gistfile1.js

$.ajax({
  url: mw.config.get('wgScriptUrl') + '/api' + mw.config.get('wgScriptExtension'),
  data: {
    action: "query",
    prop: "onlinestatus",
    onlinestatususer: mw.config.get('wgTitle'),
    format: 'json'
  },
  success: function(data) {
    // code to update the statusbar based on the returned message
  }
});