Top 25+ JavaScript Interview Questions and Answers

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Research shows that JavaScript is used in 12% of all technical interviews. However, only 42% of candidates pass these interviews. That’s why it is important to prepare in advance. And this is where our guide can help you. We’ve put together a comprehensive list of the top 25+ JavaScript interview questions with clear and straightforward answers.

Whether you’re an experienced developer or a fresher ready for your first interview, these questions cover basic and advanced JavaScript topics.

So, let’s get started!

About JavaScript

JavaScript is a programming language used in web development to make websites interactive. 

It adds behaviour to websites, letting them respond to user actions like form validation, animation, and data fetching without reloading the page. 

JavaScript works on both front-end and back-end development, making it versatile for creating responsive and engaging web applications.

According to Statista, JavaScript is the top choice for web development, with about 63.61% of developers worldwide preferring it. 

JavaScript is also in high demand in India’s tech industry due to its crucial role in web development and the rise of digital initiatives.

To find the perfect JavaScript job, use Hirist, India’s top IT job portal. Here, you can search for JavaScript jobs, choose your preferred location and experience level, and easily apply for jobs that fit your skills.

JavaScript Interview Questions for Freshers

Here are some commonly asked basic interview questions on JavaScript, along with their answers.

1. What is JavaScript? 

JavaScript is a programming language used to create interactive elements on websites. It runs in web browsers and can manipulate webpage content, respond to user actions, and interact with servers.

2. What are variables in JavaScript? 

Variables are used to store data values in JavaScript. They can hold different types of data like numbers, strings (text), and boolean (true/false) values.

3. What are the different data types in JavaScript? 

JavaScript has several data types, including, 

• Number: Represents both integer and decimal (floating-point) numbers.
• String: Represents sequences of characters or alphanumeric values.
• Boolean: Represents true or false values.
• Object: Represents collections of properties and complex values.
• Null: Represents values that are intentionally empty or unknown.
• Undefined: Indicates variables that have been declared but not assigned a value.
• Symbols: Provides unique identifiers for objects and supports special use cases.

4. How do you declare a variable in JavaScript? 

You can declare a variable using the var, let, or const keywords followed by the variable name. 

For example:

var myNumber = 10;

let myName = “Rahul”;

const PI = 3.14;

5. What is an event in JavaScript? 

An event is an action that occurs on a webpage, such as clicking a button or submitting a form. JavaScript can respond to these events by executing specific functions.

6. How do you create a function in JavaScript? 

You can create a function using the function keyword followed by the function name and parameters. 

For example:

function greet(name) {

    return “Hello, ” + name + “!”;

}

7. How do you add an event listener in JavaScript? 

You can add an event listener to an HTML element using the addEventListener method. 

For example:

document.getElementById(“myButton”).addEventListener(“click”, function() {

    alert(“Button clicked!”);

});

JavaScript Interview Questions Experienced

We have categorized the commonly asked JS programming interview questions and answers based on candidates’ experience levels.

JavaScript Interview Questions for 2 Year Experience

Here are some common JS interview questions and answers for candidates with 2 years of experience. 

8. What are closures in JavaScript?

A closure is a feature in JavaScript that allows a function to remember and access its lexical scope even when it’s executed outside that scope. It helps in maintaining private variables and creating modular code.

9. Explain the difference between let, const, and var.

var is used to declare variables with function scope, while let and const have block scope (limited to the nearest curly braces). Variables declared with const cannot be changed after assignment, but variables declared with let can.

10. What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code runs line by line and blocks further execution until each line finishes. Asynchronous code allows tasks to run in the background and doesn’t wait for them to complete before moving on.

11. How does prototypal inheritance work in JavaScript?

JavaScript uses prototypal inheritance, where objects can inherit properties and methods from other objects. Each object has a prototype object from which it inherits properties.

12. Explain the concept of event bubbling and event delegation.

Event bubbling refers to the propagation of an event from the innermost element to the outermost element in the DOM hierarchy. Event delegation is a technique where a parent element handles events triggered by its child elements.

JavaScript Interview Questions for 3 Year Experience 

Here are some important JS interview questions for candidates with 3 years of experience. 

13. What are Promises in JavaScript?

Promises are used for asynchronous programming in JavaScript. They represent the eventual completion or failure of an asynchronous operation, allowing you to handle the result or error once it’s available.

14. Explain the concept of higher-order functions.

Higher-order functions are functions that can take other functions as arguments or return functions as their results. They enable functional programming paradigms like map, filter, and reduce.

15. Explain the concept of the “this” keyword in JavaScript.

It is one of the most commonly asked JavaScript this interview questions. 

In JavaScript, the “this” keyword refers to the current execution context or the “owner” of the function being executed. The value of “this” is determined by how a function is called. It is not fixed and can change based on the invocation context.

