Create GUID / UUID in JavaScript

<script type="text/javascript">
    function guid() {
        function s4() {
            return Math.floor((1 + Math.random()) * 0x10000).toString(16).substring(1);
        }

        return s4() + s4() + '-' + s4() + '-' + s4() + '-' + s4() + '-' + s4() + s4() + s4();
    }
    alert(guid());
</script>