I am running macOS Monterey (12.2) with Safari 15.3. I am trying to use JavaScript in Safari (or Chrome) to programmatically determine the version of macOS installed.
When I run the following command:
var nAgt = navigator.userAgent;
I get the following results:
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/15.3 Safari/605.1.15
Mozilla/5.0 (Macintosh; Intel Mac OS X10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.110 Safari/537.36
User agent string is no longer reliable in future versions of Mac OS and Windows (11). You should look into client hints API for javascript. Some references for context and the future solution:
https://www.otsukare.info/2021/02/15/capping-macos-user-agent
https://help.duo.com/s/article/6627?language=en_US
https://web.dev/user-agent-client-hints/
https://browserleaks.com/client-hints
The new API doesn't work for all browsers yet. So until the world catches up, the detection for new OSes is best effort.