(a real quick guide)
3) DEFUNCT JS VERSION Obsolete JS versions from Netscape and MS JScript.
1) There is no way to determine the "JS version" reliably.
2) Which version? ECMA, browser engine, defunct version?
3) Not indicative of anything - Features are different across various JS engines.
4) Version check pretty much serves no purpose in applications.
Don't do version checks, check if the required feature is available instead.
For example, check if the Geolocation API is available if using GPS for project.