20+ JavaScript Interview Questions and Answer in 2025

Ever since its inception, JavaScript has revolutionised the domain of Web Development as we know it. JavaScript provides a set of flexible features to developers and organisations, streamlining and simplifying the Web Development process – to the extent that it is now one of the most used programming languages across the world.This widespread popularity and demand for the language have been noteworthy, prompting aspiring professionals to opt for the domain and attend JavaScript interviews. So it’s essential to narrow down the most likely JavaScript Interview Questions for anyone seeking a career in this field.

As per W3Techs, 98.6% of all websites use JavaScript, making it not only one of the most widely used languages among developers but also one of the most sought-after skills by diverse businesses.  This blog assembles a comprehensive set of JavaScript Interview Questions covering basic and advanced topics in JavaScript. 

Table of Contents 

1) Areas You Should Study for a JavaScript Interview 

2) Basic JavaScript Interview Questions and Answers 

3) Advanced JavaScript Interview Questions and Answers 

4) Tips to Crack a JavaScript Interview 

5) Conclusion 

Areas You Should Study for a JavaScript Interview 

Before we delve deeper into the top JavaScript Interview Questions and Answers, here are some of the areas you must study for a JavaScript interview. 

1) JavaScript Fundamentals: Review variables, data types, functions, and control flow. 

2) DOM Manipulation: Understand how JavaScript interacts with the Document Object Model (DOM). 

3) Asynchronous JavaScript: Learn callbacks, promises, and wait for handling asynchronous operations. 

4) Object-Oriented Programming (OOP): Familiarise yourself with OOP principles in JavaScript. 

5) Error Handling: Be prepared to discuss try...catch blocks and managing exceptions. 

6) ES6+ Features: Review modern JavaScript features like arrow functions and template literals. 

7) Design Patterns: Understand common design patterns in JavaScript. 

8) Data Structures and Algorithms: Study fundamental data structures and algorithms. 

9) Testing and Debugging: Familiarise yourself with testing frameworks and debugging techniques. 

10) Browser APIs and Events: Review common browser APIs and event handling.
 

 

Basic JavaScript Interview Questions and Answers 

This section of the blog will expand on some basic JavaScript Interview Questions and Answers. 

What is JavaScript?  

JavaScript is defined as a high-level, interpreted programming language that allows you to add interactivity and dynamic behaviour to web pages. It was developed by Brendan Eich in 1995 and is primarily used for client-side web development. All web browsers support the language, and has gained popularity due to its versatility and ease of use. 

What are the Primitive Data Types in JavaScript?  

JavaScript has six primitive data types: string, number, boolean, null, undefined, and symbol.  

1) Strings are sequences of characters which are enclosed in single quotes ('') or double quotes (""). 

2) Numbers can be integers or floating-point numbers and are used for mathematical calculations. 

3) Booleans represent either true or false values. 

4) Null represents the absence of any object value. 

5) Undefined is a variable that has been declared but has not been assigned a value. 

6) Symbols are unique and immutable values that are often used as keys in JavaScript objects. 

What is the Difference Between let, var, and const?  

JavaScript provides different ways to declare variables, namely let, var, and const. 

1) var is the older way of declaring variables in JavaScript and has function scope. It can be redeclared and reassigned throughout the function. 

2) let was introduced in ES6 and has block scope. It allows variables to be redeclared within different blocks, but not within the same block. It can be reassigned to a new value. 

3) const is also introduced in ES6 and has block scope. It is used for variables that should not be reassigned once they are declared. However, it's important to note that const does not make objects or arrays immutable; only the binding to the variable is immutable. 

Explain Hoisting in JavaScript.  

Hoisting is defined as a behaviour in JavaScript where both variable and function declarations are moved to the top of their containing scope during the compilation phase. This essentially means that you can use variables and functions before they are declared in your code.

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What is the Use of the 'this' Keyword?  

The 'this' keyword in JavaScript refers to the object that the function is being executed on or the object that owns the currently executing code. It allows you to access and manipulate the properties and methods of the object within the function. The value of 'this' is determined by how a function is called. 

What is the Difference Between '==' and '===' Operators?  

In JavaScript, the '==' operator, also known as the equality operator, compares two values after converting their types if necessary. It performs type coercion, which means it attempts to convert the operands to a common type before making a comparison. In contrast, the '===' operator, also known as the strict equality operator, checks for both value and type equality. When asked about JavaScript Operator differences, it’s important to note that '===' does not perform type coercion and requires the operands to have the same value and type for a true comparison.

How Does Scoping Work in JavaScript?  

JavaScript has two types of scopes: global scope and function scope. Variables that are declared outside of any function have global scope and can be accessed from anywhere. Variables that are declared within a function have function scope and are accessible only within that function or nested functions.  

What are JavaScript Events?  

JavaScript events are actions or occurrences that happen in the browser. Some examples of events include a user clicking on an element, hovering over an element, submitting a form, or the page finishing loading. You can use JavaScript to listen to these events and trigger specific actions or functions in response to them. Event handling in JavaScript involves using event listeners to detect and respond to events. 

What is the Purpose of the 'return' Statement in JavaScript?  

The 'return' statement is used in JavaScript functions to specify the value that the function should return when it is called. It allows a function to compute a value and provide the result back to the code that called it. When a 'return' statement is encountered in a function, the function stops executing and immediately returns the specified value. If no 'return' statement is present or if it is omitted, the function returns 'undefined' by default. 

How Does Error Handling Work in JavaScript? 

JavaScript provides mechanisms for handling errors to prevent program crashes and unexpected behaviour. The 'try...catch' statement is used to catch and handle exceptions (errors) that may occur during the execution of code within the 'try' block. If an exception is thrown within the ‘try’ block, it is caught by the ‘catch’ block, which allows you to handle the error gracefully by providing alternative code or displaying error messages. 

