code: function(screenName, options) {
        if(!screenName) {
          warn('screenview: `screenName` is required.');
          return;
        }
  
        options = options || {};
        options.screenName = screenName;
        _set.call(this, 'send', 'screenview', options);
  
        return this;
      }
docs: https://developers.google.com/analytics/devguides/collection/analyticsjs/screens