This way
<script type = "text/javascript">
window.onload = function () {
var ddlFruits = document.getElementById("<%=ddlFruits.ClientID %>");
ddlFruits.onchange = function () {
var selectedValue = ddlFruits.value;
alert(selectedValue);
};
};
</script>
<asp:DropDownList ID="ddlFruits" runat="server">
<asp:ListItem Text="Mango" Value="1" />
<asp:ListItem Text="Apple" Value="2" />
</asp:DropDownList>