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 the crypto.randomUUID() method do?

A) Decrypts a string.

B) Creates a random number.

C) Generates a unique identifier.

D) Encrypts a string.

2 / 30

2. How can you handle both the response and errors in the Fetch API?

A) Using catch() only

B) Using finally()

C) Using then() and catch()

D) Using then() only

3 / 30

3. How can you avoid blocking the event loop in JavaScript?

A) Increase the number of event listeners.

B) Split large tasks into smaller chunks and use setTimeout().

C) Minimize the use of promises.

D) Use synchronous code for all operations.

4 / 30

4. How can you detect if an element has focus?

A) element.isFocused()

B) document.activeElement === element

C) element.focused

D) document.hasFocus()

5 / 30

5. What is JSONP in the context of AJAX?

A) A replacement for XMLHttpRequest

B) A JSON parser for browsers

C) A modern AJAX API

D) A method to fetch JSON data from other domains

6 / 30

6. Which of the following methods is used to convert a JavaScript object to JSON format?

A) JSON.parse()

B) JSON.toJSON()

C) JSON.stringify()

D) JSON.convert()

7 / 30

7. Which company developed JavaScript?

A) Microsoft

B) Google

C) Sun Microsystems

D) Netscape

8 / 30

8. What is JavaScript primarily used for?

A) Designing static websites

B) Writing system-level software

C) Server-side programming

D) Adding interactivity to web pages

9 / 30

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

A) Using _ as a prefix

B) Using const

C) Using the private keyword

D) Using # as a prefix

10 / 30

10. What is the default value of a declared variable that has not been assigned a value?

A) undefined

B) 0

C) null

D) NaN

11 / 30

11. What is the purpose of minifying JavaScript files?

A) To improve code readability.

B) To detect syntax errors.

C) To add comments to the code.

D) To reduce file size for faster loading.

12 / 30

12. How do you detect which key was pressed during a keydown event?

A) event.code

B) event.key

C) event.which

D) All of the above

13 / 30

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

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

B) A method for improving performance.

C) A rule to prevent HTTP requests.

D) A browser feature to cache data.

14 / 30

14. Why should third-party libraries be used cautiously?

A) They always introduce compatibility issues.

B) They might contain vulnerabilities or malicious code.

C) They reduce code size.

D) They are harder to debug.

15 / 30

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

A) element.attachEvent()

B) element.onEvent()

C) element.bindEvent()

D) element.addEventListener()

16 / 30

16. How do you handle errors in an async function?

A) Using try...catch

B) Using catch

C) Using finally

D) None of the above

17 / 30

17. What does the this keyword represent inside an object method?

A) The parent object

B) Undefined

C) The object that owns the method

D) The global object

18 / 30

18. What is the purpose of using try...catch blocks in debugging?

A) To improve application performance.

B) To log error messages to the console.

C) To catch and handle runtime errors gracefully.

D) To prevent syntax errors.

19 / 30

19. What does the instanceof operator do in JavaScript?

A) Checks if a class has been inherited

B) Defines a property in a class

C) Creates an instance of a class

D) Checks if an object was created by a specific class

20 / 30

20. What does the preventDefault() method do in an event handler?

A) Removes the event listener.

B) Prevents the default action of the event.

C) Stops the event from propagating.

D) Stops the execution of the event handler.

21 / 30

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

A) JSON.parse()

B) JSON.read()

C) JSON.stringify()

D) JSON.convert()

22 / 30

22. How do you stop event propagation in JavaScript?

A) event.preventPropagation()

B) event.stopPropagation()

C) event.preventDefault()

D) event.clear()

23 / 30

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

A) Creates a single promise

B) Resolves multiple promises sequentially

C) Rejects all promises if one fails

D) Resolves multiple promises concurrently

24 / 30

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

A) define

B) object

C) class

D) new

25 / 30

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

A) To prevent SQL Injection attacks.

B) To simplify database queries.

C) To reduce code execution time.

D) To improve code readability.

26 / 30

26. What is the primary use of strict mode in JavaScript?

A) To enforce HTTPS.

B) To optimize performance.

C) To prevent API misuse.

D) To enhance security by catching common coding errors.

27 / 30

27. Why is it important to set an expiration time for JWTs (JSON Web Tokens)?

A) To improve token generation speed.

B) To simplify authentication.

C) To reduce server load.

D) To prevent unauthorized access after a specific time.

28 / 30

28. How does Object.entries() work in ES6?

A) Converts an object into a string

B) Returns an array of key-value pairs from an object

C) Creates an object clone

D) Merges two objects

29 / 30

29. What is the primary purpose of unit testing?

A) To test the entire application workflow.

B) To test individual components or functions.

C) To measure application performance.

D) To identify security vulnerabilities.

30 / 30

30. Which method in Chrome DevTools helps measure script execution time?

A) performance.now()

B) debugger

C) console.time() and console.timeEnd()

D) console.log()

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