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

• React Docs - Rendering Lists
• nanoid - Tiny ID generator

Output

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