How to retrieve and save HTML viewer's information in Javascript

I have a HTML homepage publicly available. Using Google Analytics, it provides some viewer's demographic information, also the OS, browser, etc.

I want to get as much information as possible from the viewer, especially OS login name or username, and save them in a .txt file. How can I do that in javascript (or any possible means embedded in my HTML)?

For instance, in Java, one would do System.getProperty("user.name"); to get user's name.