as Node.js never waits for an API to return data
• Better synchronization of code between server and client due to same code base
• Easy for web developers to start using Node.js in their projects as it is a JavaScript library

16 . What are the different API functions supported by Node.js?

There are two types of API functions. They are as follows:

• Synchronous APIs: Used for non-blocking functions
• Asynchronous APIs: Used for blocking functions

17 . What is the difference between Angular and Node.js?

18 . What are some of the most commonly used libraries in Node.js?

There are two commonly used libraries here:

• ExpressJS – Express is a flexible Node.js web application framework that provides a wide set of features to develop web and mobile applications.
• Mongoose – Mongoose is also a Node.js web application framework that makes it easy to connect an application to a database.

19 . What is the meaning of a test pyramid?

A test pyramid is a methodology that is used to denote the number of test cases executed in unit testing, integration testing, and combined testing (in that order). This is maintained to ensure that an ample number of test cases are executed for the end-to-end development of a project.

20 . What are the pros and cons of Node.js?

21 . What is the difference between Ajax and Node.js?

22 . What is the command used to import external libraries?

The “require” command is used for importing external libraries. For example – “var http=require (“HTTP”).”  This will load the HTTP library and the single exported object through the HTTP variable.

Now that we have covered some of the important beginner-level Node.js interview questions let us look at some of the intermediate-level Node.js interview questions.

Var http + require(‘http’);

23 . What does event-driven programming mean?

An event-driven programming approach uses events to trigger various functions. An event can be anything, such as typing a key or clicking a mouse button. A call-back function is already registered with the element executed whenever an event is triggered.

24 . What is an Event Loop in Node.js?

Event loops handle asynchronous callbacks in Node.js. It is the foundation of the non-blocking input/output in Node.js, making it one of the most important environmental features.

25 . What is the control flow function?

The control flow function is a common code snippet, which executes whenever there are any asynchronous function calls made, and they are used to evaluate the order in which these functions are executed in Node.js.

26 . Differentiate between process.nextTick() and setImmediate()?

The distinction between method and product. This is accomplished through the use of nextTick() and setImmediate(). next Tick() postpones the execution of action until the next pass around the event loop, or it simply calls the callback function once the event loop’s current execution is complete, whereas setImmediate() executes a callback on the next cycle of the event loop and returns control to the event loop for any I/O operations.

27 . Explain asynchronous and non-blocking APIs in Node.js.

• All Node.js library APIs are asynchronous, which means they are also non-blocking
• A Node.js-based server never waits for an API to return data. Instead, it moves to the next API after calling it, and a notification mechanism from a Node.js event responds to the server for the previous API call

28 . What is the buffer class in Node.js?

Buffer class stores raw data similar to an array of integers but corresponds to a raw memory allocation outside the V8 heap. Buffer class is used because pure JavaScript is not compatible with binary data

29 . What is piping in Node.js?

Piping is a mechanism used to connect the output of one stream to another stream. It is normally used to retrieve data from one stream and pass output to another stream

30 . What is callback hell?

• Callback hell is also popular as the pyramid of doom. It is the result of intensively nested, unreadable, and unmanageable callbacks, which in turn makes the code harder to read and debug
• Also, Improper implementation of the asynchronous logic causes callback hell

31 . What is a reactor pattern in Node.js?

A reactor pattern is a concept of non-blocking I/O operations. This pattern provides a handler that is also associated with each I/O operation. As soon as an I/O request generates, it submits to a demultiplexer

32 . What are the input arguments for an asynchronous queue?

There are two main arguments that an asynchronous queue uses. They are:

• Concurrency value
• Task function

33 . What are the different types of HTTP requests?

HTTP defines a set of request methods used to perform desired actions. The request methods include:

GET: Generally, used to retrieve the data

POST: Generally used to make a change in state or reactions on the server

HEAD: Similar to the GET method, but asks for the response without the response body

DELETE: Generally, used to delete the predetermined resource

34 . What is WASI?

The WASI class implements the WASI system called API and extra convenience methods for interacting with WASI-based applications. Moreover, every WASI instance represents a unique sandbox environment. Each WASI instance also must specify its command-line parameters, environment variables, and sandbox directory structure for security reasons.

