Hide tab or field based on User roles in CRM 2011 using javascript

function UserHasRole(roleName) {

var serverUrl = Xrm.Page.context.getServerUrl();
var oDataEndpointUrl = serverUrl + "/XRMServices/2011/OrganizationData.svc/";
oDataEndpointUrl += "RoleSet?$top=1&$filter=Name eq '" + roleName + "'";
var service = GetRequestObject();

if (service != null)
{
service.open("GET",oDataEndpointUrl, false);
service.setRequestHeader("X-Requested-Width", "XMLHttpRequest");
service.setRequestHeader("Accept", "application/json,text/javascript, */*");
service.send(null);
var requestResults = eval('(' +service.responseText + ')').d;

if (requestResults != null && requestResults.results.length == 1)
{
    var role = requestResults.results[0];
    var id = role.RoleId;

//Get Current User Roles
    var currentUserRoles = Xrm.Page.context.getUserRoles();  

//Check whether current user roles has the role passed as argument
for (var i = 0; i < currentUserRoles.length;i++)
{
    var userRole = currentUserRoles[i];  
if (GuidsAreEqual(userRole, id))
{
return true;
}
}
}
}

return false;
}

function GetRequestObject()
{
if (window.XMLHttpRequest)
{
return new window.XMLHttpRequest;
}
else
{
try
{
return new ActiveXObject("MSXML2.XMLHTTP.3.0");
}
catch (ex) {
return null;
}

}

}

function GuidsAreEqual(guid1, guid2) {

var isEqual = false;

if (guid1 == null || guid2 == null){

isEqual = false;

}

else {

isEqual = (guid1.replace(/[{}]/g, "").toLowerCase() == guid2.replace(/[{}]/g, "").toLowerCase());

}
return isEqual;

}

function salesmanagerrole() {
    var field = Xrm.Page.ui.controls.get("fieldname");
    var tabvisible = Xrm.Page.ui.tabs.get("tabname");
    if (UserHasRole('Sales Manager')) {
        tabvisible.setVisible("true");
        field.setVisible("true");
    }
}