Which statement about keys in a list is best practice?

Which statement about keys in a list is best practice?

• A. Always use the array index as the key.
• B. Use stable, unique identifiers from your data (like id) and avoid indices when items can reorder.
• C. Keys are optional; React can render without them.
• D. Keys should be generated randomly for each render.

Answer: (B)

Stable, unique keys let React identify each list item across renders so it can efficiently update, move, or reuse DOM elements. The best practice is to use identifiers from your data that are stable and unique (like an id). This keeps the key tied to the item itself, so even if the list order changes or items are added or removed, React can track which item is which and preserve any local state, focus, or animations correctly. Using the array index as a key can lead to problems when items reorder or are inserted, because the same index might refer to a different item on subsequent renders, causing incorrect updates and potential state mix-ups. Generating random keys each render would make React treat every render as a brand-new element, killing the benefits of keys. For static, never-changing lists, an index might be harmless, but in real dynamic lists the stable-id approach is the reliable default.

Stable, unique keys let React identify each list item across renders so it can efficiently update, move, or reuse DOM elements. The best practice is to use identifiers from your data that are stable and unique (like an id). This keeps the key tied to the item itself, so even if the list order changes or items are added or removed, React can track which item is which and preserve any local state, focus, or animations correctly. Using the array index as a key can lead to problems when items reorder or are inserted, because the same index might refer to a different item on subsequent renders, causing incorrect updates and potential state mix-ups. Generating random keys each render would make React treat every render as a brand-new element, killing the benefits of keys. For static, never-changing lists, an index might be harmless, but in real dynamic lists the stable-id approach is the reliable default.