Javascript Interview Questions?

JavaScript became very popular for making websites more dynamic and interactive, especially in the past five years.

It's well-known for front-end development, but it's also used for back-end development with NodeJS. Additionally, it's handy for making mobile apps, desktop apps, and progressive web apps.

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Basic Javascript Interview Questions & Answers

Below are some of the top JavaScript interview questions for freshers (with proper answers).

1. What is JavaScript?

JavaScript, also known as JS, is a popular dynamic programming language used for front-end development. The primary purpose of using JS on web pages is to show things in a dynamic manner.

In front-end development, HTML and CSS are used to show static content on a web page. In scenarios where you need to show dynamic content, animation, interactive maps, etc., the role of JavaScript comes into play. Developers call it the third layer, followed by HTML and CSS.

JavaScript is a widely used programming language on the internet, especially for creating dynamic and interactive features on websites like adding new content or changing colors without refreshing the page. It's not only used in web development but also in servers, browsers, and even games.

2. What is JavaScript Promise?

A JavaScript Promise is a way to handle asynchronous operations in a more organized and structured manner. It represents a value that might be available now, in the future, or never. Promises are commonly used for tasks like making network requests or reading files

3. What is JavaScript closure?

A JavaScript closure is like a self-contained function that has access to variables from the outer or surrounding function, even after the outer function has finished executing. In simple terms, it's a way to store and use data from an outer function within an inner function. Closures are often used to create private variables, maintain state, and control access to data in JavaScript.

4. What is JavaScript array?

A JavaScript array is like a special container that can hold multiple pieces of data, such as numbers, text, or other objects. These pieces of data, called elements, are stored in a list-like structure, and each element has a unique position or index in the array. You can use arrays to organize and work with collections of related information.

5. What are the characteristics of an array in JavaScript?

Arrays in JavaScript have several key characteristics:

  1. Resizable
  2. Zero-indexed
  3. Ability to build shallow copies
  4. Feature to store collection of different items or data types

6. What is JavaScript function?

A JavaScript function is like a reusable set of instructions that you can define and name. It's a block of code that can perform specific tasks when you call it in your program. Functions are handy because they allow you to write the code for a particular task once and then use it multiple times throughout your program. This makes your code more organized and easier to maintain.

7. What is JavaScript object?

A JavaScript object is a data structure that stores related information as key-value pairs. It allows you to group data and functions together. Objects are fundamental in JavaScript, representing everything from DOM elements in web pages to custom data structures. They enable the creation of complex and organized data in JavaScript programs.

9. What is isNan() function in JavaScript?

The full form of NaN is Not a Number. So, in simple terms, the isNaN() is a function in JavaScript that is used to check whether a value is a number or not once it is converted to a number.

If the value is Not a Number, it will show true. It should be noted that this function first changes the values to numbers and then checks whether it is NaN or not.

10. What is the difference between client-side and server-side JS?

Client-Side Server-Side
Users can see the source code Users can’t see the source code
It serves the requests of the users and shows the content accordingly. It is used to manage the database and other backend aspects.
Dependent on browser No dependency on browsers
Runs on the web browsers Runs on the web servers
Less data security Strong data security

11. Is JavaScript case-sensitive?

Yes. It is a case-sensitive language.

12. What is JavaScript DOM?

DOM stands for Document Object Model. It is an interface in coding that is used to represent the data and content of a web document. It helps in enabling programs to modify the style, content, and structure of a web document.

For instance, you can call the web page a document. If you want to edit this page in JavaScript, then the role of DOM comes into play.

13. What is JavaScript event?

Every action that occurs in code or programming is an event. For instance, the loading of a page is an event. Clicks by the end users on a button are an event. The role of events in JavaScript is to make the program understand that some action has been taken and to react accordingly to that action.

14. What is JavaScript callback?

The callback is a function in JavaScript. Its role is to work as an argument by getting passed to other functions. When it is passed to the other functions, those functions will also have the argument executed.

Since it is passed in functions to call back the arguments in more functions as well, its name is justified. It must be noted that you can use callback in other functions only if they are already executed.

15. What is BOM in JavaScript?

BOM in JavaScript stands for "Browser Object Model." It refers to the set of objects and functions provided by web browsers to interact with and control the browser itself. This includes things like managing browser windows, handling cookies, and interacting with the browser's history

16. What are comments in JavaScript?

JavaScript comments are like notes you write within your code to explain things or leave messages. You can use them to add information, suggestions, or warnings. The important thing to remember is that the JavaScript engine completely ignores these comments, so they don't affect how your code works.

