Are you ready for this?

Ten questions, free, marked as you go, written at exactly the level of the React Certification. Nothing is recorded and no account is needed — it exists so you can decide for yourself rather than take our word for it.

The real paper is 60 questions in one sitting with a 70% pass mark and no partial credit. It is not a certificate of attendance, and roughly a third of it is foundations — the point is the whole language, not the parts you enjoyed.

Question 1 of 10

Rendering lists and conditionals

items is a list a user can reorder by dragging. Why is key={index} a problem here specifically?

{items.map((item, index) => (
  <Item key={index} data={item} />
))}

Pick an answer to see whether it is right. You cannot change it afterwards — that is the point.