Node.js

 Node.js is a leading server-side platform for creating applications. With the growing demand, the job in this field is rapidly increasing. So, if you are planning to switch to this career, it is better to start by preparing for Node.js interview questions and answers.

Here are some of the most asked Node.js interview questions and answers for freshers, intermediate-level, and experienced professionals that you should know to ace your interview.

Node.js Interview Questions And Answers For Freshers

Let’s look at some Node.js interview questions and answers for freshers:

Q1: What is Node.js?

Ans: Node.js is an open-source library and cross-platform to run web applications. It is popular to create server-side web applications, perfect for data-intensive applications, as it uses an asynchronous and event-driven model. Besides this, it enables the developer to implement JavaScript code on any machine outside a browser.  

Q2: What is the purpose of Node.js?

Ans: Node.js is single-threaded by nature and used for non-blocking and event-driven servers. It is mainly used to develop:

• Network applications
• Real-time web applications
• Distributed systems
• General-purpose applications 

Q3: What are the benefits of using Node.js?

Ans: Here are the significant benefits of using Node.js: 

• Node.js has been considered a full-stack JavaScript for serving server-side applications and clients. Hence, the companies do not need to hire separate back-end or front-end developers, saving money and time.  
• Node.js is highly extensible, allowing you to customise and extend your Node.js further based on the requirements. You can also transmit data between the web server and the client with JSON. Besides this, it is also enabled with built-in APIs for developing HTTP, DNS, TCP etc., servers. 
• Node.js is famous for offering excellent performance and simultaneously transferring ample data. It is one of the perfect choices for back-end development that decodes the JavaScript code using Google’s VS JavaScript engine. Its web development engine helps add the machine’s code directly, making it easier and faster to write and execute server code. 
• Node.js was built for scalability, allowing numerous nodes to run simultaneously and interact with each other. It is superior to other web back-end development solutions, where the cross-platform runtime environment will promptly balance the workload.  
• Node.js has a caching module feature that reduces the loading period and enables developers to execute the codes without any error. The caching module also helps the web application get a request easily and then transmit it to in-app memory, which enables the user to get results on web pages quickly without waiting for a long time.  

Q4: Define the term I/O?

Ans: The short form for input/output refers to the program’s interaction with the system’s disk and network. Some primary examples of I/O are reading/writing data from/to a disk, talking to databases, and making HTTP requests. These are slow compared to retrieving memory or working on the CPU. 

Q5: What is a callback in Node.js?

Ans: A callback is a function called when a task is completed. It permits other codes to run in the meantime and prevents further blocking. It is an asynchronous platform, one of the reasons why Node.js highly relies on callbacks. All the APIs created in the library are written to the support callbacks. 

Q6: What is NPM?

Ans:  NPM, which stands for Node Package Manager, is responsible for handling all packages and modules for Node.js. Its two significant functions are: 

• It provides online storage for Node.js packages or modules, which can be searched on search.nodejs.org. 
• It provides command-line usability to download Node.js packages and handles Node.js versions and dependencies. 

Q7: What is the difference between Angular and Node.js?

Ans: Here are the significant differences between Angular and Node.js: 

Angular	Node.js
An open-source web application development framework	A cross-platform run-time environment for applications
Written in typescript	Written in C, C++, and JavaScript languages
Used for creating single-page web applications for client-side	Used for developing fast and scalable server-side networking applications
Best for creating interactive and creative web applications	Best for developing small-size applications
Ideal for developing real-time applications	Ideal in situations when something scalable and faster is required

Q8: Explain Node.js web application architecture.

Ans: A web application in node.js is determined in four layers: 

Client Layer: This component includes web browsers, applications, or mobile browsers that can create an HTTP request to the web server.
Server Layer: This component includes the web server that can interrupt the request made by clients and pass responses to them. 

Data Layer: This component contains databases or any other data source. 

Business Layer: This component comprises the application server used by the webserver to do the needed processing. The business layer interacts with the data layer through databases or external programs in this layer. 