16. What is the event loop in JavaScript, and how does it work?

The event loop is a mechanism that handles asynchronous operations in JavaScript. It continuously checks the call stack and task queue, moving tasks from the queue to the stack for execution when the stack is empty.

17. What is the difference between == and === in JavaScript?

== is used for loose equality comparison and performs type coercion if needed, while === is used for strict equality comparison (checking both value and type) without type coercion.

JavaScript Interview Questions for 5 Years Experience

Here are some important JS interview questions for candidates with 5 years of experience. 

18. What are generators in JavaScript, and how are they used?

Generators are functions that can be paused and resumed at certain points using the yield keyword. They allow for asynchronous-like behaviour in synchronous code and are often used for iterative algorithms and handling streams of data.

19. Explain the concept of ES6 modules in JavaScript.

ES6 modules provide a way to organize and structure code into reusable components. They use import and export statements to define dependencies between modules, allowing for better code organization and maintainability.

20. How does JavaScript handle memory management and garbage collection?

JavaScript uses automatic memory management through garbage collection. It allocates memory when objects are created and automatically frees up memory when objects are no longer in use or unreachable by the program. 

Common garbage collection algorithms include reference counting and mark-and-sweep.

21. What is the difference between function declaration and function expression?

Function declarations are hoisted and can be called before they are defined in the code, while function expressions are not hoisted and must be defined before they are called. Function expressions can also be anonymous (no function name).

22. What are some best practices for optimizing JavaScript performance?

Some best practices include,

• Minimizing DOM manipulation
• Using efficient data structures like arrays and objects
• Reducing unnecessary function calls
• Caching variables for reuse
• Using browser developer tools to profile and optimize code

JavaScript Interview Questions for 10 Years Experience

Here are some important JavaScript interview questions for candidates with 10 years of experience. 

23. What are the differences between classical inheritance and prototypal inheritance in JavaScript?

Classical inheritance involves defining classes and creating instances of those classes using constructors and inheritance hierarchies. 

Prototypal inheritance, on the other hand, uses prototypes to share behaviour between objects, allowing for more flexible and dynamic object creation.

24. What are the advantages and disadvantages of using closures in JavaScript?

Closures in JavaScript allow functions to retain access to variables from their parent scopes, enabling encapsulation and private data. 

However, excessive use of closures can lead to memory leaks if not managed properly, as they maintain references to outer scope variables.

25. What are some common design patterns used in JavaScript development?

Common design patterns in JavaScript include the Singleton pattern, Factory pattern, Module pattern, Observer pattern, and Promises pattern. 

These patterns help in structuring and organizing code to promote reusability, maintainability, and scalability.

26. How does JavaScript handle error handling, and what are the best practices for managing errors in production code?

JavaScript uses try-catch blocks to handle synchronous errors and .catch() methods for handling Promise rejections in asynchronous code. 

Best practices for managing errors include,

• Logging errors with relevant information
• Implementing graceful error recovery strategies
• Using tools like error monitoring services to track and analyze production errors.

Advanced JavaScript Interview Questions

Take a look at these advanced JavaScript programming questions and answers.

27. Explain the concept of lexical scoping in JavaScript.

Lexical scoping in JavaScript means that the scope of a variable is determined by its position within the source code’s lexical structure. Inner functions have access to variables declared in their outer function’s scope.

28. What is the NaN property in JavaScript?

The NaN property in JavaScript represents a value that is “Not-a-Number.” It indicates that a value is not a valid number. If a calculation or operation results in a value that is not a number, it will return NaN. 

To check if a value is NaN, you can use the isNaN() function. It’s important to know that isNaN() converts the given value to a number and then checks if it equals NaN.

29. What are the various types of errors in JavaScript?

JavaScript errors can be categorized into three types:

• Load time errors: These errors occur while a web page is loading. They include syntax errors that are identified dynamically during the loading process.
• Runtime errors: These errors happen when a command is used incorrectly in HTML.
• Logical errors: These errors result from flawed logic within a function, leading to unexpected outcomes or incorrect behaviour during execution.

30. How is DOM used in JavaScript?

The DOM (Document Object Model) in JavaScript controls how different elements in a web page interact with each other. 

It allows developers to manipulate objects like links and paragraphs on a web page, enabling actions such as adding or removing them dynamically. Using the DOM API simplifies web development compared to other models. 

Also Read - Top 25+ HTML CSS JavaScript Interview Questions and Answers

JavaScript Problem Solving Questions

These are some important problem-solving JavaScript interview questions and answers. 

