From the [http://msdn.microsoft.com/en-us/library/hbxc2t98%28VS.85%29.aspx JScript Homepage] (edits mine): :'''JScript''' is the Microsoft implementation of the ECMA 262 language specification (ECMAScript Edition 3). With a few minor exceptions (to maintain backwards compatibility), JScript is a full implementation of the ECMA standard.

JScript has some Microsoft-specific extensions meant to aid in using the language as a local programming language on the Windows platform. A short (and probably incomplete) list can be found on the Wikipedia page.