That isn't true. All important parts of what we call "Java Script" are well-defined at central, publicly accessible places.
However, one needs to know that the standardized part of Java Script is called ECMAScript, whose current version (5th edition) is available at:
http://www.ecma-international.org/publications/standards/Ecm...
One also needs to know that the DOM API is not specified by ECMA but by the W3C:
The DOM standard consists of 3 parts, all of which provide an appendix for ECMAScript:
http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/ecma-scri...
http://www.w3.org/TR/2000/REC-DOM-Level-2-Core-20001113/ecma...
http://www.w3.org/TR/2004/REC-DOM-Level-3-Core-20040407/ecma...
One might ask why all those JavaScript tutorials don't link to the standard, and why so many of them don't even mention the correct technical term "ECMAScript". No offense intended, but I guess they simply don't know any better. Although they ought to know.
For those who don't like to read technical standards, there are also nice secondary resources like the Mozilla Developer Network: