ทำยังไงจะนำคำสั่ง Javascript ที่ได้จาก php ไปใช้ครับ

<label>CATEGORY*</label><select name="ddlcategory" id="ddlcategory" onchange="dropdown()">
<option value="choose">CHOOSE CATEGORY</option>
<?php
include("conn.php");
$sql = "SELECT * FROM category";
$qry = mysql_query($sql) or die("qry");
while($data = mysql_fetch_array($qry)){
echo "<option value=\"" . $data['id_cat'] . "\" id=\"" . $data['name_cat'] . "\">" . $data['name_cat'] . "</option>";
}
?>
</select>
<br><br>
<label>TYPE</label><select name="ddltype" id="ddltype">
<option value="choose">CHOOSE TYPE</option>
</select>
<span id="mySpan"></span>

function dropdown(){
var category = document.getElementById("ddlcategory").value;
var type = document.getElementById("ddltype");
var country = document.getElementById("ddlcountry");
while(type.length > 0){
type.remove(0);
country.remove(0);
}
url = "dropdown.js.php";
pmeters = "cat=" + category; 

HttPRequest.open('POST',url,true);

HttPRequest.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
HttPRequest.setRequestHeader("Content-length", pmeters.length);
HttPRequest.setRequestHeader("Connection", "close");
HttPRequest.send(pmeters);
 
 
HttPRequest.onreadystatechange = function()
{
if(HttPRequest.readyState == 4) // Return Request
{

document.getElementById("mySpan").innerHTML = HttPRequest.responseText;
var js = HttPRequest.responseText; //รับค่ากลับมาจาก dropdown.js.php อันนี้แหละครับ ที่เป็นปัญหา


alert(js);
}
}
}

<?php
header("Content-Type:text/javascript");
include("conn.php");
$cat = $_POST['cat'];

$sql = "SELECT * FROM type WHERE id_cat = '$cat';";
$qry = mysql_query($sql);

$i = 0;
while($data = mysql_fetch_array($qry)){
$js = <<<JS
var opt = new Option('{$data['id_typ']};', '{$data['name_typ']}');
var type = document.getElementById("ddltype").options[$i] = opt;
JS;

echo $js; // ส่งค่านี้กลับไป ajax_framework.js
$i ++;
}
?>