Copy billing to shipping using jQuery

gistfile1.html

<form>
  <fieldset>
    <legend>Billing</legend>
    <input type="text" name="first_name" />
    <input type="text" name="last_name" />
    <input type="text" name="billing_address_1" />
    <input type="text" name="billing_address_2" />
    <input type="text" name="billing_city" />
    <input type="text" name="billing_state" />
    <input type="text" name="billing_zip" />
    <select name="billing_country">
      <option value="...">...</option>
    </select>
  </fieldset>
  <fieldset>
    <legend>Shipping</legend>
    <label><input type="checkbox" id="same_as_billing" /> Same as billing</label>
    <input type="text" name="shipping_first_name" />
    <input type="text" name="shipping_last_name" />
    <input type="text" name="shipping_address_1" />
    <input type="text" name="shipping_address_2" />
    <input type="text" name="shipping_city" />
    <input type="text" name="shipping_state" />
    <input type="text" name="shipping_zip" />
    <select name="shipping_country">
      <option value="...">...</option>
    </select>
  </fieldset>
</form>

<script src="jquery.js"></script>
<script>
  $("#same_as_billing").on("change", function(){
    if (this.checked) {
      $("[name='shipping_first_name']").val($("[name='first_name']").val());
      $("[name='shipping_last_name']").val($("[name='last_name']").val());
      $("[name='shipping_address_1']").val($("[name='billing_address_1']").val());
      $("[name='shipping_address_2']").val($("[name='billing_address_2']").val());
      $("[name='shipping_city']").val($("[name='billing_city']").val());
      $("[name='shipping_state']").val($("[name='billing_state']").val());
      $("[name='shipping_zip']").val($("[name='billing_zip']").val());
      $("[name='shipping_country']").val($("[name='billing_country']").val());
    }
  });
</script>

but if country state and city are the dependent dropdown how to do it