Because JSS is evaluated as JavaScript JSS processing works in all environments where JavaScript is available.
Node.js
Node.js brings googles V8 JavaScript engine to the command line. Therefore you can use node.js to process JSS.
OpenWGA
OpenWGA is an open source java based content management system using the Mozilla JavaScript engine rhino as scripting and expression engine. The latest version of OpenWGA supports JSS.
Your Browser
Last but not least because your browser also supports JavaScript you can use JSS just inside the browser.