JavaScript Questions

Saurabh Gupta

Score: 🔥 27

Beginner

Took 11 minutes

Programming Language

JavaScript

90%

Questions Attempted

10 / 10

Correct Answers

Incorrect Answers

Points Earned

🔥 9

Question 1

What will the following code snippet log to the console?

console.log(5 + '5');

10
55
Error
NaN

Explanation:

JavaScript performs type coercion in this case. The number 5 is implicitly converted to a string, resulting in string concatenation rather than numerical addition.

Question 2

Which of these is NOT a valid way to create a custom error in JavaScript?

throw new Error('This is a custom error')
throw 'This is a custom error'
throw { message: 'This is a custom error' }
throw new CustomError('This is a custom error')

Explanation:

While you can throw strings or objects as errors, 'CustomError' is not a built-in JavaScript error type. To create custom errors, you'd typically extend the base Error object.

Question 3

Which event is fired when a user clicks on an HTML element?

onmouseover
onmousedown
onclick
onload

Explanation:

The onclick event is specifically triggered when an element is clicked.

Question 4

Which data type represents a sequence of characters in JavaScript?

number
string
boolean
object

Explanation:

In JavaScript, the 'string' data type is specifically designed to store and manipulate textual data, representing sequences of characters.

Question 5

What is the purpose of event.preventDefault() in form handling?

Stops the form from submitting
Clears the form fields
Validates the form data
Submits the form data asynchronously

Explanation:

event.preventDefault() prevents the default behavior of an event. In form handling, it stops the form's default submission action, allowing you to handle it using JavaScript.

Question 6

What value is returned by confirm('Are you sure?') if the user clicks the 'Cancel' button?

true
false
null
undefined

Explanation:

The confirm() function returns a boolean value. If the user clicks 'OK', it returns true; if they click 'Cancel', it returns false. This allows you to branch your code based on the user's response.

Question 7

Which event is commonly used for handling form submissions?

'click'
'change'
'submit'
'input'

Explanation:

The 'submit' event is triggered when a form is submitted, allowing you to handle data validation or submission logic.

Question 8

What is the difference between delete obj.property and obj.property = undefined?

delete completely removes the property, while setting it to undefined keeps the property but with no value.
They are functionally the same.
delete sets the property to null, while setting it to undefined removes it entirely.
You cannot set a property to undefined in JavaScript.

Explanation:

delete removes a property from an object. Setting a property to undefined leaves the property in the object but with an undefined value.

Question 9

How do you set the 'src' attribute of an image element with the ID 'profilePic' to 'new-image.jpg'?

document.getElementById('profilePic').setAttribute('src', 'new-image.jpg');
document.getElementById('profilePic').src = 'new-image.jpg';
document.querySelector('#profilePic').src = 'new-image.jpg';
document.querySelector('#profilePic').setAttribute('src', '/new-image.jpg');

Explanation:

While setAttribute works, directly setting the src property is a more concise and common approach for image elements in JavaScript.

Question 10

How can you loop through the properties of an object myObject?

for (let key in myObject) {...}
for (let i = 0; i < myObject.length; i++) {...}
myObject.forEach(property => {...})
while (myObject.hasNext()) {...}

Explanation:

The for...in loop is used to iterate over the properties (keys) of an object.