react-key-prop

React Key Prop

Goal

Give React stable item identity. Use keys tied to data, not render position.

Rules

Always add key to rendered list items.
Prefer unique, stable IDs from data.
Generate missing IDs once when data is created or loaded.
Use the same key while the same item exists.
Do not generate keys during render.
Do not use Math.random() or Date.now().
Do not use useId() for list keys.
Avoid array index keys for dynamic lists.

Good Pattern

{todos.map((todo) => (
  <li key={todo.id}>{todo.text}</li>
))}

Missing IDs

Create IDs once, before render:

const itemsWithIds = data.map((item) => ({
  ...item,
  id: crypto.randomUUID(),
}));

Index Exception

Index key is acceptable only when all are true:

List is static.
Items are never inserted or removed from the middle.
Order never changes.
Items have no internal state.

Failure Mode

Index represents position. When order changes, React reuses the wrong component. Result: stale state, wrong input values, broken animations, or unexpected focus.

References

Output

Stable key choice.
Note when data lacks IDs.
Fix for any index, random, date, or useId key.