Essential JavaScript Interview Questions You Should Know

Updated 4 July 2026 · Intervuz

Introduction

JavaScript has become a fundamental language for web development, and mastering it is essential for developers looking to succeed in technical interviews. Whether you are a fresher or a seasoned professional, being well-prepared for JavaScript interview questions can significantly improve your chances of landing the job. This article covers some essential interview questions and answers to help you prepare effectively.

1. What is JavaScript?

JavaScript is a high-level, interpreted programming language that conforms to the ECMAScript specification. It is widely used for creating interactive web applications and can be run on various platforms, including web browsers and servers.

2. What are the data types supported by JavaScript?

JavaScript supports several data types, including:

• Primitive Types: Number, String, Boolean, Null, Undefined, Symbol, and BigInt.
• Non-Primitive Types: Objects (including arrays and functions).

3. Explain the concept of 'hoisting'.

Hoisting is a JavaScript mechanism where variables and function declarations are moved to the top of their containing scope during the compile phase. This means that you can use variables and functions before they are declared in your code.

4. What is the difference between '=='' and '==='?

The '==' operator compares two values for equality, but it performs type coercion if the values are of different types. The '===' operator, on the other hand, checks for both value and type equality, meaning no type conversion occurs. It is generally recommended to use '===' to avoid unexpected results.

5. What is a closure in JavaScript?

A closure is a function that retains access to its lexical scope, even when the function is executed outside that scope. This allows for data encapsulation and can be useful for creating private variables.

6. What are promises and async/await in JavaScript?

Promises are objects that represent the eventual completion (or failure) of an asynchronous operation and its resulting value. The 'async/await' syntax allows you to write asynchronous code that looks synchronous, making it easier to read and maintain.

7. Explain the 'this' keyword in JavaScript.

The 'this' keyword refers to the context in which a function is called. Its value can vary depending on how a function is invoked. In the global context, 'this' refers to the global object (window in browsers), while in strict mode, it is 'undefined'.

8. What is the event loop?

The event loop is a mechanism that allows JavaScript to perform non-blocking operations despite being single-threaded. It enables the execution of asynchronous code by placing it in a queue and processing it after the current execution stack is empty.

9. What are arrow functions, and how do they differ from regular functions?

Arrow functions are a concise syntax for writing function expressions. They do not have their own 'this', making them ideal for preserving the context of 'this' from the enclosing scope. However, they cannot be used as constructors and do not support the 'arguments' object.

10. How can you improve the performance of a JavaScript application?

To improve performance, consider the following strategies:

• Minimize DOM manipulation.
• Use event delegation.
• Optimize loops and avoid unnecessary calculations.
• Utilize caching and memoization.
• Load scripts asynchronously.

Conclusion

Preparing for JavaScript interviews involves understanding key concepts and practicing your responses to common questions. To gain confidence and improve your interview skills, consider practicing out loud with Intervuz, where you can interact with our AI interviewer, Vika. With the right preparation, you can approach your next interview with confidence and clarity.