Most-asked 15+ JavaScript Interview Questions and Answers [2025 Updated]

I have prepared a detailed list of the Top 15+ JavaScript interview questions and answers to guide you in impressing the hiring managers. I have picked each JavaScript interview question carefully by taking guidance from development professionals depending on what might be asked in the next interviews. 

Read this article thoroughly and increase your opportunities of cracking the development interviews. This blog post will offer you a concrete overview of JavaScript, a popular programming language.  

Let’s start this, 

Q 1. What do you mean by JavaScript?

The most dynamic programming language, JavaScript, is majorly applied for creating versatile and highly user-intuitive websites. This enables developers to make responsive pages to different user activities such as typing, clicks and movements of the mouse. JavaScript is specially applied to manipulate page elements, animate graphics, manage multimedia and validate different forms. This runs within the web browser. 

Q 2. What are the different types of data supported by JavaScript?

There are different types of data supported by JavaScript, and they are Primitive data types and Non-primitive data types.

Primitive data types are those that can be immutable. Strings, null, undefined, numbers, Biglnt, booleans and symbols are among primitive data types.

On the other hand, Non-primitive data types are those that store sets of data or different functionalities. Functions, objects and arrays are among non-primitive data types.

Q 3. What are events in JavaScript?

Events in JavaScript are referred to as activities or functions that occur on a web page. Some instances of Events are form submission, clicking a button, pressing a key, and more! JavaScript enables web developers to manage such Events with the assistance of Event Listeners. For instance, you can run a function while a user makes a click. Such Events help a web page become highly engaging and versatile.

Q 4. What are Closures in JavaScript?

The function Closure in JavaScript is used to remember the parent functions and variables that are highlighted in its outside area. This is remembered even after the task has been conducted and completed. It enables easy access to the outside function’s variables from the inside section.

Q 5. Differentiate Regular functions from Arrow functions.

In JavaScript, Regular functions are known as to the element that calls them so that they can have their separate setting. For some cases, Arrow functions are simpler and more brief as they take this from the adjacent code rather than owning it themselves.

Q 6. Can you state the benefits of using JavaScript?

There are plenty of benefits of this dynamic programming language, and they are;

• This is easy to use.
• JavaScript is way faster than other languages and can manage complex calculations. Due to its function of client-side code execution, it is speedier than other server-side languages.
• It has built-in security features due to Angular and React frameworks.
• It is a very lightweight programming language; hence, there is no need to install heavy software. 
• JavaScript offers cross-browser compatibility.
• It offers broad library support.
• This is interactive and dynamic and can handle features such as mouse movements, clicks and keyboard inputs.
• It supports third-party add-ons, making it easier for developers to create stunning JavaScript applications.

Q 7. Can you explain the functionality of the type of operator?

The type of operator in JavaScript is employed to figure out the type of a variable. It assists in guaranteeing that specific variables are used correctly by offering a string that defines the type of the operand including ‘number’, ‘boolean’ and ‘string’. This is a popular type-checking operator and it helps in returning the string that indicates the specific type of operand one passes to it. 

Q 8. State the difference between defined and not defined.

The element variable in JavaScript is considered to be ‘defined’ when it has been stated and assigned a value. Viewing the variable in your code will enable you to check this. When a variable is ‘not defined’—that is, not clearly declared—any request to access it will end in a ReferenceError. It is also acceptable for the variable to be set up but not configured, in which event it has been defined, but its actual value is undefined.

Q 9. What do you mean by Strict Mode?

The Strict Mode in JavaScript refers to the specific mode that enables you to select a more demanding version of the programming language. It eliminates some harmful features and boosts the amount of errors that JavaScript delivers. For example, it prevents deployments to read-only attributes and the employ only of variables that are undeclared. This also limits programmers in making code that is simple and less likely to have errors. By inserting ‘use strict’ at the start of your program or function, you will be able to activate the Strict Mode.

Q 10. Can you differentiate search from indexOf?

Yes, I can. 

