Encoder

Converts a Scala value into the representation expected by a DOM definition.

For a custom HtmlAttr, DomType is String. The attribute calls encode when := is evaluated, and the HTML renderer escapes that encoded string later. An encoder should therefore describe representation only: it neither needs to pre-escape HTML nor implies semantic sanitization of URLs, CSS, JavaScript, or custom attribute values.

Custom attribute definitions can pair an encoder with htmlAttr:

val priority = htmlAttr("data-priority", Encoder[Int, String](_.toString))
div(priority := 2)

The attribute constructor validates the custom name. The encoder controls only its value and must return a non-null string for a string-valued custom attribute. The library's BooleanAsAttrPresenceEncoder is a specialized exception interpreted by HtmlAttr.