Top 50 JavaScript Interview Questions and Answers

By Indeed Editorial Team

Published 27 September 2021

The Indeed Editorial Team comprises a diverse and talented team of writers, researchers and subject matter experts equipped with Indeed's data and insights to deliver useful tips to help guide your career journey.

JavaScript (JS) is a programming language used to increase the amount of user interaction on a webpage. With a constant update in its coding techniques and the rise of applications and websites, JS developers need to keep upgrading their skill set. Acquiring a thorough understanding of what interviewers ask for in a JavaScript job role increases your chances of securing the job. In this article, we look at 50 JavaScript interview questions and answers.

How do I prepare for a JavaScript interview?

Preparing for a JavaScript interview involves researching less familiar topics with a solution-oriented approach. Additionally, the following points can help you do well:

  • understanding JS concepts, such as hoisting, callbacks, promises and IIFE

  • practising interview-based coding exercises beforehand

  • knowing its origin and future scope in the programming industry

  • showcasing your knowledge regarding up-to-date JS features

  • coming up with two code solutions for a problem

While this list is endless, aspiring JS developers should also study the company's standard coding practices and apply them during the interview.

Related: What Is JavaScript?

Top 18 JavaScript interview questions and answers

Here are some questions you can read through to help you prepare:

1. What is the difference between JavaScript and JScript?

This question helps gauge your foundational understanding of the programming language.

Example: “Both the languages are similar, except the fact that Jscript is a variant of JavaScript.”

2. How do you use JavaScript to identify the OS in a client machine?

This shows your ability to work with clients and also tests the basics of your client-server knowledge in JavaScript programs.

Example: “The navigator.userAgent or navigator.appVersion properties help identify the client's OS.”

3. How does the Unshift method contribute to JavaScript?

Such questions will test your fundamental knowledge about using arrays in JavaScript.

Example: The Unshift method is more like the push method that works at the start of the array. We can use the Unshift method to add one or more components before the array's start.”

4. How does the Push method contribute to JavaScript?

This question tests your detailed knowledge of how to modify the number of elements in an array. Focus on regulating array ecosystems.

Example: “By using several arguments, it is useful to append one or more components to the array's end. You can also alter an array's length with the Push method.”

Related: jQuery Interview Questions and Answers: General and Expert Level

5. Name some common JavaScript data types

When answering this question, give an example for each data type if you can.

Example: “Common JS data types are boolean, number, object, string, and undefined. These can be divided in three categories of primitive, composite, and special.”

6. How does a confirmation box and alert box vary?

JS interviewers ask this question to know how effectively could you create code that simplifies the message for the user.

Example: “Both these boxes are used to communicate a message to the user. While the alert box has only one option to click on (OK), the confirmation box has two (OK and CANCEL).”

7. Name the three pop-up boxes that exist in JS

This question indicates your ability to create interactive and followable interfaces for users.

Example: “The three pop-up boxes in JS include Alert, Confirm and Prompt.”

8. What are two ways to read and write a file using JS?

This JS interview question helps analyse your basic usability metric with the programming language.

Example: “A file can be read or written on JS through either a webpage and Active X objects, or through JS extensions.”

Related: How To Become a Front-End Developer: A Complete Guide

9. What exactly are global variables?

This tests the candidate's insights on reducing the usage of local variables and saving up on memory.

Example: “If the var keyword, which is used to declare a locale object or variable, is omitted, then a global variable is declared. Additionally, global variables exist all across the code to eliminate any limitations.”

10. What does negative infinity mean in JS?

Despite being more of a value-related answer, negative infinity also helps in looping.

Example: “When any negative number is divided by zero, you will get a JS number called negative infinity. Also, when divided by itself, it will return NaN.”

11. What is the purpose of NaN?

NaN stands for “not a number” and can help identify whether a value is a valid number or not. For example, finding the square root of a negative number returns NaN.

Example: “NaN represents non-number values. Simply put, it helps identify invalid numbers.”

12. Name the three looping structures in JS

Loops are a highly utilised feature in any programming language and indicate the coder's ability to cut down longer codes into shorter ones.

Example: “For, While and Do-while are the three looping structures.”

13. What would be the output of 5+9+”2” in JS?

This question tests your idea of how numeric calculations occur in JS programming.

Example: “As 2 is placed within a string, it will be concatenated to the sum of 5+9. Thus, the output is 142.”

Related: 11 AngularJS Interview Questions You Should Prepare For

14. What is DOM in JS?

DOM helps in managing web documents through code. Using DOM in JS, the coder gets access to the components of a webpage.

Example: “Document Object Model is automatically created by the web browser once the page has loaded. Each and every element of the webpage can be accessed and modified from the DOM.”

15. What are the window.onload and onDocumentReady features?

Both these functions are related to the loading of the webpage and code. Explain your answers in terms of which feature manipulates the code quicker.

Example: “While onDocumentReady downloads the code right before the DOM is ready, it permits the coder to manipulate the code early. Whereas, window.onload doesn't begin till all webpage information isn't ready. Moreover, it causes delays before the code is run.”

16. What are break and continue statements?

Both these terminologies come under the looping aspect of programming. Give an example of how do the break and continue statements affect loops.

Example: “While the break statement prompts an exit from a running loop, the continue statement permits the same loop to keep running, with a fresh start.”

17. Name the three types of errors that flag in JS?

These errors are automatically generated and categorised into three types. If you can portray your code-resolving abilities with a diversified knowledge on how these errors arise then you'll create a good impression.

Example: “The three errors types in JS are Load-time, Runtime and Logic errors. Any of these errors will interupt the smooth functioning of the program.”

18. Name the two basic groups of data types in JS

This explains your flexibility when it comes to using a large variety and number of variables.

Example: “The two basic groups of data types in JS are Primitive and Reference types. Primitive types represent only number and Boolean types, whereas reference types consist of more complex data types such as strings and dates.”

Related: HTML Interview Questions and Answers for Professionals

32 additional JavaScript interview questions

As promised, here are 30 additional JS interview questions that will strengthen your preparation for the upcoming interview:

  1. Can you remove duplicates from a JS array? If so, then how?

  2. How can the setTimeout function be used?

  3. What's the method to perform a 301 redirect in JS?

  4. Explain the meaning of scope chain.

  5. Is JS or ASP faster?

  6. What is an undefined value in JS?

  7. How does Void(0) work?

  8. What does NULL mean in JS?

  9. Differentiate between “==” and “===”?

  10. Can the style/class of an element be altered?

  11. Describe a for-in loop.

  12. What factors affect an anonymous function?

  13. Describe what event bubbling is.

  14. What is the difference between a web garden and a web farm?

  15. What are the properties of screen objects?

  16. Explain with an example what the unshift() method is.

  17. Why is innerHTML content continually refreshed?

  18. Describe the important JS Array Method properties.

  19. Describe in detail the concept of OOPS in JS?

  20. What issues do coders face in JS Unit Testing?

  21. What are the solutions for JS Unit Testing?

  22. How are the Divide-and-Conquer and QuickSort algorithms related?

  23. What is External JS? Give an example.

  24. How do Cookies contribute to JS?

  25. What's a JS multiplication table?

  26. What are the unescape() and escape() functions used for?

  27. Please elaborate on the Eagle eye test.

  28. How does the blur function help?

  29. What are deferred scripts in JS.

  30. How do ViewState and SessionState vary?

  31. How do you insert comments in JS?

  32. What does the “==” operator indicate?

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