17. What is runtime environment in JavaScript?

The runtime environment is where a program runs. It can use things like objects and libraries to execute the code. Developers typically use two types of JavaScript environments: one for web browsers and another for Node.js.

The web browser environment uses a JavaScript engine, web features, an event system, and a queue for tasks.

18. What are the advantages of JavaScript?

JavaScript is awesome because:

  1. It works on all web browsers, so you can use it everywhere.
  2. It makes websites interactive and dynamic, so you can click, drag, and do cool stuff.
  3. You can use it to validate forms and give users helpful feedback.
  4. It's easy to learn, and there are tons of resources to help you.
  5. Many companies need JavaScript developers, so it's a useful skill to have.

19. What is JavaScript used for?

JavaScript is used for making websites interactive and dynamic. It's the language behind things like online forms, games, and real-time updates on web pages.

JavaScript is a programming language that is primarily used for making websites more interactive and dynamic. It allows web developers to create features like interactive forms, games, animations, and real-time updates on web pages.

  1. Creating Interactive Web Pages
  2. Web-based Game Development
  3. Web App Development
  4. MObile App Development
  5. Web Servers

20. Who developed JavaScript?

JavaScript was developed by Brendan Eich in 1995 while he was working at Netscape Communications Corporation.

21. What are the disadvantages of JavaScript?

JavaScript is a widely used programming language for web development, but it does have some disadvantages, including:

  1. Browser Compatibility:
  2. Security Risks:
  3. Client-Side Execution:
  4. Performance
  5. No Multithreading:
  6. Lack of Strong Typing:
  7. Complexity
  8. Lack of Offline Support:
  9. Search Engine Optimization (SEO):

22. What is the difference between JavaScript and Java?

JavaScript and Java are two different programming languages, despite their similar names. Here are the key differences:

Below, we have curated a tabular comparison of Java vs JavaScript so that it becomes easier for you to understand the main differences.

Java JavaScript
  • Object-oriented language
  • Object-based scripting language
  • Used for complicated tasks and processes
  • Not suitable for complicated tasks
  • Requires code compilation
  • Interpreted, text-based code
  • Independent language
  • Used in conjunction with HTML
  • Strongly typed; requires variable declaration
  • Loosely typed; no strict data type requirements
  • Static typing
  • Dynamic typing
  • High memory consumption
  • Low memory consumption
  • Saved as bytecode (.java)
  • Saved as source code (.js)
  • Uses threads for concurrency
  • Uses events for concurrency
  • .java
  • .js
  • Supports multithreading
  • Doesn't support multithreading
  • Objects are based on classes
  • Objects are based on prototypes
  • Requires JDK (Java Development Kit)
  • Can be run with a text editor
  • Primarily used for backend development
  • Can be used for both front-end and back-end
  • 23. What was the first name of JavaScript?

    JavaScript was originally called "Mocha" when it was first created by Brendan Eich in 1995.

    Later, it was briefly named "LiveScript" before it got its current name, JavaScript, which was a marketing move to associate it with the popular Java programming language of the time. Despite the name changes, the language's core features and functionality remained largely the same.

    24. When was JavaScript invented?

    JavaScript was invented in 1995. It was created by Brendan Eich while he was working at Netscape Communications.

    The development of JavaScript was a response to the need for a programming language that could be used to add interactivity to websites. It was initially called "Mocha" and later "LiveScript" before being officially named JavaScript.

    JavaScript quickly gained popularity and became a fundamental part of web development, allowing web developers to create dynamic and interactive websites.

    25. What is JavaScript ES6?

    ES6 stands for ECMAScript 6. It is a version of JavaScript which was released in 2015. Hence, it is also called ES 2015.

    There are plenty of new features in JavaScript ES6 compared to the prior versions. These are intended to write better and clean code, while helping developers to get more done with less code.

    26. What is a named function in JS?

    A named function in JavaScript is just a function that has been given a name. You give it a name using the "function" keyword. This is an important topic for JavaScript beginners and could be a common question in interviews.

    27. What is an anonymous function?

    Anonymous functions are like secret agents of code - they don't have names, and they're declared without any labels.

    In JavaScript, when you create an anonymous function, you can't use it immediately. To use it, you need to store it in a variable first.

    28. What is the difference between JavaScript and JScript?

    JavaScript and JScript are often used interchangeably because they are similar scripting languages, but there are some differences between them:

    JavaScript JScript
    A trademark of Oracle Corp. Microsoft owns JScript
    No support for active content creation Supports active content creation
    Can manage multi-browser compatibility Supports Internet Explorer only
    Code runs in browser Code runs only in Internet Explorer
    Can’t access objects of the browser Can access objects of Internet Explorer
    Widely used Not popular as it is supported by Internet Explorer only

    29. How many ways are there to create an object in JavaScript?

    There are three ways to create a JavaScript object:

    Ways Syntax
  • Object literal
  • object = { property1: value1, property2: value2, ...propertyN: valueN }
  • Using new keyword
  • var objectname = new Object();
  • Using object constructor
  • var objectname = new Object();
  • 30. What is JavaScript prototype?

