DropDown binding issue in asp .net

Question

I am trying to append the values to dropdown using below line of javascript code the values that i get in the option are correct but the issue is with the code in my aspx page 

ddlCountries.appendChild(option);

Below is my aspx code

<ext:ComponentColumn runat="server">
  <Items>
     <ext:DropDownField runat="server" ID="ddlCountries" SingleSelect="true"></ext:DropDownField>
  </Items>
</ext:ComponentColumn>