get_clientDataKeyValue

The property returns an Array of objects representing the values of the ClientDataKeyNames for the RadListView items.

<telerik:RadListView ID="RadListView1" runat="server" ClientDataKeyNames="ID">
</telerik:RadListView>
<telerik:RadCodeBlock ID="RadCodeBlock1" runat="server">
    <script type="text/javascript">
        function GetClientDataKeyValueForFirstItem() {
            var clientDataKeyValues = $find("<%= RadListView1.ClientID %>").get_clientDataKeyValue();
            var itemIndex = 0; 
            var firstItemKeyValue = clientDataKeyValues[itemIndex]["ID"];
        }
    </script>
</telerik:RadCodeBlock>

This method is supported only with server-side data binding. If you are working with client-side binding, see the following KB article: Item drag and drop with client data binding.