For the built-in objects, the reference is in the ECMAScript spec:
http://www.ecma-international.org/publications/standards/Ecma-262.htm
For the host objects, use MDC and MSDN:
https://developer.mozilla.org/en/gecko_dom_reference
http://msdn.microsoft.com/en-us/library/ms533050(VS.85).aspx
Also, there is the Webkit DOM Reference (but it seems somewhat sloppy):
http://developer.apple.com/library/safari/#documentation/AppleApplications/Reference/WebKitDOMRef/index.html
Then again there are the web-standards defined by W3C. You can use my W3 Viewer to browse them:
http://www.w3viewer.com
(The standards of interest are in the "JavaScript" and "DOM" categories)