However, both of them are applied to search within a string, and the search() and indexOf() function in different ways. The position of a substring’s initial appearance is returned by indexOf() after searching for it. If the substring will not be able to be located, the returning value is -1. On the other hand, search() employs regular expressions and offers the index of the first match or -1 in the case that no match is found there. As it can manage different patterns, search() is more effective.

Q 11. Explain the differences between an object and an array.

A huge set of key-value pairs, with the specific keys typically being symbols or strings and the values being any data type, is referred to as an object in JavaScript. Objects are not organized in any specific order. On the contrary, arrays are huge sets of values that are organized in a particular sequence and are indexed by specific numbers that initiate at 0. Related data should be stored in objects, while ordered data is stored effectively in arrays. Index places are employed to get easy access to array values, and keys are employed to access the values of objects. 

Q 12. What do you mean by Hoisting in JavaScript?

Hoisting in JavaScript refers to the act of relocating function and variable declarations comes first due to their scope before the code execution. This also means that different functions and variables can be addressed before declaration. However, their duties are not triggered; only the declarations are addressed. For example, variables set with a value will be back undefined if used prior to their actual task, while a function can be addressed prior to its declaration even.

Q 13. What are the differences between String() and Substring()? 

Any data type, like objects, numbers, or booleans, can be transformed into a string with the application of the String() function. It is used to ensure a variable contains a string format. On the contrary, the substring() method eliminates or restricts a section of a string among two specified indices. It does not alter or modify the original string; rather, it returns a new value. Therefore, substring() alters the content of any strings, while String() changes specific data types.

Q 14. State the purpose of the setTimeout() function in JavaScript.

setTimeout () refers to the specific function of JavaScript that pauses the running of the specific function or code because of a specific amount of time that is predetermined already. This is measured in Milliseconds. It allows a programmer to start a function in a predetermined amount of time. 

One function is SetTimeout(() => alert(“Hello”), 3000). This will highlight a notification after three seconds. Time-based functions such as animations, the latest updates, and user interactions can be managed perfectly with its assistance.

Q 15. Can you explain how you can remove duplicates in an array?

You must apply a Set to remove duplicates in an array from JavaSript. Turning an array to a Set and back to an array removes duplicates, as a set only handles single values by nature. Take uniqueArray = […new Set(array)] for instance. Filter() is another major technique that continues through the array, saving just the unique values while contrasting each other.

Q 16. Explain the term BOM.

The term BOM refers to the Browser Object Model. It enables JavaScript to connect with the browser despite the content of the page by reflecting the setting in which a web page is offered. The browser window, location, history, pop-ups, and size can all be managed by employing the techniques and features that the BOM provides. It helps in managing not only the document data but also the functionality of the browser.

Q 17. What do you mean by DOM? State the usability of it. 

The term DOM refers to the Document Object Model. This interface highlights the structure of an HTML document as a network of objects. JavaScript automatically uses, alters, and updates the content of the web page with the help of the DOM. Without having to reboot the entire web page, developers can alter HTML tags, attributes, fonts, and formats on the web page by communicating with the DOM.

Q 18. What do you mean by callback function in Java?

The callback function in JavaScript refers to the function that is executed immediately after receiving data as input to another task. Asynchronous programming often applies it to handle complex functions such as user input waiting and file reading. The callback is launched to complete the task once the primary function has been completed. This makes it feasible to do functions such as API calls or animations without triggering the primary thread to start working.

Q 19. Explain the usage of Window Objects.

The browser window or tab is depicted by JavaScript’s Window object. It provides specific features and processes for communicating with the browser setting. It can be employed, for example, to present alarms, adjust the window, new tab launching, and access the dimensions of the browser. As the Window object is open in the browser, any script that is currently running in the browser can have direct access to it. 

Q 20. Can you state how to create an object in JavaScript?

With the help of literal notation, you will be able to create an object in JavaScript. This is a very popular and common method for object creation. For instance, let person = { name: ‘ Paul’, age: 25};.