JavaScript Quiz Test Online (30 random Questions) 2025

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 Promise.resolve() do?

A) Creates a pending promise

B) Rejects a promise

C) Resolves a pending promise with a given value

D) Waits for a promise to resolve

2 / 30

2. How do you handle errors in the Fetch API?

A) Using a catch() block

B) Using try...catch

C) Using onerror

D) Using then()

3 / 30

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

A) element.innerHTML = ""

B) element.clear()

C) element.deleteChildren()

D) element.removeChildren()

4 / 30

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

A) Avoid debugging tools.

B) Disable source maps in production.

C) Use unminified files.

D) Store source maps in cookies.

5 / 30

5. 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 modify an object's prototype

D) To seal an object

6 / 30

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

A) Use synchronous code for all operations.

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

C) Increase the number of event listeners.

D) Minimize the use of promises.

7 / 30

7. What does the FileReader API do?

A) Reads files from a remote server.

B) Reads files from the local file system.

C) Deletes local files.

D) Writes data to local files.

8 / 30

8. What is the main drawback of callbacks?

A) All of the above

B) Callback hell (nested callbacks)

C) Difficult error handling

D) Poor readability

9 / 30

9. What is method overriding in JavaScript?

A) A subclass providing its own implementation of a method inherited from a parent class

B) Defining multiple methods with the same name

C) Calling a method multiple times

D) Changing the return type of a method

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10. What is the main purpose of console.trace() in debugging?

A) To stop code execution.

B) To display the stack trace of a function call.

C) To measure execution time.

D) To log all variables in scope.

11 / 30

11. What does AJAX stand for?

A) Asynchronous JavaScript and XML

B) Advanced JavaScript and XHTML

C) Asynchronous Java and XML

D) Asynchronous JSON and XML

12 / 30

12. What should you do before deploying a JavaScript application?

A) Minify and compress files.

B) Scan for vulnerabilities using automated tools.

C) All of the above.

D) Perform thorough testing.

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

A) To improve token generation speed.

B) To prevent unauthorized access after a specific time.

C) To reduce server load.

D) To simplify authentication.

14 / 30

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

A) Caching information

B) API keys

C) Non-sensitive data

D) User preferences

15 / 30

15. Which method is used to open a request in XMLHttpRequest?

A) start()

B) beginRequest()

C) open()

D) openRequest()

16 / 30

16. How do you stop event propagation in JavaScript?

A) event.clear()

B) event.preventDefault()

C) event.stopPropagation()

D) event.preventPropagation()

17 / 30

17. Which company developed JavaScript?

A) Sun Microsystems

B) Microsoft

C) Google

D) Netscape

18 / 30

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

A) To restrict cookies to HTTPS connections only.

B) To allow cookies to be accessed over HTTP.

C) To prevent cookies from expiring.

D) To encrypt cookie data.

19 / 30

19. Which keyword is used to define a constant in JavaScript?

A) const

B) let

C) constant

D) var

20 / 30

20. Which HTTP response code indicates a successful request?

A) 500

B) 301

C) 404

D) 200

21 / 30

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

A) The parent object

B) The global object

C) Undefined

D) The object that owns the method

22 / 30

22. What is Cross-Site Scripting (XSS)?

A) A technique to improve web application performance.

B) A technique for caching web pages.

C) A method for secure data encryption.

D) An attack where malicious scripts are injected into web pages.

23 / 30

23. What is the output of console.log(1 + "2" + 3);?

A) 6

B) "6"

C) 33

D) "123"

24 / 30

24. What is the difference between == and === in JavaScript?

A) == checks value only, === checks both value and type.

B) === checks value only, == checks both value and type.

C) Both compare only values.

D) Both compare only types.

25 / 30

25. 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.

26 / 30

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

A) setTimeout()

B) executeLater()

C) setRepeater()

D) setInterval()

27 / 30

27. Which method is used to store data in the browser's local storage?

A) sessionStorage.setItem()

B) storeData()

C) localStorage.setItem()

D) browserData.set()

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

A) Both B and C

B) let newArr = oldArr;

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

D) let newArr = oldArr.slice();

29 / 30

29. What does the DOMContentLoaded event signify?

A) The DOM is fully constructed.

B) The document and CSS are loaded but not images.

C) The document and all resources (images, CSS, etc.) are loaded.

D) The DOM is partially constructed.

30 / 30

30. What is a class in JavaScript?

A) A template for creating objects

B) An array-like structure

C) A function that returns an object

D) A built-in data type

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