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Why Do We Use the Word “Debugger” in JavaScript?

The word “debugger” is used in JavaScript to refer to a tool that can step through JavaScript code line by line. This can help debug JavaScript code, which refers to finding and fixing errors. To use it, the JavaScript console in the browser needs to be opened. Then, debugger commands can be used to comb through the code line by line.

What is Implicit Type Coercion in JavaScript? 

Implicit type coercion is a JavaScript concept that automatically converts a value from one type to another. This conversion follows a priority order that typically begins with Strings, numbers, and finally, booleans. For instance, when adding a string to a number, JavaScript will implicitly coerce the number to a string before performing the addition operation because strings have the highest priority in type coercion. For example, when the number 7 is combined with the string '11' using the addition operator, the result is the string '711'.

What is the Purpose of the bind() Method in JavaScript? 

The bind() method is used to create a new function with a specified this value and an initial set of arguments. It allows for setting the context of a function permanently.

What is the Difference Between Function Declarations and Function Expressions? 

Function expressions are defined by assigning a function to a variable, while Function declarations are defined using the function keyword. Another difference is that, unlike function expressions, function declarations are hoisted.

What is Memoization in JavaScript? 

Memoization is a technique for improving the performance of JavaScript code. It stores the results of expensive calculations in a cache, allowing the JavaScript code to avoid re-performing the expensive calculations in case the same input is provided again.

What is Recursion in JavaScript? 

Recursion is a programming technique where a function calls itself. It's useful for solving problems like calculating the Fibonacci sequence or finding a number's factorial.

What is a Callback Function in JavaScript?

A callback function is passed as an argument to another function, which is then invoked inside the outer function. It enables event-driven or asynchronous programming.

Advanced JavaScript Interview Questions and Answers 

This section of the blog will expand on some advanced JavaScript Interview Questions and Answers. 

What are Closures in JavaScript?  

Closures are a powerful and often misunderstood concept in JavaScript. A closure is created when an inner function references variables from its outer function, even after the outer function has finished executing.  

The inner function retains access to the variables and their values, forming a closure. Closures are commonly used to create private variables and encapsulate data within functions, providing a way to maintain state and preserve data integrity. 

Explain the Concept of Prototypal Inheritance 

Prototypal inheritance is a fundamental concept in JavaScript. Unlike classical inheritance in other programming languages, JavaScript uses prototypal inheritance, which is based on objects instead of classes. Each object in JavaScript has an internal link to another object called its prototype. 

When you try to access a property or method on an object, and it doesn't exist in the object itself, JavaScript looks for it in the object's prototype. This chain of prototypes allows objects to inherit properties and methods from their prototypes, forming a prototype chain.

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How Does Event Delegation Work in JavaScript?  

Event delegation is a technique used in JavaScript to handle events efficiently, especially for dynamically generated content or a large number of elements. Instead of attaching event listeners to individual elements, you attach a single event listener to a parent element that contains the target elements.  

When an event occurs, it bubbles up the DOM tree, and the event listener on the parent element captures the event and determines the target element. By using event delegation, you can reduce the number of event listeners and improve performance, especially when dealing with a large number of elements. 

What is the Event Loop in JavaScript?  

The event loop is a crucial component of JavaScript's runtime environment that manages the execution of code and the handling of events. It ensures that JavaScript remains single-threaded while handling potentially long-running operations asynchronously. The event loop continuously checks for tasks in the event queue and processes them in a sequential manner, similar to how a JavaScript for Loop iterates through a set of instructions repeatedly. 

When an event occurs, or an asynchronous operation completes, the corresponding callback or promise resolution is added to the event queue. The event loop picks up these tasks and executes them one by one, allowing JavaScript to handle I/O operations, timers, and other asynchronous events efficiently. 

How Does Asynchronous Programming Work in JavaScript?  

JavaScript uses asynchronous programming to handle time-consuming tasks without blocking the execution of other code. Asynchronous operations, pertaining to making API requests or reading files, are executed concurrently, allowing other code to run while waiting for the completion of the asynchronous task.   

JavaScript provides several mechanisms for asynchronous programming, including callbacks, promises, and async/await. Callbacks are the traditional approach, where a function is passed as an argument to be called once the asynchronous task is complete. Promises provide a cleaner syntax and allow the chaining of asynchronous operations. Async/await, introduced in ES2017, allows writing asynchronous code that resembles synchronous code, making it easier to read and understand. 

Tips to Crack a JavaScript Interview 

This section of the blog will give you some essential tips you must consider in order to crack a JavaScript interview. 

1) Master the Fundamentals: Have a strong grasp of JavaScript basics, including variables, functions, scope, and control flow. 

2) Practice Coding: Solve JavaScript exercises regularly to improve problem-solving skills. 

3) Review Common Questions: Study common JavaScript Interview Questions and understand the underlying concepts. 

4) Stay Updated: Keep up with modern JavaScript features and popular frameworks. 

5) Showcase Projects: Highlight personal or open-source JavaScript projects to demonstrate practical experience. 

6) Demonstrate Problem-solving Skills: Break down complex problems and explain your thought process. 

7) Communicate Clearly: Articulate ideas clearly and ask thoughtful questions. 

8) Research the Company: Understand the company's products and culture to tailor your responses. 

9) Stay Calm and Confident: Approach the interview with a positive mindset and maintain confidence in your abilities.

Conclusion 

All in all, preparing for a JavaScript Interview requires a solid understanding of the language's fundamentals and advanced concepts. By familiarising yourself with basic and advanced JavaScript Interview Questions, practicing coding, and staying updated with latest trends in JavaScript development, you can confidently tackle any JavaScript Interview. We hope this blog guides you towards career excellence in the field of Java.

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