Eduraize | JavaScript Quiz 01 - Test Your JS Knowledge with 25 Essential Questions

01. Which of the following is a correct way to declare a variable in JavaScript (modern and recommended way)?

var myVariable;

variable myVariable;

let myVariable;

constant myVariable;

Explanation: The let keyword declares a block-scoped variable introduced in ES6.

02. What will typeof null return?

"object"

"null"

"undefined"

"function"

Explanation: typeof null returns "object" due to a legacy bug in JavaScript.

03. Which of the following is used to check strict equality in JavaScript?

=

==

===

!=

Explanation: === checks for both value and type equality.

04. What does the isNaN() function do?

Checks if a value is not a number

Converts a value to a number

Rounds a number to the nearest integer

Checks if a number is infinite

Explanation: isNaN() returns true if the value is not a number.

05. Which of the following is a looping structure in JavaScript?

if

switch

for

try

Explanation: The for loop is a control flow statement for iteration.

06. What does undefined mean in JavaScript?

The variable is declared but not assigned a value.

The variable is not declared.

The variable contains null.

The variable is an error.

Explanation: undefined indicates a declared variable without an assigned value.

07. What will console.log(2 + "2") output?

4

22

NaN

undefined

Explanation: The + operator concatenates when one operand is a string. The number 2 is coerced to "2", then concatenation happens.

08. What is the default value of a function parameter in JavaScript?

null

0

undefined

false

Explanation: If no value is passed, parameters are undefined by default.

09. Which keyword is used to return a value from a function?

return

yield

exit

break

Explanation: The “return” keyword specifies the value to be returned by the function.

10. What is the output of console.log("10" - 5)?

NaN

-5

10

5

Explanation: The - operator converts the string "10" to a number (numeric subtraction)

11. What does the filter() method do in JavaScript?

Filters elements by key

Removes duplicates

Modifies the original array

Returns a new array with elements passing a condition.

Explanation: # The filter() method in JavaScript is used to create a new array that contains only the elements that pass a certain condition.
# filter() creates a new array with all elements that pass a test provided by a callback function.

12. How do you access the second element of an array arr?

arr[1]

arr(2)

arr.2

arr{1}

Explanation: Arrays are zero-indexed, so the second element is at index 1 like this arr[1].

13. What is the output of console.log(3 ** 2)?

6

9

8

27

Explanation: # The ** operator calculates the power of a number (exponentiation).
# 3**2 means 3 raised to the power of 2, which is calculated as: 3x3=9

14. How do you create an object in JavaScript?

let obj = Object();

let obj = {};

let obj = [];

let obj = new Object[];

Explanation: {} is the shorthand for creating an object literal.

15. Which method can be used to merge two arrays in JavaScript?

concat()

append()

merge()

combine()

Explanation: The concat() method merges two or more arrays.

16. What does the map() method return?

A new array

The original array

A single value

A modified object

Explanation: map() creates a new array populated with the results of calling a function on every element.

17. What is the purpose of setTimeout()?

Executes a function immediately

Executes a function after a delay

Stops code execution

Executes a function repeatedly

Explanation: setTimeout() delays the execution of a function.

18. What does this refer to in JavaScript?

The current function

The prototype object

The global object

The current object

Explanation: “this” refers to the object it belongs to in the current context.

19. Which function is used to remove the last element from an array?

remove()

pop()

shift()

splice()

Explanation: The pop() method removes and returns the last element of an array.

20. How do you handle exceptions in JavaScript?

try/catch

catch/throw

throw/handle

try/finally

Explanation: The try/catch block handles exceptions and allows error recovery.

21. What does Object.freeze() do?

Makes an object immutable

Deletes all properties of an object

Adds new properties to an object

Converts an object to a string

Explanation: Object.freeze() prevents modification of properties in an object.

22. What is the purpose of async/await in JavaScript?

To delay code execution

To lock JavaScript execution

To execute functions in parallel

To handle asynchronous operations more easily

Explanation: async/await allows writing asynchronous code in a synchronous-like manner.

23. Which method is used to convert JSON to a JavaScript object?

JSON.parse()

JSON.stringify()

JSON.objectify()

JSON.convert()

Explanation: JSON.parse() converts a JSON string into a JavaScript object.

24. What will let {a, b} = {a: 10, b: 20}; do?

Create a new object with a and b

Assign undefined to a and b

De-structure the object into a and b

Throw an error

Explanation: Object de-structuring extracts values into variables.

25. What is a "closure" in JavaScript?

A function inside another function

A way to close a variable

A method to define classes

A reserved keyword

Explanation: A closure gives a function access to its outer scope.

Js Quiz 01

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