Test for network connectivity

network_test.js

// We'll assume we aren't online.
var online = false;

// Assume we're a packaged app with systemXHR permissions.
// See: https://developer.mozilla.org/en-US/docs/Web/Apps/App_permissions for more.
var request = new window.XMLHttpRequest({mozSystem: true});
request.open('GET', 'http://mozilla.com', true);

request.addEventListener('load', function(event) {
   console.log('We seem to be online!', event);
   online = true;
});

request.addEventListener('error', function(event) {
   console.log('We are likely offline:', event);
});

request.send(null);