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

What does the window.open() method do?

A) Opens a new browser tab or window.

B) Navigates to a new URL in the same window.

C) Closes the current window.

D) Reloads the current page.

2 / 30

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

A) Page reload

B) Data fetching from a server

C) Form submission

D) File download

3 / 30

What is Cross-Site Request Forgery (CSRF)?

A) An attack that steals cookies.

B) A vulnerability in JavaScript frameworks.

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

D) A type of browser exploit.

4 / 30

What is the purpose of Promise.race()?

A) Resolves with the fastest promise

B) Resolves with all promises' results

C) Rejects all promises if one fails

D) Pauses until all promises are complete

5 / 30

How can you detect when the DOM is fully loaded?

A) Using the DOMContentLoaded event on the document object.

B) Using the onready event.

C) Using the load event on the window object.

D) Using the readyState property.

6 / 30

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.

7 / 30

Which symbol is used to create private class fields in ES6+?

A) private keyword

B) #

C) @

D) _

8 / 30

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

A) setRequestHeader()

B) appendHeader()

C) customHeader()

D) addHeader()

9 / 30

How can developers avoid exposing sensitive information in source maps?

A) Use unminified files.

B) Disable source maps in production.

C) Store source maps in cookies.

D) Avoid debugging tools.

10 / 30

How can you determine the type of device being used?

A) Using the window.deviceType property.

B) Using the screen.type property.

C) Using the device.check() method.

D) Using the navigator.userAgent property.

11 / 30

What should you do before deploying a JavaScript application?

A) All of the above.

B) Scan for vulnerabilities using automated tools.

C) Perform thorough testing.

D) Minify and compress files.

12 / 30

What does HTTPS provide for web applications?

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

B) A debugging tool for developers.

C) Faster loading speed.

D) Automatic code optimization.

13 / 30

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

A) There is no difference.

B) Both stop the event from propagating.

C) Both prevent the default behavior.

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

14 / 30

What is asynchronous JavaScript?

A) Code executed only after page load

B) Code executed in parallel without blocking

C) Code executed line by line in order

D) Code executed without using variables

15 / 30

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

A) Use setTimeout.

B) Use .then() or await.

C) Use Promise.reject.

D) Use a synchronous loop.

16 / 30

Which method is used to test asynchronous code in Jest?

A) expectAsync()

B) testAsync()

C) wait()

D) done()

17 / 30

How do you stop event propagation in JavaScript?

A) event.preventDefault()

B) event.clear()

C) event.stopPropagation()

D) event.preventPropagation()

18 / 30

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

A) super();

B) base();

C) parent();

D) constructor();

19 / 30

How can you monitor changes to the size of an element?

A) Using the MutationObserver API.

B) Using the IntersectionObserver API.

C) Using the performance.now() method.

D) Using the ResizeObserver API.

20 / 30

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

A) To prevent syntax errors.

B) To improve application performance.

C) To log error messages to the console.

D) To catch and handle runtime errors gracefully.

21 / 30

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

A) Filters elements based on a condition

B) Mutates the original array

C) Transforms each array element and returns a new array

D) Sorts the array

22 / 30

What does the window.location object represent?

A) The browser's history.

B) The browser's settings.

C) The user's geolocation.

D) The URL of the current document.

23 / 30

What does the credentials option in the Fetch API specify?

A) Redirect behavior

B) Request timeout

C) Include cookies in requests

D) Cache policy

24 / 30

Which operator is used to check for inequality in JavaScript?

A) !==

B) !=

C) None of the above

D) Both A and B

25 / 30

What happens when a return statement is used inside a forEach loop?

A) It throws an error.

B) It exits the function containing the loop.

C) The loop stops.

D) It skips to the next iteration.

26 / 30

Which JavaScript method can help prevent XSS by escaping HTML?

A) clean

B) sanitize

C) encode

D) escape

27 / 30

Which symbol is used to assign a value to a variable in JavaScript?

A) =

B) :

C) ===

D) ==

28 / 30

Which property is used to access or change the content of an HTML element?

A) innerHTML

B) innerText

C) outerHTML

D) innerContent

29 / 30

What does the Notification API do?

A) Tracks the user's activity.

B) Updates the webpage title.

C) Displays browser notifications.

D) Sends notifications to the user's email.

30 / 30

Which JavaScript API helps sanitize user inputs?

A) Fetch API

B) JSON.stringify

C) DOMParser

D) Sanitizer API

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