JavaScript Interview Prep: 35+ Must-Know Questions and Expert Answers (2025) – MTA Skills

Master the key concepts and techniques required to excel in any JavaScript interview. This guide is crafted by experts to help you tackle challenging questions with confidence and ease.

Preparing for a JavaScript interview can often feel overwhelming due to the vast array of topics and concepts you need to master. That’s why we’ve created “JavaScript Interview Prep: 35+ Must-Know Questions and Expert Answers.” This will help you to navigate through the most frequently asked interview questions with confidence. Let’s dive in and get ready to impress your future employers!

1. What are the recommendations to create new object?

It is recommended to avoid creating new objects using new Object(). Instead you can initialize values based on its type to create the objects

• Assign {} instead of new Object()
• Assign “” instead of new String()
• Assign 0 instead of new Number()
• Assign false instead of new Boolean()
• Assign [] instead of new Array()
• Assign /()/ instead of new RegExp()
• Assign function (){} instead of new Function()

Example:

2. How do you define JSON arrays?

JSON arrays are written inside square brackets and arrays contain JavaScript objects. For example, the JSON array of users would be as below:

3. How do you generate random integers?

You can use Math.random() with Math.floor() to return random integers.

Example: If you want generate random integers between 1 to 10, the multiplication factor should be 10

4. What is a Regular Expression?

A regular expression is a sequence of characters that forms a search pattern. You can use this search pattern for searching data in a text. These can be used to perform all types of text search and text replace operations.

Syntax:

Example: The regular expression or search pattern with case-insensitive username would be

5. What are modifiers in regular expression?

Modifiers can be used to perform case-insensitive and global searches. Let’s list down some of the modifiers

Example: Let’s take an example of global modifier

6. What are regular expression patterns?

Regular Expressions provide a group of patterns in order to match characters. Basically, they are categorized into 3 types

1. Brackets: Used to find a range of characters e.g.
   1. [abc]: Used to find any of the characters between the brackets(a,b,c)
   1. [0-9]: Used to find any of the digits between the brackets
2. (a|b): Used to find any of the alternatives separated with |
3. Metacharacters: These are characters with a special meaning. For example, below are some use cases,
   1. \d: Used to find a digit
   1. \s: Used to find a whitespace character
4. \b: Used to find a match at the beginning or ending of a word
5. Quantifiers: These are useful to define quantities. For example, below are some use cases
   1. n+: Used to find matches for any string that contains at least one n
   1. n*: Used to find matches for any string that contains zero or more occurrences of n
   1. n?: Used to find matches for any string that contains zero or one occurrences of n

7. How do you search a string for a pattern?

You can use the test() method of regular expression in order to search a string for a pattern, and return true or false depending on the result

Example:

8. What are modules and why are they useful?

Modules are reusable pieces of code that can be imported and exported between different files in a project. They help in organizing code, making it more maintainable and scalable. By using modules, you can avoid global namespace pollution and manage dependencies more effectively. In JavaScript, you can use import and export statements to work with modules.

9. How do you import and export modules in JavaScript?

In JavaScript, you can import and export modules using the import and export statements. To export a module, you can use export before a function, variable, or class, or use export default for a single default export. To import a module, you use the import statement followed by the name of the exported module and the path to the module file.

Example:

10. What are the different types of errors in JavaScript?

Answer: In JavaScript, there are three main types of errors: syntax errors, runtime errors, and logical errors. Syntax errors occur when the code violates the language’s grammar rules, such as missing a parenthesis. Runtime errors happen during code execution, like trying to access a property of undefined. Logical errors are mistakes in the code’s logic that lead to incorrect results but don’t throw an error.

11. Explain “call stack” in Event Loop.

The place where your code is executed. Each function call gets pushed onto the stack and popped off when completed.

12. How do you handle errors using try…catch blocks?

To handle errors using try…catch blocks, you wrap the code that might throw an error inside a try block. If an error occurs, the control is transferred to the catch block where you can handle the error. Optionally, you can use a finally block to execute code regardless of whether an error occurred or not.

Syntax:

13. What is the purpose of the finally block?

The finally block in JavaScript is used to execute code after a try and catch block, regardless of whether an error was thrown or caught. It ensures that certain cleanup or finalization code runs no matter what.

