Ajax call

getDocumentVersionMarkup: function DocumentVersions_getDocumentVersionMarkup(doc)
      {
    	  var json = null;
    	  Alfresco.util.Ajax.jsonGet(
                  {
                      url: Alfresco.constants.PROXY_URI + "api/sites/" + Alfresco.constants.SITE + "/memberships/" + encodeURIComponent(Alfresco.constants.USERNAME),
                      successCallback:
                      {
                          fn: function(response)
                          {
                              try {
                                  json = JSON.parse(response.serverResponse.responseText);
                                  Alfresco.util.PopupManager.displayMessage(
                                          {
                                             text: json
                                          });
                              } catch (e) {
                                  // handle error
                              }

                          },
                          scope: this
                      },

                      failureCallback:
                      {
                          fn: function(response)
                          {
                              // handle failure case
                          },
                          scope: this
                      }
                  });

    	 // Calling membership webscript for getting role of the current user
    	 //var json = remote.call("/api/sites/" + page.url.templateArgs.site + "/memberships/" + encodeURIComponent(user.name));
    	 Alfresco.logger.debug("Printing json object from Alfresco logger " + json);
    	 //console.log()("Printing json object from console.log " + json);

         var downloadURL = Alfresco.constants.PROXY_URI + 'api/node/content/' + doc.nodeRef.replace(":/", "") + '/' + doc.name + '?a=true',
            html = '';

         html += '<div class="version-panel-left">'
         html += '   <span class="document-version">' + $html(doc.label) + '</span>';
         html += '</div>';
         html += '<div class="version-panel-right">';
         html += '   <h3 class="thin dark" style="width:' + (Dom.getViewportWidth() * 0.25) + 'px;">' + $html(doc.name) +  '</h3>';
         html += '   <span class="actions">';
         if (this.options.allowNewVersionUpload)
         {
            html += '   <a href="#" name=".onRevertVersionClick" rel="' + doc.label + '" class="' + this.id + ' revert" title="' + this.msg("label.revert") + '">&nbsp;</a>';
         }
   	  	 if (json.role == "SiteViewer")
   	  	 {
   	  		 Alfresco.logger.debug("User has a Viewer role. Hiding donwload button");
   	  		 // do nothing
   	  	 }else{
   	  		 html += '   <a href="' + downloadURL + '" class="download" title="' + this.msg("label.download") + '">&nbsp;</a>';
   	  	 }
         /*html += '      <a href="' + downloadURL + '" class="download" title="' + this.msg("label.download") + '">&nbsp;</a>';*/
         html += '      <a href="#" name=".onViewHistoricPropertiesClick" rel="' + doc.nodeRef + '" class="' + this.id + ' historicProperties" title="' + this.msg("label.historicProperties") + '">&nbsp;</a>';
         html += '   </span>';
         html += '   <div class="clear"></div>';
         html += '   <div class="version-details">';
         html += '      <div class="version-details-left">'
         html += $userAvatar(doc.creator.userName, 32);
         html += '      </div>';
         html += '      <div class="version-details-right">';
         html += $userProfileLink(doc.creator.userName, doc.creator.firstName + ' ' + doc.creator.lastName, 'class="theme-color-1"') + ' ';
         html += Alfresco.util.relativeTime(Alfresco.util.fromISO8601(doc.createdDateISO)) + '<br />';
         html += ((doc.description || "").length > 0) ? $html(doc.description, true) : '<span class="faded">(' + this.msg("label.noComment") + ')</span>';
         html += '      </div>';
         html += '   </div>';
         html += '</div>';

         html += '<div class="clear"></div>';
         return html;
      },

      /**
       * Called when a "onRevertVersionClick" link has been clicked for a version.
       * Will display the revert version dialog.
       *
       * @method onRevertVersionClick
       * @param version
       */
      onRevertVersionClick: function DocumentVersions_onRevertVersionClick(version)
      {
         // Find the version through the index and display the revert dialog for the version
         Alfresco.module.getRevertVersionInstance().show(
         {
            filename: this.latestVersion.name,
            nodeRef: this.options.nodeRef,
            version: version,
            onRevertVersionComplete:
            {
               fn: this.onRevertVersionComplete,
               scope: this
            }
         });
      },


Response I am getting


{
	"role": "SiteManager",
	"isMemberOfGroup": false,
	"authority":
	{
		"authorityType": "USER",
		"fullName": "admin",
		"userName": "admin",
		"firstName": "Administrator",
		"lastName": "",
		"url": "\/alfresco\/s\/api\/people\/admin"
	},
	"url": "\/alfresco\/s\/api\/sites\/testing\/memberships\/admin"
}