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. How can you optimize the rendering performance of a web application?

A) Increase CSS file size.

B) Minimize DOM manipulations and reflows.

C) Use synchronous JavaScript everywhere.

D) Disable browser caching.

2 / 30

2. What does the history.pushState() method do?

A) Redirects to a new URL.

B) Fetches the current page title.

C) Modifies the browser history without reloading the page.

D) Clears the browser history.

3 / 30

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

A) Checks if a string contains a substring

B) All of the above

C) Works on both arrays and strings

D) Checks if an array contains a specified value

4 / 30

4. Which method is used to remove the last element from an array?

A) shift()

B) splice()

C) remove()

D) pop()

5 / 30

5. What does the static keyword do in a class?

A) Creates a constant variable

B) Defines a method accessible only on the class itself, not instances

C) Creates an instance of a class

D) Makes a property immutable

6 / 30

6. Which of the following best describes load testing?

A) Testing individual components in isolation.

B) Debugging memory leaks in the code.

C) Measuring the application's performance under high user load.

D) Testing an application for security vulnerabilities.

7 / 30

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

A) appendHeader()

B) setRequestHeader()

C) addHeader()

D) customHeader()

8 / 30

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

A) @

B) #

C) private keyword

D) _

9 / 30

9. What does setTimeout() do in JavaScript?

A) Stops code execution for a set time

B) Executes code repeatedly

C) Executes a function after a delay

D) Executes a function immediately

10 / 30

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

A) addEventListener()

B) addListener()

C) registerEvent()

D) attachListener()

11 / 30

11. What is the purpose of the AbortController API in JavaScript?

A) To stop animations on a webpage.

B) To cancel asynchronous operations like fetch().

C) To clear browser history.

D) To halt DOM rendering.

12 / 30

12. What does querySelector() do?

A) Selects elements by ID only.

B) Selects elements by class only.

C) Selects the first matching element.

D) Selects all elements of a given type.

13 / 30

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

A) element.onEvent()

B) element.bindEvent()

C) element.addEventListener()

D) element.attachEvent()

14 / 30

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

A) splice()

B) push()

C) pop()

D) unshift()

15 / 30

15. How can you write a multi-line comment in JavaScript?

A) /* Comment */

B) /* Comment // Comment */

C) // Comment

D)

16 / 30

16. What is the purpose of the timeout property in XMLHttpRequest?

A) To specify the time before the request is automatically aborted

B) To log the request duration

C) To control the server's response time

D) To delay the request

17 / 30

17. What is the purpose of reduce() in JavaScript?

A) Reduces array size

B) Concatenates arrays

C) Filters elements

D) Executes a reducer function on each element, resulting in a single output

18 / 30

18. What is the default HTTP method used by AJAX?

A) PUT

B) DELETE

C) POST

D) GET

19 / 30

19. Which method can be used to find the length of a string in JavaScript?

A) str.length()

B) str.length

C) str.size

D) str.size()

20 / 30

20. What does the fetch() method do?

A) Creates DOM elements

B) Executes functions asynchronously

C) Sends HTTP requests and returns a promise

D) Fetches variables from memory

21 / 30

21. How can you remove an item from localStorage?

A) localStorage.eraseItem()

B) localStorage.removeItem()

C) localStorage.delete()

D) localStorage.clearItem()

22 / 30

22. What is the use of arrow functions in ES6?

A) All of the above

B) Declares functions with lexical this binding

C) Declares functions with a simpler syntax

D) Supports this context binding

23 / 30

23. What does the navigator.geolocation.getCurrentPosition() method do?

A) Fetches the current time.

B) Updates the user's address.

C) Sets the user's location.

D) Retrieves the user's current location.

24 / 30

24. What is polymorphism in JavaScript?

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

B) Creating multiple instances of a class

C) None of the above

D) Inheriting properties from multiple classes

25 / 30

25. How do you declare a variable in JavaScript?

A) let

B) const

C) All of the above

D) var

26 / 30

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

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

B) Closes the current window.

C) Reloads the current page.

D) Opens a new browser tab or window.

27 / 30

27. Which of the following is NOT a valid DOM event?

A) change

B) mouseover

C) click

D) hover

28 / 30

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

A) They might contain vulnerabilities or malicious code.

B) They reduce code size.

C) They always introduce compatibility issues.

D) They are harder to debug.

29 / 30

29. What is the responseType property used for in XMLHttpRequest?

A) To set the request headers

B) To define the status code of the response

C) To define the method used in the request

D) To define the format of the response

30 / 30

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

A) To prevent SQL Injection attacks.

B) To reduce code execution time.

C) To simplify database queries.

D) To improve code readability.

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