Example:

14. What is lazy loading and why is it beneficial in web development?

Lazy loading is a design pattern used to defer the loading of non-essential resources at the time of the initial page load. Instead, these resources are loaded as they are needed. This improves the performance and user experience of a web application by reducing initial load time and conserving bandwidth.

15. How do you copy properties from one object to other?

You can use the Object.assign() method which is used to copy the values and properties from one or more source objects to a target object. It returns the target object which has properties and values copied from the source objects.

Syntax:

Example:

16. How can you get the list of keys of any object?

You can use the Object.keys() method which is used to return an array of a given object’s own property names, in the same order as we get with a normal loop.

Example:

17. What will be printed in the following code?

Answer: Option 2

Explanation: In JavaScript, arrays are reference types. Both a and b point to the same array in memory. Modifying b also affects.

18. How many ways an HTML element can be accessed in JavaScript code?

There are six possible ways to access HTML elements in JavaScript which are:

1. getElementById() Method: It is used to get the element by its id name

2. getElementsByClass() Method: It is used to get all the elements that have the given classname

3. getElementsByTagName() Method: It is used to get all the elements that have the given tag name

4. querySelector() Method: This function takes CSS style selector and returns the first selected element

5. querySelectorAll() Method: This function takes a CSS style selector and returns all elements that match the selector

6. getElementsByName() Method: It is used to get all elements with a given name attribute

19. What are the features of JavaScript?

JavaScript offers a wide range of features as follows:

• Dynamic typing
• Prototypal inheritance
• First class functions
• Closures
• Asynchronous programming with promises
• Event driven programming

20. What benefits does JavaScript offer compared to other web technologies?

JS gives the following advantages over other technologies:

• Cross-platform compatibility
• Rich interface user experience
• Reduced server load with client-side processing
• Enhanced interactivity and responsiveness
• Large ecosystem with numerous libraries and frameworks

21.  What is a class in JavaScript?

A class in JavaScript is a blueprint for creating objects with predefined properties and methods. It was introduced in ECMAScript 6 (ES6) and provides a more structured and clean way of defining object-oriented patterns in JavaScript.

22. How do you define a class in JavaScript?

You can define a class using the class keyword

23. What is the difference between a class and a constructor function?

These are following differences

• A class is a more modern syntax for defining objects and creating instances, whereas a constructor function is an older way of doing the same.
• Classes use the class keyword and have a constructor method for initializing objects. Constructor functions use the function keyword and typically have this keyword to define properties.
• Classes have built-in methods like extends for inheritance and static for defining static methods, which are not inherently available in constructor functions.

24. What are the differences between classes and objects?

• A class is a blueprint for creating objects. It defines properties and methods that the created objects will have
• An object is an instance of a class. It is a concrete entity with the properties and methods defined by the class

25. What are the differences between client-side and server-side JavaScript?

Client-Side JavaScript

• Runs in: Browser
• Purpose: Enhances user experience (e.g., animations, form validation)
• Execution: On user’s device
• Security: Code is visible and can be manipulated

Server-Side JavaScript

• Runs in: Web server
• Purpose: Handles backend operations (e.g., database interactions, server logic)
• Execution: On server
• Security: Code is hidden and more secure

Key Differences

• Location: Client-side in browser, server-side on server
• Use Cases: Client-side for user interaction, server-side for data management
• Visibility: Client-side code is visible, server-side code is not
• Processing: Client-side on user’s device, server-side on server

26. What is the difference between “method” and “function” in JavaScript?

In JavaScript, both methods and functions are blocks of code designed to perform specific tasks, but they have distinct roles and contexts

Function: A standalone block of code that performs a specific task and can be invoked independently.

Syntax:

Usage: Functions can be used to execute general actions and can be called by their name from anywhere in your code.

Method: A function that is a property of an object and operates on the data contained within that object.

Syntax:

Usage: Methods are invoked as properties of an object and typically manipulate or operate on the object’s data.

Example:

P.S. while all methods are functions, not all functions are methods. Methods are specific to objects and often used to manipulate the object’s data, while functions are more general and can be used for a wide variety of tasks.

