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

A) Use the triggerOnce() method.

B) Use the oneTimeEvent() function.

C) Use the once option in addEventListener().

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

2 / 30

2. Which of the following best describes load testing?

A) Testing individual components in isolation.

B) Measuring the application's performance under high user load.

C) Testing an application for security vulnerabilities.

D) Debugging memory leaks in the code.

3 / 30

3. What is a Same-Origin Policy (SOP)?

A) A method for improving performance.

B) A policy restricting how scripts interact with content from different origins.

C) A rule to prevent HTTP requests.

D) A browser feature to cache data.

4 / 30

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

A) Modifies the browser history without reloading the page.

B) Clears the browser history.

C) Redirects to a new URL.

D) Fetches the current page title.

5 / 30

5. How do you create a new promise?

A) new Promise()

B) new Async()

C) Promise.create()

D) Promise.start()

6 / 30

6. Which of the following is used to declare a variable in JavaScript?

A) All of the above

B) const

C) let

D) var

7 / 30

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

A) Using async

B) Using the Promise constructor

C) Using await

D) Using Promise.race()

8 / 30

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

A) document.newElement()

B) document.addElement()

C) document.generateElement()

D) document.createElement()

9 / 30

9. What does HTTPS provide for web applications?

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

B) Automatic code optimization.

C) Faster loading speed.

D) A debugging tool for developers.

10 / 30

10. What is the purpose of minifying JavaScript files?

A) To improve code readability.

B) To detect syntax errors.

C) To reduce file size for faster loading.

D) To add comments to the code.

11 / 30

11. Which readyState value indicates that the request is complete?

A) 1

B) 3

C) 4

D) 2

12 / 30

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

A) element.onEvent()

B) element.attachEvent()

C) element.addEventListener()

D) element.bindEvent()

13 / 30

13. Which storage type is most vulnerable to XSS attacks?

A) cookies

B) sessionStorage

C) cache

D) localStorage

14 / 30

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

A) "number"

B) "object"

C) "NaN"

D) "undefined"

15 / 30

15. Which company developed JavaScript?

A) Google

B) Microsoft

C) Sun Microsystems

D) Netscape

16 / 30

16. How do you ensure a function runs only after a specific promise is resolved?

A) Use setTimeout.

B) Use a synchronous loop.

C) Use Promise.reject.

D) Use .then() or await.

17 / 30

17. What is the purpose of the splice() method?

A) None of the above.

B) To remove and/or add elements to an array.

C) To remove elements from an array.

D) To add elements to an array.

18 / 30

18. Which method adds an event listener to an element?

A) attachListener()

B) registerEvent()

C) addEventListener()

D) addListener()

19 / 30

19. 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

20 / 30

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

A) Array.prototype.copy()

B) Array.copy()

C) [...array]

D) Array.clone()

21 / 30

21. Which JavaScript method can help prevent XSS by escaping HTML?

A) sanitize

B) clean

C) encode

D) escape

22 / 30

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

A) MediaStream API

B) Web Animations API

C) Performance API

D) Event Loop API

23 / 30

23. How can you make a function return a promise in ES6?

A) By calling .then() inside the function

B) By adding async before the function name

C) By using await inside the function

D) By wrapping the return value in Promise.resolve()

24 / 30

24. JavaScript is ________.

A) Case-sensitive for keywords only

B) Case-sensitive

C) Not case-sensitive for variables

D) Case-insensitive

25 / 30

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

A) Boolean

B) Number

C) String

D) Float

26 / 30

26. Which of these is an example of an asynchronous task in AJAX?

A) File download

B) Form submission

C) Data fetching from a server

D) Page reload

27 / 30

27. What is lazy loading in performance optimization?

A) Minimizing HTTP requests.

B) Loading all assets upfront.

C) Preloading all images in the background.

D) Delaying loading of resources until they are needed.

28 / 30

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

A) text/plain

B) multipart/form-data

C) application/json

D) application/x-www-form-urlencoded

29 / 30

29. How do you declare a variable in JavaScript?

A) const

B) let

C) All of the above

D) var

30 / 30

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

A) To block all external scripts.

B) To prevent HTTP requests.

C) To allow specific inline scripts to execute securely.

D) To encrypt inline scripts.

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