Top JavaScript Interview Questions and Answers (Updated 2024)

JavaScript (JS) is one of the most widely used programming languages for web development. It is lightweight, interpreted, and essential for building dynamic and interactive web applications. Created by Brendan Eich in 1995, JavaScript powers both client-side and server-side code. This versatility has made it a must-have skill for front-end and full-stack developers alike. Whether you are a fresher or an experienced developer preparing for a JavaScript interview, this comprehensive guide will cover key JavaScript concepts, from basic syntax to advanced features and best practices.

Table of Contents

1. JavaScript Interview Questions for Freshers
2. JavaScript Intermediate Interview Questions
3. JavaScript Interview Questions for Experienced Developers
4. Additional Advanced JavaScript Topics
5. Resources for Preparing JavaScript Interviews

JavaScript Interview Questions for Freshers

Here are some common JavaScript questions frequently asked in interviews for freshers and entry-level roles, especially in front-end development positions.

1. What is the difference between Java and JavaScript?

• JavaScript is a client-side scripting language primarily used for creating interactive web pages.
• Java is a general-purpose object-oriented programming language with a “write once, run anywhere” philosophy.

JavaScript is lightweight and used within web browsers to manipulate HTML and CSS, whereas Java is a compiled language that runs on a variety of platforms via the Java Virtual Machine (JVM).

2. What are JavaScript Data Types?

JavaScript has several data types, which can be categorized into three major groups:

• Primitive Types:
  • Numbers
  • Strings
  • Booleans
  • Symbols
  • Undefined
  • Null
• Composite Types:
  • Objects
  • Arrays
  • Functions

3. Which symbol is used for comments in JavaScript?

• Single-line comment: //
• Multi-line comment: /* */

// This is a single-line comment
/* 
  This is a multi-line comment
*/

4. What will be the result of 3 + 2 + "7"?

JavaScript evaluates the expression from left to right:

• First, 3 + 2 = 5.
• Then, 5 + "7" becomes a string concatenation, resulting in "57".

5. What is the use of isNaN() in JavaScript?

isNaN() is a built-in function that checks whether a value is “Not-a-Number.” It returns true if the value is NaN, otherwise it returns false.

console.log(isNaN('hello')); // true
console.log(isNaN(123)); // false

6. What are undeclared and undefined variables?

• Undefined: Variables that are declared but not assigned a value.
• Undeclared: Variables that have not been declared with var, let, or const. Accessing them results in a runtime error.

7. How do you create new elements dynamically in JavaScript?

JavaScript’s createElement() method is used to create HTML elements dynamically.

<button onclick="create()">Click Here!</button>

<script>
  function create() {
    let element = document.createElement('div');
    element.textContent = "Hello, JavaScript!";
    document.body.appendChild(element);
  }
</script>

8. What is null in JavaScript?

null represents the absence of any value. It is intentionally assigned to indicate that a variable has no value.

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JavaScript Intermediate Interview Questions

For developers with a few years of experience, intermediate-level JavaScript questions will focus on more in-depth understanding, including ES6+ features and DOM manipulation.

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

• == is the loose equality operator, which performs type coercion before comparing.
• === is the strict equality operator, which compares both value and type.

console.log(2 == '2'); // true (due to type coercion)
console.log(2 === '2'); // false (different types)

10. What are the differences between var, let, and const?

• var: Function-scoped, can be re-declared and updated.
• let: Block-scoped, cannot be re-declared but can be updated.
• const: Block-scoped, cannot be re-declared or updated (used for constants).

11. What is event delegation in JavaScript?

Event delegation is a technique where a single event listener is attached to a parent element and handles events for all its child elements. This is useful for dynamically added elements.

12. Explain closures in JavaScript.

A closure is a function that remembers its lexical scope even when the function is executed outside of its original scope.

function outerFunction() {
  let counter = 0;
  return function innerFunction() {
    counter++;
    return counter;
  };
}
const increment = outerFunction();
console.log(increment()); // 1
console.log(increment()); // 2

13. How can you handle asynchronous operations in JavaScript?

You can handle asynchronous operations using:

• Callbacks
• Promises
• Async/Await

14. Explain the working of setTimeout and setInterval.

• setTimeout: Executes a function after a specified delay.
• setInterval: Repeatedly executes a function after a specified interval.

setTimeout(() => console.log("Executed after 2 seconds"), 2000);
setInterval(() => console.log("Executed every 2 seconds"), 2000);

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JavaScript Interview Questions for Experienced Developers

For experienced developers, interviews often include questions on advanced JavaScript concepts, design patterns, and performance optimizations.

15. What are Promises and how do they work?

A Promise represents the eventual completion or failure of an asynchronous operation. It has three states:

• Pending
• Fulfilled
• Rejected

javascriptCopy codeconst myPromise = new Promise((resolve, reject) => {
  let condition = true;
  if (condition) resolve("Success!");
  else reject("Failure!");
});

myPromise
  .then(result => console.log(result))
  .catch(error => console.log(error));

16. What are JavaScript Modules?

Modules allow you to encapsulate code and only expose the variables or functions that should be available for other parts of your application. JavaScript modules are implemented using export and import statements.

17. Explain async and await.

async functions enable asynchronous, promise-based behavior in a cleaner syntax. The await keyword pauses the execution until a Promise is resolved.

async function fetchData() {
  let response = await fetch('https://api.example.com/data');
  let data = await response.json();
  console.log(data);
}

18. What is the event loop in JavaScript?

The event loop handles the execution of multiple tasks, including promises and asynchronous callbacks. It continuously checks the call stack and task queue and processes tasks accordingly.

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Additional Advanced JavaScript Topics

19. What are WeakMap and WeakSet?

• WeakMap: A collection of key-value pairs where keys are weakly referenced, meaning they can be garbage-collected.
• WeakSet: Similar to Set, but it holds weakly referenced objects that can be garbage-collected.

20. How does garbage collection work in JavaScript?

JavaScript uses automatic garbage collection via reference counting and the mark-and-sweep algorithm to free up memory by removing unused objects.

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Resources for Preparing JavaScript Interviews

• Books:
  • Eloquent JavaScript by Marijn Haverbeke
  • JavaScript: The Good Parts by Douglas Crockford
• Online Platforms:
  • MDN Web Docs
  • LeetCode
  • GeeksforGeeks

Preparing for a JavaScript interview requires a solid understanding of the fundamentals, as well as hands-on coding practice. Familiarizing yourself with commonly asked questions, advanced topics, and real-world problem-solving will significantly increase your chances of success.