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

Answer

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

Answer

To define a new class in JavaScript

Answer

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

Answer

Sorts an array in ascending order

Answer

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

Answer

To sort array elements based on a custom comparator

Answer

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

Answer

To validate whether a string is properly formatted JSON

Answer

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

Answer

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

Answer

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

Answer

To reverse the order of elements in an array

Answer

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

Answer

A reusable HTML component

Answer

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'

Answer

'object'

Answer

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

Answer

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

Answer

A restriction that prevents objects from sharing methods

Answer

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

Question 21

What is the DOM in the context of web development?

Accepted Answer

A programming interface that represents an HTML document as a structured tree of objects that JavaScript can interact with

Answer

A database used to store website content

Answer

A programming language used to style web pages

Answer

A protocol used for transferring files over the internet

Question 22

What is the purpose of the JavaScript 'const' keyword?

Accepted Answer

It declares a variable whose binding cannot be reassigned after it is set

Answer

It declares a variable that can be reassigned freely

Answer

It declares a variable that only exists inside loops

Answer

It permanently deletes a variable after use

Question 23

What does the JavaScript 'Array.isArray()' method check?

Accepted Answer

Whether a given value is actually an array

Answer

Whether an array is empty

Answer

Whether a value is a number

Answer

Whether a variable has been declared

Question 24

What is the purpose of the JavaScript 'Object.keys()' method?

Accepted Answer

It returns an array containing the property names of a given object

Answer

It returns an array of the values stored in an object

Answer

It permanently removes all properties from an object

Answer

It converts an object into a string

Question 25

What is the difference between synchronous and asynchronous code in JavaScript?

Accepted Answer

Synchronous code runs in order, blocking further execution until it finishes, while asynchronous code allows other code to run while it waits

Answer

Synchronous code always runs faster than asynchronous code

Answer

Asynchronous code can never fail or throw an error

Answer

There is no real difference between the two

Question 26

What does the JavaScript 'localStorage' object allow a web page to do?

Accepted Answer

It stores key value data in the browser that persists even after the page is closed and reopened

Answer

It stores data only for the current tab session

Answer

It permanently sends data to a remote server

Answer

It stores data that automatically expires after one minute

Question 27

What is the purpose of the JavaScript 'Array.prototype.find()' method?

Accepted Answer

It returns the first element in an array that satisfies a given testing function

Answer

It returns every element in an array that matches a condition

Answer

It removes the first matching element from the array

Answer

It counts how many elements satisfy a condition

Question 28

What does the JavaScript 'try...catch' block do?

Accepted Answer

It lets a program attempt to run code and handle any errors that occur without crashing

Answer

It permanently prevents any code from ever throwing an error

Answer

It only works with asynchronous code

Answer

It automatically retries a failed function call three times

Question 29

What is the purpose of the JavaScript 'Number.parseInt()' function?

Accepted Answer

It converts a string into an integer, parsing digits until it reaches a non-numeric character

Answer

It converts a number into a string

Answer

It rounds a decimal number to the nearest whole number

Answer

It checks whether a variable is a number

Question 30

What is the purpose of the JavaScript 'export' and 'import' keywords used with ES modules?

Accepted Answer

They allow code from one file to be shared with and used in another file

Answer

They are used only to import external CSS files

Answer

They permanently combine multiple files into a single file automatically

Answer

They are used only for importing images

Question 31

What does the JavaScript '||' operator do when used between two values?

Accepted Answer

It returns the first truthy value, or the last value if none of them are truthy

Answer

It always returns true regardless of the operands

Answer

It converts both values into numbers before comparing them

Answer

It only works with boolean values, never other types

Question 32

What is the purpose of the JavaScript 'Array.prototype.sort()' method by default?

Accepted Answer

It sorts array elements as strings, based on their character order, unless a custom comparison function is provided

Answer

It always sorts numbers correctly from smallest to largest without any extra configuration

Answer

It permanently removes duplicate elements while sorting

Answer

It only works on arrays containing numbers

Question 33

What is a callback function in JavaScript?

Accepted Answer

A function passed as an argument to another function, to be executed at a later point

Answer

A function that can only be called once during a program's lifetime

Answer

A function that automatically calls itself repeatedly forever

Answer

A special type of variable declaration

Question 34

What does the JavaScript 'Array.prototype.includes()' method check?

Accepted Answer

Whether a specified value exists anywhere within an array

Answer

Whether an array is completely empty

Answer

Whether every element in an array is the same

Answer

Whether an array has been sorted already

Question 35

What is the purpose of destructuring assignment in JavaScript?

Accepted Answer

It allows values from arrays or properties from objects to be unpacked into distinct variables in a concise way

Answer

It permanently merges two arrays into one

Answer

It converts an object into a JSON string

Answer

It removes properties from an object automatically

Question 36

What is the purpose of the JavaScript 'Set' object?

Accepted Answer

It stores a collection of unique values, automatically ignoring duplicate entries

Answer

It stores key value pairs like a dictionary

Answer

It always keeps its elements sorted numerically

Answer

It can only hold string values

Question 37

What does the JavaScript 'window' object represent in a browser environment?

