ECMAScript (the language "behind" JavaScript) is under active development and the current progress and active standard is of course documented.

You find a link on this page: https://www.ecma-international.org/publications/standards/Ecma-262.htm

Please note: This document is absolutely NOT meant to be learned by heart or to be used as a developer. Rather it's important to browser vendors.

Still, if you're interested in getting a glimpse behind the scenes, this document might be interesting.