Q9: How many types of API functions are there in Node.js?

Ans:  There are two types of API functions in Node.js: 

• Asynchronous, Non-blocking API functions: These API functions operate non-simultaneously, which means that Node.js will request data to the API, but it will not get intercepted till the data is obtained. Rather, it will move to the following API after appealing it, and a notification from a Node.js event will reply to the server for the previous API request.  Simply put, these functions enable working ahead while managing the requests. For example, Online forums, emails, etc.   
• Synchronous, Blocking API functions: This is opposite to synchronous blocking functions, which means that it will make the calling system for a response. Accordingly, when a system uses these API functions, it will expect to obtain immediate data after the requests are made. These APIs are beneficial and used where the availability and connectivity are high, and low delay is expected.  Simply put, the application will appeal and wait for a response until the time value is returned. For example, Instant messaging, video meetings, etc. 

Q: What is the difference between front-end and back-end development?

Ans: Here are the significant difference between front-end and back-end development: 

Front-end Development	Back-end Development
The development of an application on the front end refers to the client side of an application.	The development of an application on the back end refers to the server side of an application.
It is the part of web applications where users can see and interact.	It includes everything behind the scenes that cannot be seen or interacted with.
It includes everything that adds to the visual elements of a web application.	It includes a web server that communicates with the database to assist the users’ requests.
Some examples of front-end development technologies are CSS, HTML, JavaScript, AngularJS, Bootstrap, jQuery, and React.js.	Some examples of back-end development technologies are Java, PHP, Python, C++, Node.js, etc.
Used for SEO	Used for backup
Some front-end framework examples are AngularJS, React.js, jQuery, Sass, etc.	Some back-end framework examples are Xpress, Django, Rails, Laravel, Spring, etc.

Node.js Interview Questions And Answers For Intermediate Level

Here are some  Node.js interview questions and answers for the intermediate-level candidate that you should know: 

Q1: How can you manage the packages in the Node.js project?

Ans: You can manage the packages in the Node.js project by utilising numerous package installers and their designed files accordingly. However, most of them use NPM or yarn, where both provide nearly all libraries of JavaScript along with other numerous features of controlling environment-specific configurations. You can use package.json and package-lock.json to support the libraries’ versions installed in one project.  

Q2: What is REPL in the context of Node.js?

Ans: The full form of REPL in Node.js context is Read, Eval, Print, and Loop. It shows a computer environment like a window console or Linux where any command can take place so the system can respond with an output. Here are the tasks performed by REPL: 

• Read: Here, the reader’s input is read, dissected into JavaScript data structure and then stored in the memory. 
• Eval: The data structure is received and evaluated. 
• Print: The final result is printed. 
• Loop: Loops the given command till the time CTRL+C is pressed twice. 

Q3: What are the challenges faced in Node.js?

Ans:  These are the major challenges faced in Node.js: 

• Vast stack: Node.js does not offer any particular custom to develop an application. The frameworks used by Node.js are not opinionated and do not provide any specific guidelines for writing the codes. It is one of the reasons why each application needs a different and individual approach and experienced programmers for developing and maintaining the codes. 

• Technical errors: Several problems in Node.js occur because of a lack of practice. The open-source libraries and frameworks are highly active and provide numerous ways to build applications. However, having many options can be a major problem for developers with zero experience, leading to application structure incompatibility.  

• Scalability problems: Node.js is a single-threaded approach that makes the scaling a bit confusing. However, you might need to divide the complex applications with CPU/MEM-heavy computations into smaller microservices, which manage different operations. Hence, when you create a scalable Node.js application, you must keep it small. 

• Poor documentation: Documentation is one of the essential parts of any IT project. It gives an insight into how applications work and informs the developers about the components, how they are relatable, and what their significant function is. Poor documentation will extend the developing time and make the entire process difficult to understand. It can create multiple problems when it comes to the implementation of new features and performance.  

Q4: What are the key features of Node.js?