    A prototype in JS is one of the most useful objects using which developers can inherit one object’s features to another. You can say that a prototype is linked to all the objects and functions in JS. That’s what it makes capable of inheriting the features of an object.

    31. What is the role of JavaScript callback function?

    Since numerous actions in JS are asynchronous, these actions are not capable of holding a program from running before completion. Using the callback function, we can define when to run the code.

    32. What is the use of window object in JS?

    In JavaScript, the window object shows that a window is open in the browser. This object supports all the popular web browsers; therefore, the objects, functions, and variables are members of the window object.

    Moreover, you can say that the global variables of JS are considered properties, whereas the global functions are considered methods of the window.

    33. What is the use of history object in JavaScript?

    The use of the JS history object is to find the web URLs that a user has visited in the browser. In addition, it also helps in browsing the last and next pages, as well as specific pages that a user has visited.

    As an important property of the window object, the history object is accessible by:

    1. window.history
    2. history

    34. How to use JavaScript in HTML?

    The primary purpose behind using JavaScript on HTML pages is to make them interactive and dynamic.

    For adding JS code to HTML code, the <script> tag is used. The JS code that you want to add to the HTML is used within the <script> tag, whether you want to place it in the <head> or <body> section. 

    This method is recommended for small codes. However, for JS scripts that are heavy, it would be great to opt for a dedicated JavaScript file. It helps you to reduce the overall maintenance of the site, differentiates the JS code and HTML code, and optimizes the loading speed.

    35. What are the benefits of using JavaScript comments?

    If you have been a developer using JS for quite some time now, then this can be among the top JavaScript interview questions for experienced professionals. You must know that there are a couple of good reasons to use comments in JavaScript.

    1. Adding information about the program or specific lines of code, helping the users to get the scope or use of that code.
    2. There are several instances when you need to write a few lines of code on a temporary basis. On such occasions, you can add a comment to the code like “to be removed later on”. And when you want to remove it, the comment helps you easily find that code, instead of searching or reading every line of code.

    36. What is null and undefined in JavaScript?

    null in JS is an object which is used to represent that a variable doesn’t have any value. This is not available by default. The developer needs to do it intentionally.

    Whereas, undefined is a global object in JS which means that the declared variable has not been assigned any value.

    37. What is void in JavaScript?

    As you might know, the meaning of void is empty. In the case of JS, the void is actually an operator with its role when a function doesn’t return any result. The result it returns will be undefined.

    38. Why do we use strict mode in JS?

    The purpose of using strict mode in JavaScript is to create silent errors. It helps in building an expression named ‘use strict’ so that we can easily enable the strict mode.

    Using it, we can modify the normal JS semantics, such as converting the silent errors to throw errors. As a result, silent errors can be avoided. In addition, the performance of the code can be improved in several cases when we use the strict mode.

    39. What is JavaScript hoisting?

    It is a technique in JS that helps in using a variable, function, or class before the code execution or declaring these things.

    What hoisting in JavaScript does is prioritize the variables, classes, and functions by shifting them to the top level. As a result, you can find their values without code execution. It will not show any errors.

    In simple words, the role of JS hoisting is to have the feature to call functions, variables, or classes even if you have not defined them in your code.

    40. Which are the best JavaScript frameworks?

    The top 10 popular frameworks of JavaScript are:

    1. ReactJS
    2. AngularJS
    3. Vue.JS
    4. NodeJS
    5. BackboneJS
    6. EmberJS
    7. Meteor
    8. Polymer
    9. Mithril
    10. Aurelia

    41. What is an argument object?

    An argument object in JavaScript is a set of multiple arguments stored in the form of arrays. This is used at the time of invoking the functions.

    It is an in-built JS object that can be accessed only inside a function. Also, this function needs to have the values of the arguments being passed.

    42. How can you use JS to find the operating system of the client machine?

    We can make use of the navigator.appVersion string to find the client OS version.

    43. What is the best way for exception handling in JavaScript?

    JS exception handling is performed using try, catch, finally, and throw keywords. Developers can use try or catch blocks to handle exceptions easily.

    44. Explain the role of the debugger keyword in JS.

    The debugger keyword in JavaScript is used to halt the program’s execution at the point you implement it. It helps you to control the code execution and its flow in a manual manner.