How to call web service using jquery

<script type="text/javascript">
      function TestMethod1()
       {
           $.ajax(
            {
                type: "GET",
                url: "http://localhost/DD/WebService1.asmx/HelloWorld",
                data: "{}",
                contentType: "application/json",
                dataType: "json",
                async: false,
                cache: false,

                success: function (result) {
                    alert(result.d);
                },
                error: function ()
                { alert(&quot;Unavailable&quot;); }
            });
        }
</script >