# namespace

Emit an `@namespace` rule for the selectors of one module.

`namespace` binds a prefix to a namespace URI, so `global` selectors in the same module can write `prefix|element`. The compiler isolates each module's namespaces, so declarations never change selectors in other modules.

```ts title="src/svg.ts"
import { global, namespace } from 'zyzz/web'

// Binds `svg|` for selectors in this module
namespace({ prefix: 'svg', uri: 'http://www.w3.org/2000/svg' })

global({ 'svg|a': { color: 'blue' } })
```

## Signature

```ts
// A URI, with an optional prefix
namespace(options)
```

## Parameters

### options.uri

* **Type:** `string`

The namespace URI. It is an identity and is never fetched or published as an asset. An empty string selects elements with no namespace.

```ts
// Restricts unprefixed selectors in this module to HTML elements
namespace({ uri: 'http://www.w3.org/1999/xhtml' })
```

### options.prefix

* **Type:** `string`
* **Default:** `undefined`

The selector prefix, as a CSS identifier, including Unicode and escaped spellings. Omitting it declares the default namespace for the module's unprefixed selectors. The last declaration for each prefix applies throughout the module.

```ts
import { global, namespace } from 'zyzz/web'

// Selects MathML elements with `m|`
namespace({ prefix: 'm', uri: 'http://www.w3.org/1998/Math/MathML' })

global({ 'm|math': { fontSize: '1.125rem' } })
```

## Returns

`void`. The compiler erases the call and emits the rule with a generated prefix, rewriting the module's selectors to match.

## Errors

TypeScript rejects unknown options and calls without `uri`.

```ts
import { namespace } from 'zyzz/web'

// The URI is required
namespace({ prefix: 'svg' })
// error: Argument of type '{ prefix: string; }' is not assignable to parameter of type 'Options & Record<never, never>'.
// Property 'uri' is missing in type '{ prefix: string; }' but required in type 'Options'.
```

The compiler reports `Source.ExtractError` for an invalid prefix and for namespace combinations it cannot isolate. Native builds reject the call with `Native.CompileError`.