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 is the responseType property used for in XMLHttpRequest?

A) To define the status code of the response

B) To define the format of the response

C) To define the method used in the request

D) To set the request headers

2 / 30

2. What is the purpose of a constructor in a class?

A) To initialize properties when an object is created

B) To define methods

C) To inherit another class

D) To destroy an object

3 / 30

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

A) constructor();

B) base();

C) parent();

D) super();

4 / 30

4. Which JavaScript API helps sanitize user inputs?

A) Sanitizer API

B) Fetch API

C) JSON.stringify

D) DOMParser

5 / 30

5. Which company developed JavaScript?

A) Netscape

B) Google

C) Microsoft

D) Sun Microsystems

6 / 30

6. How do you convert a callback-based function into a promise?

A) Using await

B) Using the Promise constructor

C) Using async

D) Using Promise.race()

7 / 30

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

A) JSON.read()

B) JSON.stringify()

C) JSON.convert()

D) JSON.parse()

8 / 30

8. What is the purpose of the Blob object?

A) To represent binary data.

B) To compress data.

C) To create HTML elements.

D) To store cookies.

9 / 30

9. What does the Promise constructor accept?

A) An array of values

B) A callback function with resolve and reject parameters

C) A string argument

D) Another promise

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10. Which HTTP response code indicates a successful request?

A) 404

B) 200

C) 500

D) 301

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

A) Minimize DOM manipulations and reflows.

B) Use synchronous JavaScript everywhere.

C) Increase CSS file size.

D) Disable browser caching.

12 / 30

12. What is the purpose of the readyState property in XMLHttpRequest?

A) Logs the request errors

B) Tracks the progress of the request

C) Indicates the server's response time

D) Specifies the HTTP method

13 / 30

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

A) executeLater()

B) setInterval()

C) setRepeater()

D) setTimeout()

14 / 30

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

A) Using _ as a prefix

B) Using the private keyword

C) Using const

D) Using # as a prefix

15 / 30

15. What does the Notification API do?

A) Sends notifications to the user's email.

B) Displays browser notifications.

C) Updates the webpage title.

D) Tracks the user's activity.

16 / 30

16. What does console.assert() do in debugging?

A) Displays debugging information in a separate file.

B) Stops code execution.

C) Throws an error if the condition is false.

D) Logs all variable values.

17 / 30

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

A) text

B) json

C) xml

D) blob

18 / 30

18. What is the correct syntax to display an alert box in JavaScript?

A) alert("Hello");

B) msg("Hello");

C) alertBox("Hello");

D) display("Hello");

19 / 30

19. What is the purpose of the timeout property in XMLHttpRequest?

A) To control the server's response time

B) To delay the request

C) To log the request duration

D) To specify the time before the request is automatically aborted

20 / 30

20. What is the purpose of Promise.all()?

A) Rejects all promises if one fails

B) Resolves multiple promises sequentially

C) Resolves multiple promises concurrently

D) Creates a single promise

21 / 30

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

A) Clears the browser history.

B) Modifies the browser history without reloading the page.

C) Fetches the current page title.

D) Redirects to a new URL.

22 / 30

22. Which HTTP method is idempotent?

A) PUT

B) POST

C) DELETE

D) GET

23 / 30

23. Which of the following statements is true about the 'this' keyword in JavaScript?

A) It refers to the parent object of the current function.

B) It refers to the object that owns the function being executed.

C) It always refers to the global object.

D) It refers to the prototype of the current object.

24 / 30

24. What is the purpose of Object.freeze()?

A) To create a prototype

B) To make an object immutable

C) To inherit properties

D) To delete an object

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25. What is the purpose of the focus event in JavaScript?

A) It triggers when an element gains focus.

B) It triggers when a user types inside an element.

C) It triggers when a mouse hovers over an element.

D) It triggers when an element loses focus.

26 / 30

26. Which of the following is NOT a valid DOM event?

A) hover

B) change

C) mouseover

D) click

27 / 30

27. What does the filter() method do?

A) Filters elements based on a condition

B) Mutates the original array

C) Transforms array elements

D) Removes duplicates from an array

28 / 30

28. What is destructuring in ES6?

A) Copying objects

B) Concatenating arrays

C) Breaking arrays or objects into smaller parts

D) Modifying array elements

29 / 30

29. Which of the following methods removes all child nodes of a DOM element?

A) element.innerHTML = ""

B) element.deleteChildren()

C) element.clear()

D) element.removeChildren()

30 / 30

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

A) Avoid debugging tools.

B) Store source maps in cookies.

C) Use unminified files.

D) Disable source maps in production.

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