callback function that processes the Web Service return value as undefined

below given line of code is used to call webservice method in js file......
 

GetFields(fieldName,_PartnershipID, myEditor.onGetFieldsCompleted,myEditor.onGetFieldsFailed);

 
LISTNER WHEN THE WEB SERVICE CALL IS RETURNED SUCCESSFULLY

myEditor.onGetFieldsCompleted= function (result)
        {
 
            if (result != null)
            {
                if(result.length > 0)---here in result iam getting undefined
                {
                    do something   
              }
                 else
                 {
                    do something
                 }
            }
        }