# WithKey<T>

> Attaches an optional stable render identity to any feature type.

> **WithKey**\<`T`\> = `T` & `object`

Attaches an optional stable render identity to any feature type.

**`key` is runtime-only.** It exists solely in memory while the renderer is
running and must never be written to a `.well` file. It is not part of the
`.well` v2 spec — persisting it would produce a non-conformant file.

## Type Parameters

### T

`T`

## Type Declaration

### key?

> `optional` **key?**: `string` \| `number`