The following is the order in which control flow statements processes function calls:

• Handling execution and queue
• Data collection and storage
• Also, concurrency handling and limiting
• Also, execution of the next piece of code

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

REPL is Read, Eval, Print and Loop. REPL is a computer environment similar to a Windows console or Linux shell. Here, you can enter any command that meets with a system-response output. The REPL environment is also bundled with Node.js by default. Also, it performs the following tasks:

• Reads user inputs, converts them into JavaScript data-structures, and stores them.
• Evaluates data structures.
• Also, prints the final output
• Loops the provided command until CTRL + C is pressed two times.

36 . What is Libuv?

Libuv is a most popular library present in Node.js. It also complements the asynchronous I/O functionality of Node.js.

Following are some of the features of Libuv:

• File system event handling
• Child forking and handling
• Also, asynchronous UDP and TCP sockets
• Asynchronous file handling and operations

37 . Describe the error-first callback in Node.js.

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 while 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, then you can also call a callback function with null as the first argument.

38 . What is the difference between spawn and fork methods in Node.js?

The spawn() function creates a new process and also launch it using the command line. What it does is that it creates a node module on the processor. Node.js invokes this method when the child processes return data.

The following is the syntax for the spawn() method:

child_process.spawn(command[, args][, options])

Coming to the fork() method, you can also consider it as an instance of the already existing spawn() method. Spawning ensures that there is more than one active worker node to handle tasks at any given point in time.

The following is the syntax for the fork() method:

child_process.fork(modulePath[, args][, options])

39 . Explain LTS releases of Node.js.

You can apply LTS or Long-Term Support to release lines you support and also maintain by the Node.js project for an extended period. Moreover, there are two types of LTS: 

1. Active, which you can actively maintain and upgrade, and also
2. Maintenance line nearing the end of the line and also maintained for a short period.

40 . What is the use of middleware in Node.js?

A middleware is a simple function that has the ability to handle incoming requests and also outbound response objects. The use of middleware is primarily for the following tasks:

• Execution of code (of any type)
• Moreover, updating request and response objects
• Completion of request–response iterations
• Also, Calling the next middleware

41 . What are global objects in Node.js?

Global objects are objects with a scope that is accessible across all of the modules of the Node.js application. However, there will not be any need to include the objects in every module. You also need to declare one of the objects as global. You do this so that you can provide any functions, strings, or objects access across the application.

42 . Use of assert in Node.js?

The use of Assert is to explicitly write test cases and also to verify the working of a piece of code. The following code snippet denotes the usage of assert:

var assert = require('assert');
function add(x, y) {
return x + y;
}
var result = add(3,5);
assert( result === 8, 'three summed with five is eight');

43 . What are stubs in Node.js?

Stubs are simply functions that you can use to assess and also analyze individual component behavior. When running test cases, stubs are useful in providing the details of the functions executed.

44 . Use of ExpressJS.

The most popular use of ExpressJS is to design a framework using Node.js. Express.js also uses a management point that controls the flow of data between servers and server-side applications.

Being lightweight and flexible, Express.js also provides users with lots of features used to design mobile applications.

45 . What is the crypto module in Node.js? How to use it?

The crypto module in Node.js is most popular for cryptography, i.e., it includes a set of wrappers for the open SSL’s hash, HMAC, sign, decipher, cipher, and verify functions.

For instance, here’s an example of using a 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'); 

console.log(encryptd);

Let’s also 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'); 

console.log(decryptd);

46 . What are streams in Node.js?

Streams are a set of data entities in Node.js. You can consider these similar to the working of strings and also likearray objects. Moreover, these streams are most optimal for continuous read/write operations across a channel. But, if there is no channel, then you can’t push all of the data to the memory at once. Hence, using streams will make it easy to process a large set of data in a continuous manner.

47 . Explain readFile and createReadStream in Node.js.

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

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

The createReadStream process is partially buffered. It also treats the entire process as a series of events. However, in this process, it splits the whole files into chunks that you can easily process and send as a response individually in succession. Unlike readFile, createReadStream is effective when it comes to reading and processing large files.

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