SharePoint 2010 - Verify if current user is a member of a Group using group name (CSOM Javascript)

SP.currentUserIsMemberOf.js

  var currentUserIsMemberOf = function(groupName){
      var found = false;
      var dfd = $.Deferred(function(){
         SP.SOD.executeOrDelayUntilScriptLoaded(function(){
            context = new SP.ClientContext.get_current();
            allGroups = context.get_web().get_siteGroups();
            context.load(allGroups);
            context.load(allGroups, 'Include(Users)');

            context.executeQueryAsync(
               function(){
                  var groupInfo;
                  var groupsEnumerator = allGroups.getEnumerator();
                  while (groupsEnumerator.moveNext()) {
                      var group = groupsEnumerator.get_current();               
                      if(group.get_title() == groupName) {
                          var usersEnumerator = group.get_users().getEnumerator();
                          while (usersEnumerator.moveNext()) {
                             var user = usersEnumerator.get_current();   
                             if(user.get_id() == _spPageContextInfo.userId) {
                                found = true;

                                break;
                             }
                          }
                      }
                   }
                  dfd.resolve(found);
               },
               function(){
                  dfd.reject(args.get_message());
               }
            );
         }, 'sp.js');
      });
      return dfd.promise();
   }

SP.currentUserIsMemberOf.usage

currentUserIsMemberOf("Members of Demo").done(function(result){ 
                                                      alert(result) 
                                                    });

Require jQuery. or implement your own Deferred object like this: http://stackoverflow.com/questions/18096715/implement-deferred-object-without-using-jquery