Accepted Answer

The global object representing the browser window, giving access to things like the DOM, location, and timers

Answer

A single HTML element on the page

Answer

A specific database used for storing cookies

Answer

A function used to open new files

Question 38

What is the purpose of the JavaScript 'Array.prototype.slice()' method?

Accepted Answer

It returns a shallow copy of a portion of an array without changing the original array

Answer

It permanently removes elements from the original array

Answer

It combines two arrays into a single new array

Answer

It reverses the order of elements in an array

Question 39

What does the JavaScript 'setTimeout()' function do?

Accepted Answer

It schedules a function to run once after a specified delay, measured in milliseconds

Answer

It repeatedly runs a function forever at fixed intervals

Answer

It pauses the entire program until the delay finishes

Answer

It cancels a previously scheduled function

Question 40

What is the purpose of the JavaScript 'Array.prototype.some()' method?

Accepted Answer

It checks whether at least one element in an array passes a given test, returning true or false

Answer

It checks whether every element in an array passes a given test

Answer

It removes elements that fail a given test

Answer

It counts exactly how many elements pass the test

Question 41

What is the purpose of strict mode, enabled with 'use strict' in JavaScript?

Accepted Answer

It enforces stricter parsing and error handling rules, catching common mistakes that would otherwise be silently ignored

Answer

It permanently disables all functions in a file

Answer

It only affects how numbers are displayed on the page

Answer

It is required before any variable can be declared

Question 42

What is the purpose of the JavaScript 'Promise.all()' method?

Accepted Answer

It takes an array of Promises and resolves once all of them have resolved, or rejects if any one of them fails

Answer

It runs Promises one after another instead of at the same time

Answer

It only works with exactly two Promises

Answer

It cancels every Promise passed to it

Question 43

What does the JavaScript 'Array.prototype.join()' method do?

Accepted Answer

It combines all elements of an array into a single string, separated by a specified separator

Answer

It merges two separate arrays into one array

Answer

It splits a string into an array of substrings

Answer

It permanently sorts array elements alphabetically

Question 44

What is the difference between 'null' and '0' when used in a boolean context in JavaScript?

Accepted Answer

Both are treated as falsy values, but they represent conceptually different things, an intentional absence of value versus the number zero

Answer

'null' is always treated as true while '0' is always treated as false

Answer

'0' is treated as truthy while 'null' is treated as falsy

Answer

There is no difference between them in any context

Question 45

What is the purpose of the JavaScript 'querySelector()' method?

Accepted Answer

It returns the first element in the document that matches a specified CSS selector

Answer

It returns every element in the document regardless of any selector

Answer

It creates a brand new HTML element

Answer

It removes an element from the page

Question 46

What is the purpose of the JavaScript 'Array.prototype.flat()' method?

Accepted Answer

It flattens nested arrays into a single array up to a specified depth

Answer

It permanently removes all numbers from an array

Answer

It converts an array into an object

Answer

It duplicates every element in an array

Question 47

What will typeof NaN return in JavaScript?

Accepted Answer

"number"

Answer

"undefined"

Answer

"nan"

Answer

"object"

Question 48

What will typeof NaN return in JavaScript?

Accepted Answer

"number"

Answer

"undefined"

Answer

"nan"

Answer

"object"

JavaScript Essentials Quiz

JavaScript MCQ Questions

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

What is a closure in JavaScript?

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

What is 'hoisting' in JavaScript?

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

What is a Promise in JavaScript used for?

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

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

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

What is the JavaScript event loop responsible for?

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

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

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

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

What is event bubbling in the context of the DOM?

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

What is a template literal in JavaScript?

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

What is prototypal inheritance in JavaScript?

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

What is the DOM in the context of web development?

What is the purpose of the JavaScript 'const' keyword?

What does the JavaScript 'Array.isArray()' method check?

What is the purpose of the JavaScript 'Object.keys()' method?

What is the difference between synchronous and asynchronous code in JavaScript?

What does the JavaScript 'localStorage' object allow a web page to do?

What is the purpose of the JavaScript 'Array.prototype.find()' method?

What does the JavaScript 'try...catch' block do?

What is the purpose of the JavaScript 'Number.parseInt()' function?

What is the purpose of the JavaScript 'export' and 'import' keywords used with ES modules?

What does the JavaScript '||' operator do when used between two values?

What is the purpose of the JavaScript 'Array.prototype.sort()' method by default?

What is a callback function in JavaScript?

What does the JavaScript 'Array.prototype.includes()' method check?

What is the purpose of destructuring assignment in JavaScript?

What is the purpose of the JavaScript 'Set' object?

What does the JavaScript 'window' object represent in a browser environment?

What is the purpose of the JavaScript 'Array.prototype.slice()' method?

What does the JavaScript 'setTimeout()' function do?

What is the purpose of the JavaScript 'Array.prototype.some()' method?

What is the purpose of strict mode, enabled with 'use strict' in JavaScript?

What is the purpose of the JavaScript 'Promise.all()' method?

What does the JavaScript 'Array.prototype.join()' method do?

What is the difference between 'null' and '0' when used in a boolean context in JavaScript?

What is the purpose of the JavaScript 'querySelector()' method?

What is the purpose of the JavaScript 'Array.prototype.flat()' method?

What will typeof NaN return in JavaScript?

What will typeof NaN return in JavaScript?