Ans: Here are some key features of Node.js: 

• Node.js is asynchronous and event-driven, which means that if Node receives a request from some Input/Output operation, it will execute and carry out that operation in the background along with the acceptance of other requests. Hence, it will not wait for the response to the previous requests. 
• It is fast in executing code, as Node.js uses the V8 JavaScript Runtime engine used by Google Chrome. It has a covering over the JavaScript engine that makes the runtime engine faster; hence, the processing of requests in Node.js also becomes faster. 
• Node.js operates a single-thread model for event looping. The response from these events might or might not reach the server instantly. However, it does not stop other operations from happening in the background and makes Node.js highly scalable. Traditional servers create limited threads to manage the requests, whereas Node.js creates a single thread which offers service to a larger number of requests.  

• Node.js library uses JavaScript, which is also one of the essential components of Node.js for developers. Most developers know Javascript, its features, and its usage. Because of this, development in Node.js becomes easier for a developer who knows JavaScript.

Q5: What is callback hell in Node.js?

Ans:  Callback hell, also known as the Pyramid of Doom, is a kind of situation caused by nested callbacks, which are unreadable. Each callback takes statements that have been attained due to the previous callbacks. This structure makes the code hard to read and increases lesser maintainability. It is caused by the complex execution of the asynchronous argumentation. 

Q6: What is the meaning of a test pyramid?

Ans: A test pyramid is a graph that defines the ratio of unit tests, integration tests, and end-to-end tests needed to be documented for the project’s successful development. Here is the diagram: 

Q7: How many types of streams are available in Node.js?

Ans: These are the streams available in Node.js: 

• Readable: This stream is used for reading operations.
• Writable: This stream is used for writing operations. 
• Duplex: This stream is used for both reading and writing operations.  
• Transform: This stream is where output is computed according to the input. 

Q8: What is Reactor Pattern in Node.js?

Ans:  Reactor Pattern is a notion of non-blocking I/O operations. It is used to prevent the blocking of the Input/Output operations. It offers the developers a handle which is connected to each I/O operation. Once the I/O requests are generated, they are submitted to a demultiplexer, a notification interface that handles concurrency in non-blocking I/O mode. Besides this, it also collects every request as an event and puts it in a queue. 

Q9: What are LTS releases of Node.js?

Ans: LTS, also known as Long Term Support, is a version of Node.js that receives complex bug fixes and security and performance updates. Such versions are at least supported for 18 months, focusing on security and stability. The amendments in LTS versions are limited to bug fixes, npm, security upgrades, performance improvement, documentation updates, etc.  

Q10: What do you understand by middleware in Node.js?

Ans:  Middleware is a function that receives the Request and Response objects. When it comes to the request-response cycle in an application, these functions have access to multiple requests and response objects along with the next cycle function. Here are some of the most common middleware functions: 

• Implement any type of code 
• Appeal to the next middleware in the stack 
• Update and amend the request and the response objects  
• Complete the request-response process 

Node.js Interview Questions And Answers For Experienced Professionals

Q1: What are the different modules used in Node.js?

Ans: Modules are just like JavaScript libraries in Node.js applications, which have a set of functions. To add a module in a Node.js application, developers must use brackets comprising the module’s name. 

Node.js has various modules that provide the basic functionality required for developing a web application. Some of them are as follows: 

Core Modules	Description
HTTP:	The HTTP module comprises classes, events, and methods to build a Node.js HTTP server.
Url:	This module is used to add the methods for URL determining.
Util:	The util module adds utility functions needed in the applications, which is also essential for the developers.
Fs:	The fs module adds events, methods, and classes to handle the file I/O operations.
Query string:	The query string module adds the methods to work with a query string.
Stream:	This module is used to add the methods to manage streaming data.
Zlib:	This module is used to add methods to increase and decrease the size of the files used in an application.

Q2: What is libuv in Node.js?

