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. What does 'typeof' operator do in JavaScript?

A) Converts one type to another.

B) Checks variable type and compares it.

C) Checks if a variable exists.

D) Checks the data type of a value.

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2. Which API is used to make WebSocket connections?

A) Ajax

B) WebSocket

C) fetch()

D) HTTP

3 / 30

3. How do you detect which key was pressed during a keydown event?

A) All of the above

B) event.code

C) event.which

D) event.key

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4. What is Cross-Site Request Forgery (CSRF)?

A) A type of browser exploit.

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

C) An attack that steals cookies.

D) A vulnerability in JavaScript frameworks.

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5. How can you detect if an element has focus?

A) element.isFocused()

B) document.activeElement === element

C) document.hasFocus()

D) element.focused

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6. What is the purpose of Promise.all()?

A) Resolves multiple promises sequentially

B) Rejects all promises if one fails

C) Resolves multiple promises concurrently

D) Creates a single promise

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7. What does the IntersectionObserver API do?

A) Observes changes to DOM nodes.

B) Tracks visibility of elements within the viewport.

C) Observes HTTP requests.

D) Monitors resizing of elements.

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

A) The global object

B) Undefined

C) The parent object

D) The object that owns the method

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9. What is JSONP in the context of AJAX?

A) A modern AJAX API

B) A JSON parser for browsers

C) A replacement for XMLHttpRequest

D) A method to fetch JSON data from other domains

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10. What does the DOMContentLoaded event signify?

A) The DOM is fully constructed.

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

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

D) The DOM is partially constructed.

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11. What is the output of console.log(typeof null);?

A) "undefined"

B) "NaN"

C) "object"

D) "null"

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

A) localStorage.setItem()

B) browserData.set()

C) storeData()

D) sessionStorage.setItem()

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

A) Minimize the use of promises.

B) Use synchronous code for all operations.

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

D) Increase the number of event listeners.

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14. What is inheritance in JavaScript?

A) Defining methods in an object

B) A class receiving properties and methods from another class

C) Using multiple objects together

D) Copying one object to another

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15. How can you make a function return a promise in ES6?

A) By wrapping the return value in Promise.resolve()

B) By using await inside the function

C) By adding async before the function name

D) By calling .then() inside the function

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

A) Closes the current window.

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

C) Opens a new browser tab or window.

D) Reloads the current page.

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17. Which HTTP response code indicates a successful request?

A) 404

B) 301

C) 200

D) 500

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18. What is the result of console.log(typeof NaN);?

A) "undefined"

B) "object"

C) "NaN"

D) "number"

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19. How does Object.entries() work in ES6?

A) Merges two objects

B) Converts an object into a string

C) Creates an object clone

D) Returns an array of key-value pairs from an object

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20. Which of the following is not a valid JavaScript variable name?

A) name2

B) 2name

C) _name

D) $name

21 / 30

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

A) Filters elements based on a condition

B) Sorts the array

C) Transforms each array element and returns a new array

D) Mutates the original array

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22. What is a class in JavaScript?

A) A function that returns an object

B) An array-like structure

C) A built-in data type

D) A template for creating objects

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23. Which is NOT true about Promise.finally()?

A) It does not modify the resolved value.

B) It executes after the promise is settled.

C) It runs only on a rejected promise.

D) It can be used for cleanup.

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24. Which method is used to trigger a custom event?

A) fireEvent()

B) dispatchEvent()

C) triggerEvent()

D) emitEvent()

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25. What is the primary purpose of unit testing?

A) To test individual components or functions.

B) To identify security vulnerabilities.

C) To measure application performance.

D) To test the entire application workflow.

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26. How do you attach a click event to a button in JavaScript?

A) button.attach("click", function() { ... })

B) button.addEventListener("click", function() { ... })

C) button.onClick = function() { ... }

D) Both A and B

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27. 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.

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

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

B) To pause code execution.

C) To profile memory usage.

D) To display the current date and time.

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

A) element.deleteChildren()

B) element.removeChildren()

C) element.clear()

D) element.innerHTML = ""

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30. Which JavaScript method can help prevent XSS by escaping HTML?

A) clean

B) encode

C) sanitize

D) escape

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