Use attribute onchange inside asp:dropdown
<asp:DropDownList ID="ddShed" Width="120px" onchange="GetBalanceQty(this.options[this.selectedIndex].value);"
runat="server">
</asp:DropDownList>
JavaScript Function----
function GetBalanceQty(val)
{
var id = val;
}
Thanks & Regards,
satya.
please mark as answer if helpful.