JavaScript
Some other popular frontend technologies among interviewees:
tech-logo

Top 70+ JavaScript interview questions.

Total questions: 80
1. How do null and undefined differ in terms of their behavior in JavaScript?
easy

In JavaScript, both null and undefined are used to represent a lack of ...

2. What is type coercion in JavaScript and how does it work?
easy

Type coercion refers to the automatic conversion of one data type to another in JavaScript. It oc...

3. What is the purpose of using both `==` and `===` operators in JavaScript, and when would you use one over the other?
easy

In JavaScript, == and === are two operators used for comparing values. ...

4. In JavaScript, what is hoisting and how does it impact the way variables and functions are declared and used?
easy

Hoisting is a feature of JavaScript where variable and function declarations are moved to the top...

5. What is closure in JavaScript and how does it work? Provide an example.
easy

In JavaScript, closure is a function that has access to the outer function's variables even after...

6. How does the "this" keyword work in JavaScript, explain with the help of examples and what are some common pitfalls to be aware of when using it?
hard
The "this" keyword in JavaScript refers to the object that it belongs to. It allows us to access the...
7. How does JavaScript define the scope of variables? Explain with the help of an example.
medium

In JavaScript, the scope of a variable refers to the visibility and accessibility of that variabl...

8. Can you explain what arrow functions are in JavaScript and how they differ from regular functions?
medium

Arrow functions are a new syntax introduced in ECMAScript 6 (ES6) that allows for shorter, more c...

9. Can you explain how higher-order functions work in JavaScript and give a suitable example to support your explanation?
medium

In JavaScript, a function is considered higher-order when it takes one or more functions as argum...

10. How does an IIFE work in JavaScript, and why would you use it? Also provide an example
hard

An Immediately Invoked Function Expression (IIFE) is a function that is immediately executed as s...

11. What is functional programming and how does its concepts apply in JavaScript? Also provide an example
medium

Functional programming is a programming paradigm that focuses on the use of immutable data struct...

12. In JavaScript are functions treated as first class object? If yes then explain with the help of an example.
medium

Yes, in JavaScript, functions are treated as first-class objects, meaning that they can be assign...

13. Can you explain the different ways to create objects in JavaScript, including object literals, constructor functions, and prototypal inheritance?
easy

In JavaScript, there are several ways to create objects:

...
14. Can you explain the different data types available in JavaScript and their characteristics?
easy

In JavaScript, there are several built-in data types, including:


...
15. Can you explain how the event loop works in JavaScript and what it does?
hard

The event loop is a fundamental part of the JavaScript runtime environment that allows it to hand...

