split a javascript string

var xmlHttp_obj = new XmlHttpRequest( );

 xmlHttp_obj.onreadystatechange = function( ) {
                                 if ( xmlHttp_obj.readyState == 4) // ready 
                                {
                                  var cities_List = xmlHttp_obj.responseText;
                                  // what to do NEXT??
                                 
                                }
                              }

 xmlHttp_obj.open("POST", "http://localhost/" + window.location.host + "/TestASP_NET/AsyncTest.ashx", true);

xmlHttp_obj.send(null);