 Ans: libuv is a multi-platform support library of Node.js that provides asynchronous I/O support. It was initially built for Node.js, but with time, it is now used by other famous systems like Julia, Luvit, pyuv, etc. Its significant features include: 

• Asynchronous file & file system operations 
• Child processes
• File system events 
• Asynchronous TCP & UDP sockets
• Full-featured event loop backed

Q3: Is cryptography supported in Node.js?

Ans: Yes, Node.js supports cryptography through a module known as Crypto. It provides various cryptographic functionalities like deciphering, ciphering, a set of wrappers for opening SSL’s hash HMAC, verifying functions, etc. 

Q4: Define why Express ‘app’ and ‘server’ must be kept separate.

Ans: The Express ‘app’ and ‘server’ must be kept separate because it reduces coupling and covers and conceptualises the inside logic of the application. Here are some other reasons: 

• It permits testing the API in-process without making it necessary to perform the network calls 
• Faster testing execution 
• Finer separation concerns and cleaner code 
• Permits deploying the same API under different and flexible network requirements 

Q5: Explain an Event Emitter in Node.js.

Ans: Event Emitter is a type of class in Node.js, including objects that can emit events. These objects comprise an eventEmitter.on() function, and through this, more than one function can be linked to named events emitted by the object. All the functions attached to a certain event are triggered synchronously when an EventEmitter object throws an event.  

Q6: Name some of the exit codes of Node.js.

Ans: Exist codes are a group of codes used to complete a specific procedure, including the global object. Here are some exit codes which are used in Node.js: 

• Unused
• Fatal Error

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

Q7: What is the usage of buffer class in Node.js?

Ans: Buffer class in Node.js is used to manage raw binary data in an array of integers. However, it coincides with raw memory allocation situated outside the V8 heap. It is highly accessible and can be accessed in an application without adding a buffer module. Simply put, buffer class is used because JavaScript is not amicable with binary data. 

Q8: Explain Punycode in Node.js.

Ans: Punycode is an encoding syntax that converts a Unicode string of characters to a basic ASCII string of characters. It is imperative because hostnames can only understand ASCII characters/ In simple words, it is a character encoding technique defined by RFC 3492 that is mainly deliberated for use in Internationalised Domain Names. 

Q9: How is clustering used to enhance the performance of Node.js?

Ans: Clustering is used to overcome the issue of a single processor. When you start using clusters in a Node.js application, it creates various node.js processes. However, there is another parent process, which is called cluster manager. Its responsibility is to monitor the health of the individual instances of the application to ensure smooth running. 

Q10: How Node.js handles the child threads?

Ans:  Node.js is a single-threaded process, which does not reveal child threads thread management techniques. However, it is still used for some particular asynchronous I/O tasks that run in the background and do not usually implement any JS code or block the main event loop in the application. If you want to use the child threading technique in your application, you must add the ChildProcess module during development.

Interview Questions

 Are you looking for JavaScript interview questions? You have arrived at the right place. JavaScript is an open-source scripting language designed for creating applications. It is lightweight, interpreted, and works faster than many other programming languages. It is the third biggest web technology to develop online and mobile applications, games, web servers, etc., that can be used in browsers.

In this article, we will mention some of the most popular and important JavaScript interview questions for beginners, intermediate, and advanced levels. So, let’s get started!

JavaScript Interview Questions For Beginners

Here are some JavaScript interview questions for beginners that you need to know:

Q: What is JavaScript?

Ans: JavaScript is a popular open-source programming and scripting language for enhancing web pages and creating an excellent user experience. It differs from Java language and is an object-based, cross-platform, translated, and lightweight language widely used for client-side scripting.

Q: What are the benefits of JavaScript?

Ans: Like other programming languages, JavaScript also has its advantages, which are as follows:

• Speed: It is an interpreted and client-side scripting language, which reduces the time needed by other programming languages, such as Java, for compilation.
• Simple: JavaScript is an easy-to-understand and learns, where the structure is relatively simple for both users and developers. It is relatively easy to execute, saving a lot of money to build creative and dynamic web content.
• Versatile: It is a versatile language capable of both front-end and back-end development. Back-end development uses NodeJS, while several other libraries help in front-end development, like AngularJS, ReactJS, etc.
• Less Overhead: JavaScript reduces the code length, which helps in enhancing the performance of websites and web applications. The codes comprise less overhead because of multiple built-in functions for DOM, loops, access, etc.

