Call JavaScript function on DropDownList SelectedIndexChanged Event:

I have written one JavaScript function as follows:

 function CalcTotalAmt() 
 {
    ----------
    -----------
 }

I have one DropDownList,

  <asp:DropDownList ID="ddl" runat="server"  AutoPostBack="true" OnSelectedIndexChanged="ddl_SelectedIndexChanged"></asp:DropDownList>

I need to call the above JavaScript function in the DropDownList's SelectedIndexChanged Event. I tried like below;

protected void ddl_SelectedIndexChanged(object sender, EventArgs e)
{
    ddl.Attributes.Add("onchange", "return CalcTotalAmt();");
}

But the JavaScript function is not executing. How to call the JavaScript function in DropDownList Change Event?