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. What does NaN mean in JavaScript?

A) None of the above

B) Not a Number

C) Not a Name

D) Not a Null

2 / 30

2. What does the navigator.geolocation.getCurrentPosition() method do?

A) Updates the user's address.

B) Sets the user's location.

C) Fetches the current time.

D) Retrieves the user's current location.

3 / 30

3. What does Promise.resolve() do?

A) Rejects a promise

B) Creates a pending promise

C) Resolves a pending promise with a given value

D) Waits for a promise to resolve

4 / 30

4. What is the purpose of minifying JavaScript files?

A) To add comments to the code.

B) To improve code readability.

C) To detect syntax errors.

D) To reduce file size for faster loading.

5 / 30

5. What is the purpose of the Blob object?

A) To represent binary data.

B) To store cookies.

C) To compress data.

D) To create HTML elements.

6 / 30

6. What is the purpose of Object.getPrototypeOf()?

A) To create a prototype for an object

B) To retrieve the prototype of an object

C) To seal an object

D) To modify an object's prototype

7 / 30

7. How do you register an event that triggers once and then removes itself?

A) Use the once option in addEventListener().

B) Use the triggerOnce() method.

C) Use the oneTimeEvent() function.

D) Use addEventListener() followed by removeEventListener().

8 / 30

8. What is an object in JavaScript?

A) A collection of key-value pairs

B) None of the above

C) A special type of function

D) A built-in data type

9 / 30

9. How can developers avoid exposing sensitive information in source maps?

A) Disable source maps in production.

B) Store source maps in cookies.

C) Avoid debugging tools.

D) Use unminified files.

10 / 30

10. What will the following code do? document.getElementById("myDiv").style.color = "red";

A) Throw an error.

B) Change the background color of myDiv to red.

C) Change the text color of myDiv to red.

D) Do nothing.

11 / 30

11. What is the purpose of the abort() method in AJAX?

A) Closes the connection

B) Resends the request

C) Cancels an ongoing request

D) Logs errors in the request

12 / 30

12. Which function repeatedly executes code after a set interval?

A) setInterval()

B) executeLater()

C) setTimeout()

D) setRepeater()

13 / 30

13. Which API is used to measure page load times?

A) Web Animations API

B) Event Loop API

C) MediaStream API

D) Performance API

14 / 30

14. How do you toggle a class on an element?

A) element.classToggle(className)

B) element.classList.toggle(className)

C) element.toggle(className)

D) element.switchClass(className)

15 / 30

15. What is the purpose of the Secure flag in cookies?

A) To allow cookies to be accessed over HTTP.

B) To prevent cookies from expiring.

C) To encrypt cookie data.

D) To restrict cookies to HTTPS connections only.

16 / 30

16. Which of the following methods correctly copies an array in JavaScript?

A) let newArr = oldArr;

B) let newArr = oldArr.slice();

C) let newArr = [...oldArr];

D) Both B and C

17 / 30

17. How do you create a custom animation using the Web Animations API?

A) Use animation.create() method.

B) Use element.animate() method.

C) Use CSS.animate() method.

D) Use window.createAnimation() method.

18 / 30

18. Which response type is used to receive data as JSON?

A) json

B) text

C) xml

D) blob

19 / 30

19. How can you remove an existing child element in the DOM?

A) deleteChild()

B) removeElement()

C) clearChild()

D) removeChild()

20 / 30

20. How can you optimize the rendering performance of a web application?

A) Increase CSS file size.

B) Use synchronous JavaScript everywhere.

C) Disable browser caching.

D) Minimize DOM manipulations and reflows.

21 / 30

21. How do you attach a click event to a button in JavaScript?

A) button.addEventListener("click", function() { ... })

B) button.onClick = function() { ... }

C) Both A and B

D) button.attach("click", function() { ... })

22 / 30

22. What is the purpose of the Coverage tab in Chrome DevTools?

A) To identify unused CSS and JavaScript.

B) To debug event listeners.

C) To measure network requests.

D) To display memory usage.

23 / 30

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

A) parent();

B) base();

C) super();

D) constructor();

24 / 30

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

A) Reduced performance.

B) Difficulty in debugging.

C) Increased file size.

D) Exposure to XSS attacks.

25 / 30

25. JavaScript is a _______ language.

A) Assembly-level

B) Interpreted

C) Compiled

D) Low-level

26 / 30

26. What does the fetch() method do?

A) Fetches variables from memory

B) Creates DOM elements

C) Sends HTTP requests and returns a promise

D) Executes functions asynchronously

27 / 30

27. What is the default content type for POST requests in AJAX?

A) multipart/form-data

B) application/json

C) application/x-www-form-urlencoded

D) text/plain

28 / 30

28. Which of the following is not a JavaScript data type?

A) Number

B) Float

C) Boolean

D) String

29 / 30

29. What does HTTPS provide for web applications?

A) Automatic code optimization.

B) A debugging tool for developers.

C) Faster loading speed.

D) Protection against man-in-the-middle attacks.

30 / 30

30. How do you define a private field in a class?

A) Using const

B) Using _ as a prefix

C) Using # as a prefix

D) Using the private keyword

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