Javascript for enabled/disabled textbox when select others in dropdownlist

<asp:TextBox ID="txtid" runat="server"></asp:TextBox>
 
<asp:DropDownList ID="ddlid" runat="server" onchange="showReadonly();">
<asp:ListItem Text="Select" />
<asp:ListItem Text="English" />
<asp:ListItem Text="Other Course" />
</asp:DropDownList>


<script type="text/javascript">
      function showReadonly() {
      
          var txtobj = document.getElementById('<%=txtid.ClientID%>');
          var valobj1 = document.getElementById('<%=ddlid.ClientID%>').value;
      
          if (valobj1 == "Other Course") {
              txtobj.disabled = false;
              txtobj.focus();
          }
          else {
              txtobj.disabled = true;

          }
      }
</script>