RadioButtonList JS Gets the selected item

<asp:radiobuttonlist id= "RadioButtonList1" runat= "Server" >
        <asp:listitem value= "0" selected= "True" text= "Direction One" ></asp:ListItem>
        <asp:listitem value= "1" text= "Direction II" ></asp:ListItem>
        <asp:listitem value= "2" text= "direction three" ></asp:ListItem>
        <asp:listitem value= "3" text= "Direction four" ></asp:ListItem>
    </asp:RadioButtonList>

<script type= "Text/javascript" >
function checkisselected () {
var RBL = document.getElementById ("<%=RadioButtonListCosmetic.ClientID%>");
var radio = rbl.getelementsbytagname ("input");
var isselect = false;
for (var i = 0; i < radio.length; i++) {
if (radio[i].checked) {
Isselect = true;
Break
}
}
if (!isselect) {
Alert ("Please select an option.") ");
}
return isselect;
}
</script>

RadioButtonList JS Gets the selected item