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.

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

A) Using a catch() block

B) Using try...catch

C) Using then()

D) Using onerror

2 / 30

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

A) Use the oneTimeEvent() function.

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

C) Use the triggerOnce() method.

D) Use the once option in addEventListener().

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3. How do you toggle a class on an element?

A) element.classList.toggle(className)

B) element.switchClass(className)

C) element.toggle(className)

D) element.classToggle(className)

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4. What does the HttpOnly flag on cookies do?

A) Ensures cookies can only be accessed by JavaScript.

B) Encrypts cookie data.

C) Extends the lifespan of cookies.

D) Prevents cookies from being accessed by client-side scripts.

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5. What does Object.create() do?

A) Creates a new object with the specified prototype

B) Copies an object

C) Returns a stringified version of an object

D) Creates a deep clone of an object

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

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

B) To restrict API usage in JavaScript.

C) To prevent unauthorized file downloads.

D) To enforce HTTPS.

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

A) All of the above

B) Checks if a string contains a substring

C) Works on both arrays and strings

D) Checks if an array contains a specified value

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

A) To inherit another class

B) To define methods

C) To initialize properties when an object is created

D) To destroy an object

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9. Which company developed JavaScript?

A) Netscape

B) Google

C) Sun Microsystems

D) Microsoft

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10. What does the IntersectionObserver API do?

A) Monitors resizing of elements.

B) Observes changes to DOM nodes.

C) Tracks visibility of elements within the viewport.

D) Observes HTTP requests.

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

A) class

B) object

C) new

D) define

12 / 30

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

A) element.attachEvent()

B) element.bindEvent()

C) element.addEventListener()

D) element.onEvent()

13 / 30

13. 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()

14 / 30

14. What does the Notification API do?

A) Updates the webpage title.

B) Tracks the user's activity.

C) Sends notifications to the user's email.

D) Displays browser notifications.

15 / 30

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

A) base();

B) parent();

C) super();

D) constructor();

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16. How can you monitor memory usage in a web application?

A) By writing custom JavaScript functions.

B) Using the Memory tab in DevTools.

C) Using the Console tab in DevTools.

D) By enabling verbose logging.

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17. Which DevTools feature helps analyze how long each function takes to execute?

A) Network tab

B) Console

C) Application tab

D) Performance tab

18 / 30

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

A) 2

B) 1

C) 3

D) 4

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19. What does the DOMContentLoaded event signify?

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

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

C) The DOM is fully constructed.

D) The DOM is partially constructed.

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20. How do you set a custom HTTP header in an AJAX request?

A) customHeader()

B) setRequestHeader()

C) addHeader()

D) appendHeader()

21 / 30

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

A) The parent object

B) Undefined

C) The global object

D) The object that owns the method

22 / 30

22. What does the FileReader API do?

A) Reads files from a remote server.

B) Writes data to local files.

C) Deletes local files.

D) Reads files from the local file system.

23 / 30

23. What is destructuring in ES6?

A) Concatenating arrays

B) Copying objects

C) Modifying array elements

D) Breaking arrays or objects into smaller parts

24 / 30

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

A) document.generateElement()

B) document.createElement()

C) document.addElement()

D) document.newElement()

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25. What is the purpose of the performance.now() method?

A) To profile memory usage.

B) To pause code execution.

C) To measure code execution time in milliseconds with high precision.

D) To display the current date and time.

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26. What is the purpose of the abort() method in AJAX?

A) Closes the connection

B) Resends the request

C) Cancels an ongoing request

D) Logs errors in the request

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

A) Using the Promise constructor

B) Using Promise.race()

C) Using await

D) Using async

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28. Which method is used to test asynchronous code in Jest?

A) wait()

B) testAsync()

C) done()

D) expectAsync()

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29. Which operator is used to compare both value and type in JavaScript?

A) !=

B) ===

C) !==

D) ==

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30. What is a secure way to store sensitive data in the browser?

A) Plain JavaScript variables

B) Cookies without flags

C) Encrypted cookies with HttpOnly and Secure flags

D) Local storage

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