31. Find the maximum element in an array.

function findMaxElement(arr) {

    if (arr.length === 0) {

        return null; // Return null for empty array

    }

    let max = arr[0]; // Initialize max with the first element

    for (let i = 1; i < arr.length; i++) {

        if (arr[i] > max) {

            max = arr[i]; // Update max if current element is greater

        }

    }

    return max; // Return the maximum element

}

// Example usage:

let numbers = [10, 5, 8, 15, 3];

console.log(findMaxElement(numbers)); // Output: 15

32. Count the number of vowels in a given string.

function countVowels(str) {

    const vowels = ‘aeiouAEIOU’;

    let count = 0;

    for (let char of str) {

        if (vowels.includes(char)) {

            count++;

        }

    }

    return count;

}

// Example usage:

let text = “Hello World”;

console.log(countVowels(text)); // Output: 3 (e, o, o)

33. Calculate the Fibonacci sequence up to n numbers using recursion.

function fibonacci(n) {

    if (n <= 0) {

        return [];

    }

    if (n === 1) {

        return [0];

    }

    const sequence = [0, 1];

    for (let i = 2; i < n; i++) {

        sequence.push(sequence[i – 1] + sequence[i – 2]);

    }

    return sequence;

}

// Example usage:

let count = 8;

console.log(fibonacci(count)); // Output: [0, 1, 1, 2, 3, 5, 8, 13]

JavaScript Object Oriented Programming Interview Questions

Here are some important JavaScript OOP interview questions and answers.

34. What is Object-Oriented Programming (OOP)?

Object-Oriented Programming (OOP) is a way of writing programs that use objects to structure and handle data. This approach emphasizes reusing code and breaking down complex tasks into simpler parts.

35. What are some benefits of using OOP in JavaScript?

Using Object-Oriented Programming (OOP) in JavaScript offers several advantages. It enhances code organization, promotes code reuse, and facilitates easier maintenance. 

Additionally, OOP allows developers to model real-world objects and concepts more naturally within their codebase.

36. What is encapsulation in JavaScript?

Encapsulation is the bundling of data (attributes) and methods (functions) that operate on the data into a single unit called an object. It allows objects to control access to their internal state and hide implementation details from the outside world.

37. What is polymorphism in JavaScript?

Polymorphism allows objects of different classes to be treated as objects of a common superclass. 

This means that different classes can implement the same method name but behave differently based on the specific class instance. Polymorphism enhances code reusability and flexibility in OOP.

JavaScript Coding Interview Questions

Here are commonly asked coding-related JavaScript practical questions and answers.

38. Write a function to check if a given string is a palindrome (reads the same backward as forward).

function isPalindrome(str) {

    return str === str.split(”).reverse().join(”);

}

// Example usage:

let text1 = “racecar”;

let text2 = “hello”;

console.log(isPalindrome(text1)); // Output: true

console.log(isPalindrome(text2)); // Output: false

39. Write a function to find the factorial of a number.

function factorial(n) {

    if (n === 0 || n === 1) {

        return 1;

    }

    let result = 1;

    for (let i = 2; i <= n; i++) {

        result *= i;

    }

    return result;

}

// Example usage:

let num = 5;

console.log(factorial(num)); // Output: 120 (5! = 5 * 4 * 3 * 2 * 1)

40. Write a function to remove duplicate elements from an array.

function removeDuplicates(arr) {

    return arr.filter((value, index, self) => self.indexOf(value) === index);

}

// Example usage:

let nums = [1, 2, 2, 3, 4, 4, 5];

console.log(removeDuplicates(nums)); // Output: [1, 2, 3, 4, 5]

JavaScript MCQ Question and Answer

Here are some important JavaScript logical questions and answers.

41. What will the following code output?

console.log(typeof NaN);

A) “number”

 B) “NaN”

 C) “undefined”

 D) “string”

 Answer: A) “number”

42. Which keyword is used to declare a constant variable in JavaScript? 

A) var

 B) let

 C) const

 D) final

Answer: C) const

43. What does the setTimeout() function do in JavaScript? 

A) Pauses the execution of the script

 B) Sets a timer to execute a function after a specified delay

 C) Executes a function immediately

 D) Stops the execution of a function

 Answer: B) Sets a timer to execute a function after a specified delay

44. Which of the following is NOT a valid JavaScript data type? 

A) number

 B) boolean

 C) undefined

 D) decimal

Answer: D) decimal

45. Question: What will the following code output?

console.log(2 + “2”); 

A) 4

 B) “22”

 C) “4”

 D) NaN

Answer: B) “22”

Wrapping Up

So, these are the top 25+ JavaScript interview questions and answers that cover essential concepts and topics for JavaScript developers. Learning these questions will boost your confidence and help you impress the interviewers. And if you are looking for the latest JavaScript job opportunities, visit Hirist, the leading IT job portal in India.