Q: What is a Function in JavaScript?

Ans: Functions in JavaScript are just like a process, a group of statements that performs a task or calculates a value. It is a set of reusable codes which can be called anywhere in the program, eliminating the need to repeat the code. In simpler terms, it enables the programmer to divide a big program into several small and convenient functions.

Example of Function Declaration

This is an example of Function Declaration to give you a brief understanding:

Function abc(domain) {
const abc = `Contract jobs ${domain}`;
console.log(abc);
} 

Example of Function Expression

This is an example of Function Expression to give you a brief understanding:

const abc = function () {
const abc = `Contract jobs ${domain}`;
console.log(abc);
 }

Q: List some essential features of JavaScript.

Ans: JavaScript is a popular language with multiple features when developing a web application. Some of them are as follows:

• Lightweight Scripting Language: It is a lightweight scripting language, as it is made for managing data on the browser. It is not a general-purpose language, having only considered a set of libraries. Besides, it is also developed for client-side execution for web applications; thus, it is lightweight.
• Stable Control: JavaScript offers stable control to the browser instead of being completely confided on the web servers. It offers multiple browsers with additional functionalities that help reduce server load and network traffic.
• Client-side Calculations: JavaScript is a client-side scripting language, it can perform simple calculations on the browser. The browser does not need to ask for permission for every task. It is highly helpful when a user needs to perform these calculations multiple times. In such cases, connecting to the server will take much more time than executing the actual calculations.
• Generating HTML Content: JavaScript has useful features for producing HTML content for the website. It allows the developers to add texts, tables, links, images, etc., after an event comes up.

Q: What is the difference between Java and JavaScript?

Ans: Here is the significant difference between Java and JavaScript:

Java	JavaScript
Java is a strongly typed programming language.	It is a scripting language.
It is an object-oriented programming language.	It is an object-based language.
It is a standalone language.	It is not a standalone language because it needs to combine into an HTML program for implementation.
A strongly types language, making users choose the data type of a variable before declaring using it.	A loosely typed language makes users not bother about the variable’s data type before and after the declaration.
It should be merged before the execution.	It needs to be incorporated into the HTML program for implementation.
A difficult language to learn	An easy language to learn
Not required to run Java programs	Required to run JavaScript programs.
By using multi-threading, developers can perform difficult tasks	Users are not able to perform complicated tasks
It was developed by Sun Microsystems.	It was developed by Netscape.

Q: What is BOM in JavaScript?

Ans: BOM, also known as the Browser Object Model, is one of the essential elements of JavaScript on the web. It offers all the objects and functionality and lets the developers interact with the browser.

Q: What is DOM in JavaScript?

Ans: DOM stands for Document Object Model, which is the data representation of the objects that includes the structure and content of a document on the web. In simple terms, it is a programming interface for web documents, representing the page so that programs can change the style, content, and document structure.

Q: What is the real name of JavaScript?

Ans: JavaScript was developed by Brendan Eich of Netscape Communications Corporation. It was originally named Mocha, then LiveScript, and again renamed to JavaScript.

Q: What is the use of a history object in JavaScript?

Ans: The JavaScript History is a built-in Interface used to access the browser history. It contains the list of links visited by the user in a specific browser window, frame, tab, etc. It is an essential property of the JavaScript window object, which can be accessed via the window.history property.

Q: What is an anonymous function in JavaScript?

Ans: It is a function without having any name. These functions are used differently during the runtime, using the function operator rather than the function declaration. It is relatively more flexible than function declaration and can be easily used in the place of an expression.

Javascript Interview Questions For Intermediate

Here are some JavaScript interview questions for Intermediate-level that you need to know:

Q1: What is the need for debugging in JavaScript?

