It seems that the value does not exist on the client side, making it impossible to retrieve. However, there are alternative methods for obtaining the index (assuming initialization is complete).
selected index: $find("<%=cboName.ClientID%>").get_hiddenFieldControl().value;
selected index (again): $find("<%=cboName.ClientID%>").get_selectedIndex();
selected text: $find("<%=cboName.ClientID%>").get_textBoxControl().value;
From what I can gather, validating a combobox on the client side requires some reliance on either the index or text values, or implementing a server-side workaround.
In order to directly answer the question posed in the title, one approach could involve creating a javascript array server side containing each combobox value, which could then be referenced on the client side using the selected index...
codebehind:
// write combobox values to asp:literal
foreach (ListItem i in cboName.Items)
litCboValues.Text += "\"" + i.Value.Replace("\"", "\\\"") + "\", ";
litCboValues.Text = litCboValues.Text.TrimEnd(new char[] {',', ' '});
aspx:
<script>
// array of values
var cboValues = [ <asp:Literal id="litCboValues" runat="server" /> ];
// add an alert to the combobox to test
function pageLoad()
{
$find("<%=cboName.ClientID%>").get_textBoxControl().onblur = function () {
alert( cboValues[$find("<%=cboName.ClientID%>").get_selectedIndex()] );
};
}
</script>
<asp:ComboBox id="cboName" runat="server" ...