warning: this might be outdated
there is no navigator object in node.js, because it does not run in the browser. it runs in the system. "equivalent" to navigator is process. this object holds many information, e.g.
process.platform // linux
if you want to run a web browser, you have to execute it..
var sys = require('sys')
// open google in default browser
// (at least in ubuntu-like systems)
sys.exec('x-www-browser google.com')
this might not work in newer node.js versions (i have 2.x), you might have to use
var child_process = require('child_process')
child_process.exec('x-www-browser google.com')
i guess there is no simple way how to multiplatform "run" any file with its "default app", you would have to find out how to do it in each OS / desktop environment, and do it after OS detection.