JavaScript MCQs

Question 1

What is the difference between 'let' and 'var' in JavaScript?

Accepted Answer

'let' is block-scoped, while 'var' is function-scoped

Answer

'var' is block-scoped, while 'let' is function-scoped

Answer

Both keywords behave identically in every situation

Answer

'let' can only be used for numbers, not other data types

Question 2

What is a closure in JavaScript?

Accepted Answer

A function that retains access to variables from its outer scope even after that outer function has finished executing

Answer

A function that can never access variables outside itself

Answer

A method used to close a browser tab programmatically

Answer

A special type of loop used for iterating over arrays

Question 3

What is the difference between '==' and '===' in JavaScript?

Accepted Answer

'===' checks both value and type without converting types, while '==' converts types before comparing

Answer

'==' checks both value and type, while '===' converts types first

Answer

Both operators behave identically in every case

Answer

'===' can only be used to compare strings

Question 4

What is 'hoisting' in JavaScript?

Accepted Answer

A behavior where variable and function declarations are moved to the top of their scope during compilation, before code execution

Answer

A technique used to speed up loops

Answer

A method for lifting elements in the DOM visually

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A way to permanently delete unused variables

Question 5

What does the JavaScript 'this' keyword refer to inside a regular function called as a method of an object?

Accepted Answer

The object the method was called on

Answer

The global window object in every case

Answer

The function itself

Answer

The parent scope of the function

Question 6

What is a Promise in JavaScript used for?

Accepted Answer

To represent the eventual completion or failure of an asynchronous operation

Answer

To permanently store data in the browser

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To define a new class in JavaScript

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To validate a form before submission

Question 7

What does the JavaScript array method 'map()' do?

Accepted Answer

Creates a new array by applying a given function to every element of the original array

Answer

Removes elements from an array that do not match a condition

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Sorts an array in ascending order

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Combines all elements of an array into a single value

Question 8

What is the purpose of the JavaScript 'filter()' array method?

Accepted Answer

To create a new array containing only the elements that satisfy a given condition

Answer

To remove all elements from an array

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To sort array elements based on a custom comparator

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To combine two arrays into one

Question 9

What is the difference between 'null' and 'undefined' in JavaScript?

Accepted Answer

'undefined' means a variable has been declared but not yet assigned a value, while 'null' represents an intentional absence of value

Answer

'null' means a variable has not been declared at all

Answer

Both values behave identically in every comparison

Answer

'undefined' can only be used with numbers

Question 10

What is the JavaScript event loop responsible for?

Accepted Answer

Managing the execution of asynchronous callbacks by coordinating the call stack, task queue, and microtask queue

Answer

Automatically refreshing a webpage every few seconds

Answer

Detecting syntax errors before code runs

Answer

Compiling JavaScript code into machine language

Question 11

What does an arrow function in JavaScript do differently regarding 'this'?

Accepted Answer

It does not have its own 'this' binding and instead inherits 'this' from its surrounding lexical scope

Answer

It always binds 'this' to the global window object

Answer

It requires 'this' to be passed explicitly as an argument

Answer

It behaves identically to a regular function in every case

Question 12

What is the purpose of JSON.stringify() in JavaScript?

Accepted Answer

To convert a JavaScript object into a JSON formatted string

Answer

To convert a JSON string into a JavaScript object

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To validate whether a string is properly formatted JSON

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To remove whitespace from a string

Question 13

What is the reverse operation of JSON.stringify()?

Accepted Answer

JSON.parse()

Answer

JSON.decode()

Answer

JSON.reverse()

Answer

JSON.convert()

Question 14

What does the spread operator (...) allow you to do in JavaScript?

Accepted Answer

Expand an iterable, such as an array, into individual elements, for example when copying or merging arrays

Answer

Combine multiple functions into a single function

Answer

Permanently freeze an object so it cannot be changed

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Convert a string into a number

Question 15

What is event bubbling in the context of the DOM?

Accepted Answer

A behavior where an event triggered on a nested element also triggers on its parent elements, moving upward through the DOM tree

Answer

A behavior where clicking anywhere on a page refreshes it automatically

Answer

A method for creating new HTML elements dynamically

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A technique for preventing a page from loading images

Question 16

What is the purpose of the JavaScript 'reduce()' array method?

Accepted Answer

To combine all elements of an array into a single accumulated value based on a provided function

Answer

To remove duplicate elements from an array

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To reverse the order of elements in an array

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To check whether every element in an array satisfies a condition

Question 17

What is a template literal in JavaScript?

Accepted Answer

A string literal enclosed in backticks that allows embedded expressions and multi-line strings

Answer

A predefined layout for building a webpage

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A reusable HTML component

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A method for validating user input

Question 18

What does the 'typeof' operator return when used on a variable that has been declared but never assigned a value?

Accepted Answer

'undefined'

Answer

'null'

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'object'

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An error is thrown

Question 19

What is prototypal inheritance in JavaScript?

Accepted Answer

A model where objects can inherit properties and methods directly from other objects through a prototype chain

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A model where classes must be defined before any object can inherit from them

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A restriction that prevents objects from sharing methods

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A method used only for inheriting CSS styles

Question 20

What is the purpose of async/await syntax in JavaScript?

Accepted Answer

To write asynchronous code that reads and behaves more like synchronous code, while still being non-blocking

Answer

To make a function run faster by skipping certain steps

Answer

To permanently pause the entire browser until a task finishes

Answer

To convert synchronous code into a callback-based style

JavaScript MCQ Questions