16. Can you explain the concept of callbacks in JavaScript? Also provide an example to support your explanation.
medium
Callbacks are functions passed as arguments to other functions. The function receiving the callback ...
17. How do callbacks differ from other synchronous constructs like functions or loops in JavaScript, and why are they preferred in certain situations?
hard
Callbacks differ from other synchronous constructs like functions or loops in that they are asynchro...
18. Can you provide an explanation of callback hell in JavaScript, including some common scenarios where it may occur and possible solutions to address it?
hard
Callback hell refers to a programming pattern where multiple nested functions are called asynchronou...
19. What is a promise in JavaScript and how does it work?
hard
A promise in JavaScript is an object that represents the eventual completion (or failure) of an asyn...
20. How do you chain multiple Promises together in JavaScript, and what benefits does this offer?
medium
In JavaScript, we can chain multiple promises together by using the `.then()` method to handle the r...
21. What are Promise.all and Promise.race methods in JavaScript? When would you prefer to use one over the other?
medium
In JavaScript, the `Promise.all()` and `Promise.race()` methods are both used to handle multiple pro...
22. What is the difference between Promise.all and Promise.allSettled in JavaScript?
hard
Promises are a fundamental part of asynchronous programming in JavaScript. They allow us to represen...
23. What is the difference between Promise.any and Promise.race in JavaScript?
hard
Promises are an essential part of asynchronous programming in JavaScript. They allow us to represent...
24. What are the pros and cons of promises over callbacks?
medium
Callbacks and Promises are two popular ways to handle asynchronous operations in JavaScript. Each ap...
25. What are the different error handling patterns that can be used for error handling with promises?
medium
There are several error handling patterns that can be used for error handling with promises. Here ar...
26. Explain the concept of prototypal inheritance in JavaScript.
medium
In JavaScript, prototypal inheritance is a mechanism that allows an object to inherit properties and...
27. Can you provide an example to illustrate the differences between `proto` and `prototype` in JavaScript, including how they are used and what they do?
hard
In JavaScript, both `proto` and `prototype` refer to the same concept: the parent object in the prot...
28. What is the difference between object literals and constructor functions?
easy
Object literals and constructor functions are two ways of creating objects in JavaScript. They both ...
29. Explain the concept of async/await and how it simplifies asynchronous programming in JavaScript. How does it differ from using promises?
medium
Async/await is a feature introduced in ECMAScript 2015 that allows us to write asynchronous code tha...
30. Can you explain what the job queue or microtask queue is in JavaScript and how it works?
hard
A job queue is a data structure that allows multiple tasks to be processed asynchronously. In JavaSc...
31. How do you create private variables in JavaScript?
medium
In JavaScript, there are no truly private variables. However, we can use several techniques to creat...
32. Discuss the differences between event propagation and event bubbling in the DOM.
medium
In HTML, events are used to trigger specific actions when certain user interactions occur on a web p...
33. How can you access and manipulate the browser history in JavaScript?
easy
In JavaScript, we can access and manipulate the browser history using several methods provided by th...
34. Explain the usage of let and const keywords in JavaScript.
easy
In JavaScript, `let` and `const` are block-scoped keywords used to declare variables with a specific...
35. What is destructuring assignment in JavaScript?
easy
In JavaScript, destructuring assignments allow you to assign multiple values from an array or object...
36. What are modules in ES6?
medium
Modules in ES6 (ECMAScript 2015) are a way to organize code into reusable, self-contained units. The...
37. Explain the concept of 'this' keyword in JavaScript. Also, how does it work inside of the event handlers?
hard
In JavaScript, the `this` keyword refers to the object that is executing the code it belongs to. It ...
38. What is the difference between event.preventDefault() and event.stopPropagation() methods in JavaScript?
medium
In JavaScript, both `event.preventDefault()` and `event.stopPropagation()` are methods used to preve...
39. What is event delegation in JavaScript? What are the various scenarios in which it is useful?
medium
Event delegation is a technique used in JavaScript to handle multiple events on multiple elements us...
40. What is the difference between localStorage, sessionStorage and cookies in JavaScript? Explain with the help of examples when would you prefer to use one over the other?
medium
LocalStorage, sessionStorage, and cookies are all mechanisms for storing data on a client-side brows...
41. What is Cross-Origin Resource Sharing (CORS) and how do you handle it in JavaScript?
hard
Cross-Origin Resource Sharing (CORS) is a security feature implemented by web browsers that restrict...
42. What is Cross-Site Scripting (XSS) and how can we prevent it?
hard
Cross-Site Scripting (XSS) is a common security vulnerability that allows an attacker to inject mali...
43. What is Content Security Policy (CSP)? How can it help in preventing web security attacks?
medium
Content Security Policy (CSP) is a security feature implemented by web browsers that allows you to s...
44. What are template literals in JavaScript? Can they be used for string interpolation?
easy
Template literals in JavaScript are a way of creating strings using backticks (`) instead of quotes....
45. How do you perform exception handling in JavaScript?
medium
Exception handling is an essential part of any programming language that allows developers to gracef...
46. What are the various error handling statements used in JavaScript?
medium
In JavaScript, there are several error handling statements that can be used to handle exceptions and...
47. Explain the concept of breakpoints in debugging a JavaScript program.
medium
Breakpoints are an important tool for debugging a JavaScript program. A breakpoint is a special mark...
48. What are the various methods to access DOM elements in JavaScript?
easy
There are several ways to access DOM elements in JavaScript:...
49. What are event listeners in JavaScript? How can we add and remove one from an element?
easy
In JavaScript, event listeners allow you to respond to user interactions or other events that occur ...
50. What is the difference between the call, apply and bind methods in JavaScript?
easy
In JavaScript, the call(), apply(), and bind() methods are used to invoke a function with different ...
51. What is a higher order function? Explain with the help of relevant examples.
easy
A higher-order function (HOF) is a function that takes one or more functions as arguments and/or ret...
52. What is function currying? Explain with the help of an example.
medium
Function currying is a technique used in functional programming where a single function is converted...
53. What is URL encoding? How can we encode or decode a URL in JavaScript?
easy
URL encoding, also known as percent-encoding, is the process of converting special characters in a U...
54. What is a pure function? Explain with the help of an example in JavaScript.
easy
A pure function is a function that always produces the same output given the same input, and has no ...
55. What is a first order function? Explain with the help of an example in JavaScript.
easy
A first-order function is a function that does not accept other functions as arguments and does not ...
56. Discuss the importance of web page loading time and performance.
medium

