How to get SharePoint Groups using Javascript in SP2013 ?

<script type="text/javascript">
var requestUri = _spPageContextInfo.webAbsoluteUrl + "/_api/web/sitegroups";
  var requestHeaders = { "accept" : "application/json;odata=verbose" };
  $.ajax({
    url : requestUri,
    contentType : "application/json;odata=verbose",
    headers : requestHeaders,
    success : onSuccess,
    error : onError
  });

  function onSuccess(data, request){
var s='';
 for (var i = 0; i < data.d.results.length; i++)
{
s +=data.d.results[i].LoginName+'\n';
}
    alert(s);
}

 function onError(error) {
    alert("error");
  }
</script>