27. What are template engines in JavaScript?

Template engines in JavaScript enable developers to generate dynamic HTML content by embedding JavaScript code within a template. They facilitate the separation of logic and presentation, making web pages easier to manage and update. Examples include EJS, Handlebars, and Pug. These engines process the template and data to produce a final HTML output.

28. What is a “promise chain” in JavaScript?

A “promise chain” in JavaScript is a series of .then() methods chained together to handle asynchronous operations sequentially. Each .then() processes the result of the previous promise and returns a new promise, allowing for smooth execution of asynchronous tasks in a specified order.

Example:

Explanation: In this example, each step (fetching, parsing, processing, displaying) is handled in sequence, and any errors can be caught at the end with .catch().

29. What are JavaScript events?

JavaScript events are actions or occurrences that happen in the browser, which the browser can respond to. Events allow you to create dynamic, interactive web pages by executing code in response to user actions or other events.

• User Events: Triggered by user interactions
  • Click: Occurs when an element is clicked
  • Mouseover: Occurs when the mouse pointer moves over an element
  • Keydown: Occurs when a key is pressed down
• Form Events: Triggered by form interactions
  • Submit: Occurs when a form is submitted
  • Change: Occurs when the value of an input element changes
  • Focus: Occurs when an element gains focus
• Window Events: Triggered by window-related actions
  • Load: Occurs when the page has fully loaded
  • Resize: Occurs when the browser window is resized
  • Scroll: Occurs when the user scrolls the page

Example:

Explanation: when the button with the ID myButton is clicked, an alert box will appear with the message “Button was clicked!”.

30. How do you add an event listener to an element in JavaScript?

To add an event listener to an element in JavaScript, use the addEventListener method.

Syntax:

Example: Here we are adding a click event listener to a button with the ID “ myButton “

31. What is the difference between addEventListener and onclick?

Differences Between addEventListener and onclick:

addEventListener:  

• Multiple Handlers: Can add multiple event listeners to the same event
• Syntax: element.addEventListener(“event”, functionToCall)

onclick:

• Single Handler: Only one handler per event, overwrites previous ones
• Syntax: element.onclick = functionToCall

In short, addEventListener is more flexible and versatile, while onclick is simpler but limited to one handler.

32. How do you prevent the default action of an event?

To prevent the default action of an event in JavaScript, use the preventDefault() method

Example:

Explanation: In this example, clicking on the link with the ID myLink will not follow the link’s URL, and instead, it will log a message to the console

33. What are some common mouse events and how are they used?

Common mouse events in JavaScript allow you to create interactive and responsive web pages. Here are some frequently used mouse events

1. click: When the mouse button is clicked on an element. Used for buttons, links, and other interactive elements

• dblclick: When the mouse button is double-clicked on an element. Used for actions requiring a double click, such as opening a file.

• mouseover: When the mouse pointer hovers over an element. Used for creating hover effects or tooltips

34. What are JavaScript frameworks and libraries?

Here is a detailed description for both:

1. JavaScript Frameworks: Frameworks provide a structured way to build web applications, with predefined architecture, tools, and best practices e.g.

• React: User interfaces, single-page applications
• Angular: Single-page client applications using HTML and TypeScript
• Vue.js: User interfaces and single-page applications

2. JavaScript Libraries: Libraries offer specific functionalities to be used in projects without enforcing structure e.g.

• jQuery: Simplifies HTML document traversal, event handling, and animation
• Lodash: Utility functions for common programming tasks
• D3.js: Dynamic, interactive data visualizations

Key Differences

• Structure: Frameworks have a specific architecture; libraries offer individual tools
• Control: Frameworks call your code; you call library functions
• Use Case: Frameworks for entire applications; libraries for specific functionalities

35. How do you use the reduce method in arrays?

The reduce method in JavaScript processes an array’s elements to produce a single output value

Syntax:

Example: To sum all the numbers in an array

Explanation: The reduce method iterates over each element of the numbers array. It starts with the initialValue of 0 and adds each currentValue to the accumulator. By the end of the array, the accumulator holds the total sum of all the numbers.