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 encapsulation mean in JavaScript?

A) Removing methods from an object

B) Combining multiple objects into one

C) Hiding data and exposing only the necessary parts of an object

D) Creating a copy of an object

2 / 30

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

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

B) To enforce HTTPS.

C) To prevent unauthorized file downloads.

D) To restrict API usage in JavaScript.

3 / 30

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

A) appendHeader()

B) addHeader()

C) customHeader()

D) setRequestHeader()

4 / 30

4. What does the window.open() method do?

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

B) Closes the current window.

C) Opens a new browser tab or window.

D) Reloads the current page.

5 / 30

5. Which feature allows default values in functions?

A) Arrow functions

B) Spread operator

C) Default parameters

D) Rest parameters

6 / 30

6. What does AJAX stand for?

A) Asynchronous Java and XML

B) Advanced JavaScript and XHTML

C) Asynchronous JavaScript and XML

D) Asynchronous JSON and XML

7 / 30

7. What does the HttpOnly flag on cookies do?

A) Ensures cookies can only be accessed by JavaScript.

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

C) Extends the lifespan of cookies.

D) Encrypts cookie data.

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8. Which method is used to add an element to the end of an array?

A) pop()

B) splice()

C) push()

D) unshift()

9 / 30

9. What is lazy loading in performance optimization?

A) Loading all assets upfront.

B) Delaying loading of resources until they are needed.

C) Minimizing HTTP requests.

D) Preloading all images in the background.

10 / 30

10. What is JavaScript primarily used for?

A) Writing system-level software

B) Adding interactivity to web pages

C) Server-side programming

D) Designing static websites

11 / 30

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

A) Using the MutationObserver API.

B) Using the ResizeObserver API.

C) Using the IntersectionObserver API.

D) Using the performance.now() method.

12 / 30

12. Which company developed JavaScript?

A) Netscape

B) Sun Microsystems

C) Google

D) Microsoft

13 / 30

13. What is the purpose of minifying JavaScript files?

A) To add comments to the code.

B) To detect syntax errors.

C) To improve code readability.

D) To reduce file size for faster loading.

14 / 30

14. What will the following code do? document.getElementById("myDiv").style.color = "red";

A) Do nothing.

B) Change the text color of myDiv to red.

C) Change the background color of myDiv to red.

D) Throw an error.

15 / 30

15. How do you check if a property exists in an object?

A) property in obj

B) obj.hasOwnProperty(property)

C) Both A and C

D) obj.property

16 / 30

16. Which HTTP method is idempotent?

A) GET

B) POST

C) DELETE

D) PUT

17 / 30

17. What is the main drawback of callbacks?

A) Difficult error handling

B) All of the above

C) Poor readability

D) Callback hell (nested callbacks)

18 / 30

18. 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.

19 / 30

19. How can you remove an item from localStorage?

A) localStorage.eraseItem()

B) localStorage.removeItem()

C) localStorage.clearItem()

D) localStorage.delete()

20 / 30

20. What does the Notification API do?

A) Updates the webpage title.

B) Displays browser notifications.

C) Tracks the user's activity.

D) Sends notifications to the user's email.

21 / 30

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

A) Array.clone()

B) Array.prototype.copy()

C) Array.copy()

D) [...array]

22 / 30

22. What is polymorphism in JavaScript?

A) Creating multiple instances of a class

B) Inheriting properties from multiple classes

C) None of the above

D) Having multiple methods with the same name but different implementations

23 / 30

23. What does the Promise constructor accept?

A) Another promise

B) An array of values

C) A callback function with resolve and reject parameters

D) A string argument

24 / 30

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

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

B) Use the once option in addEventListener().

C) Use the oneTimeEvent() function.

D) Use the triggerOnce() method.

25 / 30

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

A) Prevents the default action of the event.

B) Stops the event from propagating.

C) Removes the event listener.

D) Stops the execution of the event handler.

26 / 30

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

A) addListener()

B) addEventListener()

C) registerEvent()

D) attachListener()

27 / 30

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

A) performance.now()

B) console.log()

C) debugger

D) console.time() and console.timeEnd()

28 / 30

28. How do you check the browser's viewport width in JavaScript?

A) document.body.width

B) screen.width

C) window.innerWidth

D) viewport.size

29 / 30

29. How can you prevent clickjacking attacks?

A) Use HTTPS.

B) Use the X-Frame-Options header.

C) Avoid using iframes.

D) Use strong passwords.

30 / 30

30. Which of these is NOT asynchronous in JavaScript?

A) fetch()

B) setTimeout()

C) Promise.all()

D) for loop

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