32. JavaScript Quiz

Which HTML element holds JavaScript code?

• <js>
• <code>
• <script>
• <javascript>

The <script> element holds JavaScript, either inline or linked with src.

Which keyword declares a value that cannot be reassigned?

• let
• const
• var
• static

const blocks reassignment. Use it by default, and let only when the value must change.

What does "5" + 2 produce?

• 7
• NaN
• An error
• "52"

When one side is text, + joins instead of adding, so the result is the string “52”.

Which operator compares both value and type?

• ===
• ==
• =
• =>

=== is strict equality. = assigns, and => is for arrow functions.

What is the index of the first item in an array?

• 1
• 0
• -1
• It depends on the array

Array positions start at 0, so the last item is at length - 1.

Which array method returns a new array containing only the items that pass a test?

• map()
• reduce()
• filter()
• find()

filter keeps matching items. map transforms every item, and reduce combines them into one value.

What does a function return if it has no return statement?

• undefined
• null
• 0
• An empty string

A function without return gives back undefined.

Which is the safest way to put text typed by a visitor into the page?

• element.innerHTML = text
• document.write(text)
• eval(text)
• element.textContent = text

textContent treats the text as plain text, so it cannot run HTML or script (XSS).

Which method returns only the first element matching a CSS selector?

• querySelectorAll()
• querySelector()
• getElementsByTagName()
• findElement()

querySelector returns one element or null. querySelectorAll returns a list.

What does event.preventDefault() do on a form’s submit event?

• Deletes the form
• Checks the form is valid
• Stops the browser’s normal submit and page reload
• Sends the form twice

It stops the browser’s default action so your own code can handle the form.

Which method turns an object into JSON text?

• JSON.stringify()
• JSON.parse()
• JSON.toObject()
• Object.toJSON()

stringify makes text from an object, and parse makes an object from text.

A fetch() request returns a 404 response. What happens to the promise?

• It is rejected automatically
• It still resolves, so you must check response.ok
• The page reloads
• It retries three times

fetch rejects only on network failure. HTTP errors such as 404 still resolve, so check response.ok.