Ans: Debugging is one of the essential and must-have skills for developers. It is the process of identifying the bugs and resolving them to enhance the efficient running of programs. JavaScript has multiple debugging strategies and tools to improve the performance of the code.

Q2: What is the role of a strict mode in JavaScript?

Ans: Strict Mode is a new feature that enables developers to place a function or program in a strict operating context. It prevents some particular actions from being taken, making JavaScript a safer option.

Q3: What is the use of Math Object in JavaScript?

Ans: Math is a built-in object with properties and methods for mathematical constants and functions, where its properties and methods are static. Unlike other objects, it has no constructor.

Q4: What is the meaning of hoisting in JavaScript?

Ans:  Hoisting in JavaScript is a concept which allows you to remove values of variables and functions even before providing value without getting errors. It enables functions to be safely used in code before their declaration.

Q5: What are the types of scopes in JavaScript?

Ans:  There are four types of scopes in JavaScript, which are as follows: 

• Block Scope: Initially, JavaScript has Global and Function scope. let, and const are the two important keywords developed by ES6, which provide Block Scope in JavaScript. To access the variables of this specific block, you need to create an object for it.   
• Local Scope: Local scope comprises things that are declared inside the code blocks and only accessible where it is declared.
• Function Scope: JavaScript also has a function scope, where each function creates a new scope. Variables explained inside a function cannot be accessed from outside the function, and variables declared with let, var, and const are just like when declared inside a function.
• Global Scope: Variables declared Globally, outside any function, can be accessed from anywhere in a program. These include Global Scope and Global variables, similar to function scope variables declared with var, let, and const, similar to when declared outside the block.

Q6: What is a callback in JavaScript?

Ans: Callback in JavaScript is a function that is passed to another function as an argument, which is then gathered inside the outset function to execute some routine and action. It is needed because multiple actions in JavaScript are asynchronous, which means they do stop the function from running till the time they are completed. Rather, it will run in the background while the rest of the code is still running.

Q7: What is the difference between Function declaration and Function expression?

Ans: Here is the significant difference between Function Declaration and Function Expression:

Function Declaration	Function Expression
Also known as a function statement, declares a function with a function keyword. However, the function declaration must have a function name.	Just like a function declaration without having any function name.
Does not require a variable assignment because they are standalone constructs and cannot be put inside a functional block.	Can be stored in a variable assignment.
Executed before any other code.	Load and execute only when the program interpreter gets the code line.
Can be accessed before and after the function explanation.	Can be accessed only after the function definition.

Q8: How can you add JavaScript code to HTML pages?

Ans: You can add JavaScript code to HTML pages in three ways, which are as follows:

• Embedding Code: For adding JavaScript code into HTML pages, you can use the <script>….</script> HTML tag that covers around JavaScript code inside the HTML program. You can also place JavaScript code in the body or head section, as it entirely relies on the design of the web page that developers use.
• Inline Code: This process is used to call a function in the HTML event attributes. There are multiple events or cases where developers have to add JavaScript code.
• External File: Developers can also make a separate file to keep the code of JavaScript with the (.js) extension and later add it to the HTML document. You can do it using the src attribute of the <script> tag. It helps maintain the web pages and also saves time from writing the same code again.

Q9: How can you delete a cookie in JavaScript?

Ans: You can delete a cookie in JavaScript in multiple ways, such as by using expire attribute, web browser, and max-age attribute.

Q10: What are the closures in JavaScript?

Ans: Closures are an essential JavaScript concept that every developer needs to know. It is a property of JavaScript that refers to independent variables, also known as free variables. It is a feature in which the inner function has access to the outer function variable. The three scope chains of closure are as follows:

• Access to the outer variables
• Access to its own scope
• Access to the global variables

JavaScript Interview Questions For Advanced Level

Here are some JavaScript interview questions for Advanced-level that you need to know:

Q: Name three JavaScript frameworks and their uses.

