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 an object in JavaScript?

A) A collection of key-value pairs

B) A special type of function

C) None of the above

D) A built-in data type

2 / 30

2. What does the target property of an event object return?

A) The element that triggered the event.

B) The type of the event.

C) The default action of the event.

D) The parent element of the event.

3 / 30

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

A) Disable browser caching.

B) Increase CSS file size.

C) Minimize DOM manipulations and reflows.

D) Use synchronous JavaScript everywhere.

4 / 30

4. What does the performance.now() method return?

A) The time since the browser was opened.

B) The user's system time.

C) The time since the page started loading.

D) The current time in milliseconds.

5 / 30

5. Which method is used to create a shallow copy of an array in ES6?

A) [...array]

B) Array.prototype.copy()

C) Array.clone()

D) Array.copy()

6 / 30

6. Which type of data should never be stored in client-side storage like localStorage?

A) Non-sensitive data

B) Caching information

C) User preferences

D) API keys

7 / 30

7. Which keyword is used to define a class in JavaScript?

A) class

B) object

C) define

D) new

8 / 30

8. Which of the following is not a valid JavaScript variable name?

A) _name

B) name2

C) 2name

D) $name

9 / 30

9. What does the fetch() method do?

A) Sends HTTP requests and returns a promise

B) Creates DOM elements

C) Fetches variables from memory

D) Executes functions asynchronously

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10. Which HTTP method is idempotent?

A) PUT

B) DELETE

C) POST

D) GET

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11. How do you add a new attribute to an element in JavaScript?

A) element.setAttribute(name, value)

B) element.addAttribute(name, value)

C) element.createAttribute(name, value)

D) element.attribute(name, value)

12 / 30

12. How can you write a comment in JavaScript?

A) /* This is a comment */

B) # This is a comment

C) /$ This is a comment $/

D)

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13. What is the difference between stopPropagation() and preventDefault()?

A) There is no difference.

B) stopPropagation() stops event bubbling, while preventDefault() stops the default action.

C) Both stop the event from propagating.

D) Both prevent the default behavior.

14 / 30

14. What does the includes() method do in ES6?

A) Works on both arrays and strings

B) All of the above

C) Checks if a string contains a substring

D) Checks if an array contains a specified value

15 / 30

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

A) It triggers when an element loses 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 gains focus.

16 / 30

16. Why should eval() be avoided in JavaScript?

A) It is outdated.

B) It reduces performance.

C) It doesn't work with modern browsers.

D) It can introduce security vulnerabilities like XSS.

17 / 30

17. What is the recommended way to validate user input?

A) hash

B) validate

C) parse

D) sanitize

18 / 30

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

A) Use window.createAnimation() method.

B) Use animation.create() method.

C) Use CSS.animate() method.

D) Use element.animate() method.

19 / 30

19. What is the purpose of the onreadystatechange event?

A) To log errors in the request

B) To initialize the AJAX request

C) To set request headers

D) To handle state changes in the XMLHttpRequest

20 / 30

20. What is the primary purpose of Content Security Policy (CSP)?

A) To enforce HTTPS.

B) To prevent unauthorized file downloads.

C) To mitigate XSS attacks by controlling resources a browser can load.

D) To restrict API usage in JavaScript.

21 / 30

21. Which of the following helps identify rendering performance issues?

A) Heap snapshots

B) Lighthouse

C) Call stack

D) Console tab

22 / 30

22. What is Cross-Site Request Forgery (CSRF)?

A) An attack where unauthorized commands are executed on behalf of an authenticated user.

B) An attack that steals cookies.

C) A type of browser exploit.

D) A vulnerability in JavaScript frameworks.

23 / 30

23. How do you declare a variable in JavaScript?

A) All of the above

B) var

C) let

D) const

24 / 30

24. JavaScript is ________.

A) Case-insensitive

B) Not case-sensitive for variables

C) Case-sensitive for keywords only

D) Case-sensitive

25 / 30

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

A) statusCode

B) readyState

C) responseCode

D) status

26 / 30

26. How can you write a multi-line comment in JavaScript?

A) /* Comment // Comment */

B) /* Comment */

C) // Comment

D)

27 / 30

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

A) Using const instead of var.

B) Event listeners not being removed.

C) Using arrow functions.

D) Unreferenced closures.

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28. What does the map() method do in functional programming?

A) Filters elements based on a condition

B) Sorts the array

C) Transforms each array element and returns a new array

D) Mutates the original array

29 / 30

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

A) "object"

B) "number"

C) "NaN"

D) "undefined"

30 / 30

30. What does HTTPS provide for web applications?

A) Automatic code optimization.

B) Faster loading speed.

C) A debugging tool for developers.

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

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