Web page loading time and performance play an important role in determining the success of a webs...

57. Are functions treated as first class objects in JavaScript? Support your answer with the help of an example in either case.
medium

Yes, in JavaScript, functions are treated as first-class objects.

...
58. What is a unary function? Explain with the help of an example.
easy

A unary function is a function that takes only one input value and returns a single output value....

59. What is memoization in functional programming? Explain with the help of an example in JavaScript.
medium

Memoization is a technique used in functional programming that allows a function to cache its int...

60. What are async iterators and generators in JavaScript?
hard

Async iterators and generators are two new features introduced in JavaScript with ES6. They provi...

61. What are dynamic imports in JavaScript?
hard

Dynamic imports in JavaScript allow you to load code asynchronously and on-demand, instead of inc...

62. Can you explain the difference between the usage of Object.entries() and Object.values()?
easy

The Object.entries() and Object.values() methods are both used to extra...

63. How does JavaScript handle big integers (BigInt)?
medium

A BigInt is a primitive type specifically designed to represent numeric values that are too large...

64. What is the Document Object Model (DOM) in JavaScript?
medium

The Document Object Model (DOM) is an API for HTML and XML documents. In other words, it's a way ...

65. What is the difference between document load and DOMContentLoaded events in JavaScript?
medium

The documentLoad event and the DOMContentLoaded event are both fired wh...

66. What are the differences between the window and the document object in JavaScript?
medium

The window object and the document object are both part of the Document...

67. What is a service worker in JavaScript?
medium

A service worker is a script that runs in the background of a web application, providing caching ...

68. How can we reuse information across service worker restarts in JavaScript?
medium

There are a few ways to reuse information across service worker restarts in JavaScript:

...
69. What are the differences between the arrow functions and the regular functions in JavaScript?
hard

Syntax and Conciseness:

...
70. Explain about the post message method in JavaScript? Also mention its utilities and restrictions.
medium

PostMessage is a method in JavaScript that enables communication between web pages hosted on diff...

71. What are server-sent events?
medium

Server-Sent Events (SSE) are a technology that allows a web server to push data to a client in re...

72. How do you ensure that your JavaScript code is cross-browser compatible?
medium

Ensuring that JavaScript code is cross-browser compatible means making sure that it works correct...

73. What is Babel and how is it used in JavaScript development?
medium

Babel is a tool that allows developers to transpile or convert modern JavaScript code into an old...

74. What are module bundlers and what is their role in modern JavaScript development?
medium

Module bundlers are tools used in modern JavaScript development to manage dependencies between fi...

75. What is tree shaking in module bundling?
medium

Tree shaking is a technique used in module bundling, which involves removing unused or unnecessar...

76. What are classes in ES6?
easy

In ECMAScript (ES) 6, a class is a blueprint or template for creating objects with similar proper...

77. What is the use of JSON.stringify() method?
easy

The JSON.stringify() method is used to convert a JavaScript object or an array into a string repr...

78. How do you redirect to a new page programmatically in JavaScript?
easy

In JavaScript, you can redirect to a new page using several methods, most commonly by modifying t...

79. What are setTimeout and setInterval timing functions in JavaScript?
easy

setTimeout() and setInterval() are two timing functions available in Ja...

80. What is a cookie in JavaScript?
easy

In JavaScript, a cookie is a small text file that is stored on a user's computer or mobile device...