Lists, Keys and Forms Quiz

Question 1

How do you render an array of items in JSX?

Accepted Answer

By calling map to turn the array into an array of elements

Answer

With a for loop inside the braces

Answer

With a repeat attribute

Answer

React renders arrays automatically without mapping

Question 2

What is a key used for?

Accepted Answer

Telling React which element in the new list matches which in the old list

Answer

Styling the element

Answer

Sorting the list

Answer

Making the item unique in the database

Question 3

Why is the array index a poor key for a list that can be reordered?

Accepted Answer

The key describes a position rather than an item, so state attaches to the wrong row when items move

Answer

Indexes are too slow to compare

Answer

React rejects numeric keys

Answer

Indexes are not unique

Question 4

What is wrong with this key?

Accepted Answer

A new key is generated on every render, so React remounts every row each time

Answer

Nothing, random keys guarantee uniqueness

Answer

Math.random can return duplicates

Answer

Keys must be strings

Question 5

What makes an input controlled?

Accepted Answer

Its value comes from state and changes are reported through onChange

Answer

It has a ref attached

Answer

It is inside a form element

Answer

It has the controlled attribute

Question 6

Which property do you read for a checkbox?

Accepted Answer

event.target.checked

Answer

event.target.value

Answer

event.target.selected

Answer

event.value

Question 7

Where should the submit handler go?

Accepted Answer

On the form's onSubmit

Answer

On the submit button's onClick

Answer

On the last input's onBlur

Answer

On the document