Get Current Logged On user using JavaScript

<script language="javascript" type="text/javascript">
_spBodyOnLoadFunctionNames.push("GetCurrentLoggedOnUserName");
function GetCurrentLoggedOnUserName()
{
var Loginname = document.getElementById("zz8_Menu").innerHTML ;
var end = Loginname.indexOf("<");
var nameOnly = Loginname.substring(8, end);
alert(nameOnly);
}
</script>