<textarea id="txt" style="display:none;">
<?xml version="1.0" encoding="UTF-8"?>
  <Response>
  <Tables Sum="3">
  <Table Name="aaa" type="">
  <Field FieldName="gid" DataType="int4" Position="0" />
  </Table>
  <Table Name="bbb" type="">
  <Field FieldName="gid" DataType="int4" Position="0" />
  </Table>
  <Table Name="ccc" type="">
  <Field FieldName="gid" DataType="int4" Position="0" />
  </Table>
  </Tables>
  </Response>
</textarea>
<script>

var xmlParse = function(str) 
{
    if (typeof ActiveXObject != 'undefined' && typeof GetObject != 'undefined') 
    {
        var doc = new ActiveXObject('Microsoft.XMLDOM');
        doc.loadXML(str);
        return doc;
    }

    if (typeof DOMParser != 'undefined') 
    {
        return (new DOMParser()).parseFromString(str, 'text/xml');
    }
    return createElement('div', null);
}

var xml = document.getElementById( 'txt' ).value;
var doc = xmlParse( xml );
var items = doc.getElementsByTagName( 'Table' );
var names = [];
for( var i = 0; i < items.length; i++ ) {
    names.push( items[ i ].getAttribute( 'Name' ) )
}
alert( names.join( ',' ) );

</script>