Preparing for a JavaScript interview, below are 15 common JavaScript interview questions that you may encounter, ranging from basic to more advanced topics.

1. What is JavaScript and why is it used?

JavaScript is a high-level, interpreted programming language widely used for web development. It allows you to create dynamic and interactive web pages by manipulating HTML and CSS elements on the client-side. JavaScript is an essential tool for front-end development and has become increasingly popular for back-end development, too, thanks to Node.js.

2. What is the difference between var, let, and const?

var is function-scoped and was the standard way to declare variables in older versions of JavaScript. However, it has some quirks that can lead to unexpected behavior, such as variable hoisting. let and const are block-scoped, introduced in ES6. let allows variable reassignment, while const does not.

3. What are closures in JavaScript?

A closure is a function that has access to its own scope, the scope of the outer function, and the global scope. Closures are used for data encapsulation, allowing you to create private variables and methods within a function.

4. How do you prevent callback hell in JavaScript?

Callback hell, or “Pyramid of Doom,” occurs when multiple nested callbacks become difficult to read and maintain. To prevent this, you can use:

Modularization of functions

5. What is the difference between null and undefined?

Both null and undefined represent the absence of a value. However, null is explicitly assigned to a variable, indicating that it has no value, while undefined is the default value for uninitialized variables.

6. What is the event loop in JavaScript?

The event loop is a fundamental concept in JavaScript’s asynchronous nature. It continually checks the call stack and message queue, executing functions from the call stack and moving queued-up events to the call stack when it’s empty.

7. What is the difference between == and ===?

== is the loose equality operator and performs type coercion, allowing for comparison between different data types. === is the strict equality operator and does not perform type coercion, requiring both the value and data type to match.

8. How does prototypal inheritance work in JavaScript?

Prototypal inheritance is a mechanism in which an object inherits properties and methods from another object, called its prototype. When you access a property or method that does not exist on the current object, JavaScript will look up the prototype chain to find it.

9. Explain the concept of hoisting in JavaScript.

Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase. However, only declarations are hoisted; initializations are not.

10. What is the spread operator and how do you use it?

The spread operator (…) allows you to expand an iterable (e.g., an array, string, or object) into individual elements or properties. It can be used for copying arrays, merging objects, or passing multiple arguments to a function.

11. What is an Immediately Invoked Function Expression (IIFE)?

An IIFE is a function expression that is immediately executed after its creation. It is used to create a private scope and avoid polluting the global namespace.

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

Arrow functions are a concise way to write function expressions. They have implicit returns, do not have their own this context, and cannot be used as constructors. Regular functions have a this context, can be used as constructors, and require explicit return statements.

13. How do you deep clone an object in JavaScript?

There are multiple ways to deep clone an object in JavaScript, such as using:

1. JSON.parse(JSON.stringify(obj))
2. Recursion
3. Libraries like Lodash

14. What is the difference between a method and a function in JavaScript?

A method is a function that is a property of an object, while a function is a standalone, reusable block of code.

15. What is async/await and how does it work?

async/await is syntactic sugar for Promises, introduced in ES8. It allows you to write asynchronous code in a more readable and synchronous-like manner. An async function returns a promise, and the await keyword is used to pause the execution of the function until the promise is resolved.

16. What are higher-order functions in JavaScript?

A higher-order function is a function that either takes one or more functions as arguments or returns a function as a result. Examples include the Array methods map, filter, and reduce.

17. What is function debouncing in JavaScript?

Debouncing is a technique used to limit the number of times a function can be executed within a specified time period. It is often used to optimize performance by delaying the execution of a function until a certain interval has passed since the last time it was invoked. For example, debouncing is often used in scenarios like input field validation, auto-suggest, and window resizing.

18. Explain the difference between debouncing and throttling.

Both debouncing and throttling are used to control the rate at which a function is executed, but they work differently. Debouncing delays the execution of a function until after a specified time has passed since the last invocation. Throttling, on the other hand, ensures that a function is executed no more frequently than a specified interval, regardless of how many times it is called.

19. How do you implement debouncing in JavaScript?

You can implement debouncing in JavaScript using setTimeout and clearTimeout functions. Here’s a simple example of a debounced function:

function debounce(func, delay) {
  let timeout;
  return function(...args) {
    const context = this;
    timeout = setTimeout(() => {
      func.apply(context, args);
    }, delay);

20. What is the difference between forEach, map, and filter methods in JavaScript?

All three methods are used to iterate over an array, but they serve different purposes:

1. forEach iterates over an array and performs a side effect for each element, returning undefined.
2. map iterates over an array, applies a function to each element, and returns a new array with the transformed elements.
3. filter iterates over an array and returns a new array containing only the elements that satisfy a specified condition.

21. How can closures be used to create private variables in JavaScript?

Closures can be used to create private variables by defining variables within a function scope, making them inaccessible from the outside. By returning an inner function that references these variables, you can create a closure that maintains access to the private variables.

function createCounter() {
  let count = 0; // private variable
  return {
    increment: function() {
      return count;

const counter = createCounter();
console.log(counter.increment()); // 1
console.log(counter.increment()); // 2

In this example, count is a private variable that can only be accessed and modified through the increment method.


Leave a Comment