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
Thank you to @Quentin ... IF I understand correctly, It appears that one can no longer get accurate reporting of the newer macOS systems via the UserAgent.
It seems much of the world simply assumed that it would be macOS 10.xx forever and therefore many sites could not handle a macOS 11.xx or a macOS 12.xx response.
There also appears to be a desire to hide underlying details of the OS for those who might use it for nefarious "fingerprinting purposes".
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.