Ans: There are various JavaScript frameworks, and here are the most popular ones:

• Angular: It is a powerful JavaScript framework which works across multiple operating systems. It works by using HTML vocabulary on different web pages. Angular is a cross-platform framework, easy to comprehend and has fewer errors via its concise and data rendering technique. Apart from this, it can work on multiple operating systems.
• Vue JS: It is another JavaScript framework usually used in building cross-platform applications and websites. Other than that, it is also used to build Single Page Applications. It is flexible, small in size, easy to understand, and provides two-way communication.
• React JS: It is an open-source and one of the best JavaScript frameworks to create responsive and active UI (user interfaces). It is also SEO-friendly and highly recommended by developers who create e-commerce websites. Its key features include one-way data-binding only, Virtual Document Model (DOM), ability to test and debug quickly.

Q: What are object prototypes?

Ans:  Prototypes are objects where JavaScript inherit features from one another. Simply, it is used to add new properties and methods to already existing objects and object constructors.

Q: What is memoisation?

Ans: Memoisation is one of the best techniques to make applications work faster and more effectively. It caches the results and retrieves the same information from the cache when it is needed next time for cashing instead of adding it again.

Q: What is the difference between the client-side and the server-side scripting?

Ans: Here are the significant differences between client-side and server-side scripting:

Client-side Scripting	Server-side Scripting
The main purpose is to give the requested output to the end-user	The main purpose is to manipulate and offer access to the requested database
It is used at the front end, and the source code is visible to the user	It is employed at the backend, where the source code is not shown to the users as its output of server-side is an HTML
Does not need interaction with the server	Requires interaction with the server
Relatively less secure because of code accessibility given to the client	Highly secure while working on the web application as the code is not visible
Executed on the remote computer system	Executed on the web server
Highly depends on the user’s browser version	Does not depend on the client
Does not provide any access to the files on web	Provides complete access to the file that is stored in the web database server
Customise web pages and offer unique websites	Reduces server load

Q: What is a WeakMap?

Ans: A WeakMap is a structure of key/value pairs that only allows objects as keys and removes them altogether with the related value once it cannot be accessed.  It permits associating data to objects in a way that does not prevent the key objects from being collected, even if the values show the key.

Q: What is Event Bubbling in JavaScript?

Ans: Event bubbling is a strategy of event circulation in the HTML DOM API when an event is in an element that is inside another element, and both of those elements have registered a handle to that event. In simple words, it is an element that triggers the event and then bubbles up other elements in the library.

Q: State the difference between declaring variables using var, let, and cont.

Ans:  Here are the major differences between var, let, and const:

var	let	const
The scope of a varvariable in functional scope	The scope of a let variable is block scope	The scope of a const variable block scope
Can be updated and re-declared into the scope	Can be updated, but it cannot be re-declared into the scope	Cannot be updated or re-declared into the scope
Can be declared without initialisation	Can be declared without initialisation	Cannot be declared without initialisation
Can be accessed without initialisation because its default value is not defined	Cannot be accessed without initialisation, else it will give ‘referenceError’	Cannot be accessed and declared without initialisation
Hoisting is done by initialising as default value	Hoisting is done, but not initialised	Hoisting is done but not initialised

Q: What are undeclared and undefined variables?

Ans: 

Undefined Variable	Undeclared Variable
An undefined variable is a variable which has been declared but does not have a value.	An undeclared variable is a variable that does not even exist in the program.

Q: What is Event Capturing in JavaScript?

Ans: Event capturing is one of the two ways of event distribution in the HTML DOM. In this process, the outermost element triggers the target element, which is the opposite of event bubbling.

Q: What is Strict mode in JavaScript?

Ans: Strict mode in JavaScript is a newly added feature which permits specific JS code to behave dynamically in a semantic order to secure it.

Conclusion 

Now you know all the essential JavaScript interview questions, you can start preparing for them. Practice makes a man perfect, so sit in front of the mirror and ask yourself questions as if you are giving your interview. If you want to learn more about JavaScript, you can get yourself enrol in JavaScript training.