In React, keys help identify which items in a list have changed, been added, or been removed. A good key gives React a hint about which elements can be reused efficiently. Using the array index as a key might seem easy, but it can cause subtle bugs and performance issues in certain situations.

Here's why using index as a key is risky:

1. Incorrect Component Reordering:
   If the list items can change their order, React will associate the wrong component state with the wrong item because it relies on the index position, not the content.

2. Problems with State:
   Suppose a list item holds some local state (like an input field). If the list changes and indexes shift, React might mistakenly preserve or overwrite the wrong state.

3. Performance Issues:
   React optimizes rendering by comparing the old and new virtual DOMs. If indexes are used as keys, React may re-render more components than necessary because it thinks items have changed when they haven't.

4. Bad User Experience:
   In interactive UIs (e.g., dynamic forms, drag-and-drop lists), using index as a key can lead to unexpected behaviors — like form inputs losing focus or wrong inputs being updated.

---

When is it (sort of) safe to use an index as a key?

• If the list is static and will never change order or items (e.g., a fixed menu).
• If the list items have no dynamic content, no local state, and no user interaction tied to them.

---

Best Practice:
Always use a unique and stable identifier from your data as the key — such as a database ID or a unique slug.

Example:

// Risky (Using Index)
{items.map((item, index) => (
  <div key={index}>{item.name}</div>
))}

// Better (Using Unique ID)
{items.map((item) => (
  <div key={item.id}>{item.name}</div>
))}

---