In React, what should the value of a key be?

In React, what should the value of a key be?

• A. The item's content
• B. The index position in the array
• C. A constant value across all items
• D. A unique identifier for each list item

Answer: (D)

The key value should uniquely identify each element in a list across renders. Keys let React track which items have changed, been added, or removed, so it can update the DOM efficiently and preserve item state where appropriate. The best approach is to use a stable, unique identifier from your data, such as item.id, as the key. This helps React correctly associate each item even if the list is reordered or items are inserted or deleted. Using the array index as a key can cause problems when the list changes, because items may shift positions and React can end up reusing the wrong components or losing focus and state. Relying on content as a key can fail if content isn’t guaranteed unique or can change, which also destroys stable identity. A constant key across all items would violate the need for uniqueness and confuse React's reconciliation. So, the right choice is a unique, stable identifier for each item.

The key value should uniquely identify each element in a list across renders. Keys let React track which items have changed, been added, or removed, so it can update the DOM efficiently and preserve item state where appropriate. The best approach is to use a stable, unique identifier from your data, such as item.id, as the key. This helps React correctly associate each item even if the list is reordered or items are inserted or deleted. Using the array index as a key can cause problems when the list changes, because items may shift positions and React can end up reusing the wrong components or losing focus and state. Relying on content as a key can fail if content isn’t guaranteed unique or can change, which also destroys stable identity. A constant key across all items would violate the need for uniqueness and confuse React's reconciliation. So, the right choice is a unique, stable identifier for each item.