JavaScript allows the user (or script developer really) to create their own objects, properties and methods. The prototypes for existing built-in objects can also be extended by the script developer.

This has generally been supported since the earliest versions of the main JavaScript interpreters. It was made available in the WebTV platform effective from the Summer 2000 release. This means that DHTML effects created by authoring tools such as DreamWeaver should work better in WebTV boxes that are shipped after this date or are upgraded in the field.