JavaScript Quiz Test Online (30 random Questions) 2026

Improve your JavaScript skills with this online quiz featuring 30 multiple-choice questions. Whether you’re a beginner or an experienced developer, this quiz covers JavaScript fundamentals, functions, objects, DOM manipulation, ES6 features, and asynchronous programming.

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30 Minutes JavaScript Quiz Test Online

Test your JavaScript knowledge with this 30-minute quiz! Covering fundamental to advanced concepts, this quiz features 30 multiple-choice questions that will challenge your understanding of variables, functions, objects, asynchronous programming, and DOM manipulation. Perfect for interview preparation and skill enhancement.

1 / 30

1. Which function is used to parse a string to an integer in JavaScript?

A) parse()

B) parseInt()

C) parseInteger()

D) getInt()

2 / 30

2. What does a 404 HTTP response code mean?

A) Server error

B) Request timeout

C) Forbidden request

D) Resource not found

3 / 30

3. What is the result of the code console.log(typeof NaN);?

A) "NaN"

B) "undefined"

C) "object"

D) "number"

4 / 30

4. How do you call the parent class constructor in a subclass?

A) base();

B) parent();

C) constructor();

D) super();

5 / 30

5. What is the purpose of the AbortController API in JavaScript?

A) To cancel asynchronous operations like fetch().

B) To stop animations on a webpage.

C) To halt DOM rendering.

D) To clear browser history.

6 / 30

6. What does the history.pushState() method do?

A) Modifies the browser history without reloading the page.

B) Redirects to a new URL.

C) Clears the browser history.

D) Fetches the current page title.

7 / 30

7. What is JSONP in the context of AJAX?

A) A replacement for XMLHttpRequest

B) A JSON parser for browsers

C) A method to fetch JSON data from other domains

D) A modern AJAX API

8 / 30

8. Which symbol is used to create private class fields in ES6+?

A) @

B) private keyword

C) _

D) #

9 / 30

9. Which JavaScript API helps sanitize user inputs?

A) DOMParser

B) JSON.stringify

C) Fetch API

D) Sanitizer API

10 / 30

10. Which method is used to parse JSON data in JavaScript?

A) JSON.parse()

B) JSON.stringify()

C) JSON.read()

D) JSON.convert()

11 / 30

11. What is polymorphism in JavaScript?

A) Creating multiple instances of a class

B) Inheriting properties from multiple classes

C) None of the above

D) Having multiple methods with the same name but different implementations

12 / 30

12. Which method is used to add an event listener to an element?

A) element.onEvent()

B) element.attachEvent()

C) element.bindEvent()

D) element.addEventListener()

13 / 30

13. How do you create a new Web Worker in JavaScript?

A) Worker.create()

B) WebWorker()

C) createWorker()

D) new Worker()

14 / 30

14. How can you dynamically create a new DOM element in JavaScript?

A) document.createElement()

B) document.newElement()

C) document.generateElement()

D) document.addElement()

15 / 30

15. What is asynchronous JavaScript?

A) Code executed without using variables

B) Code executed only after page load

C) Code executed line by line in order

D) Code executed in parallel without blocking

16 / 30

16. What does the Promise constructor accept?

A) Another promise

B) A callback function with resolve and reject parameters

C) A string argument

D) An array of values

17 / 30

17. What does setTimeout() do in JavaScript?

A) Executes code repeatedly

B) Executes a function immediately

C) Stops code execution for a set time

D) Executes a function after a delay

18 / 30

18. How can you detect when the DOM is fully loaded?

A) Using the readyState property.

B) Using the onready event.

C) Using the DOMContentLoaded event on the document object.

D) Using the load event on the window object.

19 / 30

19. Why is it important to use parameterized queries in JavaScript?

A) To simplify database queries.

B) To prevent SQL Injection attacks.

C) To reduce code execution time.

D) To improve code readability.

20 / 30

20. What will be the output of console.log(5 / 0);?

A) undefined

B) Infinity

C) 0

D) NaN

21 / 30

21. How can you prevent clickjacking attacks?

A) Avoid using iframes.

B) Use the X-Frame-Options header.

C) Use HTTPS.

D) Use strong passwords.

22 / 30

22. How can you determine the type of device being used?

A) Using the window.deviceType property.

B) Using the device.check() method.

C) Using the screen.type property.

D) Using the navigator.userAgent property.

23 / 30

23. How do you convert a string to an integer in JavaScript?

A) int("42")

B) Number.parseInt("42")

C) parseInteger("42")

D) convertToInt("42")

24 / 30

24. What is a secure way to handle sensitive API keys in JavaScript?

A) Store them in the backend server.

B) Store them in local storage.

C) Store them in plain text files.

D) Embed them in the HTML file.

25 / 30

25. What is the purpose of a nonce attribute in a CSP?

A) To prevent HTTP requests.

B) To encrypt inline scripts.

C) To block all external scripts.

D) To allow specific inline scripts to execute securely.

26 / 30

26. Which of the following is a common cause of memory leaks in JavaScript?

A) Unreferenced closures.

B) Using const instead of var.

C) Using arrow functions.

D) Event listeners not being removed.

27 / 30

27. How do you check the status of an AJAX request?

A) responseCode

B) statusCode

C) status

D) readyState

28 / 30

28. What is the purpose of the onreadystatechange event?

A) To log errors in the request

B) To set request headers

C) To initialize the AJAX request

D) To handle state changes in the XMLHttpRequest

29 / 30

29. What is a potential risk of using inline JavaScript?

A) Exposure to XSS attacks.

B) Difficulty in debugging.

C) Reduced performance.

D) Increased file size.

30 / 30

30. What does encapsulation mean in JavaScript?

A) Removing methods from an object

B) Hiding data and exposing only the necessary parts of an object

C) Creating a copy of an object

D) Combining multiple objects into one

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