使用来自AJAX的JQuery追加

I want to create a dropdown list that populated by another dropdown list. I'm using AJAX and PHP.

I have created my AJAX file like this:

<?php
if(isset($_POST['selname']))
{
    include('config.php');
    $clientId = $_POST['selname'];

    $query  = "SELECT tv.*, v.* FROM t_vorder tv LEFT JOIN m_vehicle v ON tv.tv_vehicleid = v.v_id WHERE tv_orderid = '$clientId'";
    $result = mysqli_query($conn, $query);
    while($row = mysqli_fetch_array($result, MYSQLI_ASSOC))
    {
        $namek = "";
        $namek .= $row['v_id'];
        if($row['v_jenis'] != "" || !empty($row['v_jenis']))
        {
            $namek .= ' - '.$row['v_jenis'];
        }
        if($row['v_platno'] != "" || !empty($row['v_platno']))
        {
            $namek .= ' - '.$row['v_platno'];
        }
        if($row['v_merk'] != "" || !empty($row['v_merk']))
        {
            $namek .= ' - '.$row['v_merk'];
        }
        $xx .= "<option value='$row[v_id]'>$namek</option>";
    }
    return $xx;
    exit;
}
?>

After that, I called this AJAX file to my main program, here's my JQuery code:

function getVehicle()
    {
        var selname = $("select[name=noorder]").val();
        $('#combobox2').html('');
        $.ajax({ url: "getVehicle.php",
            data: {"selname":selname},
            type: 'post',
            dataType: "json",
            success: function(output) {
              console.log(output);
                $('#combobox2').append(output);
            }

        });
    }

And last is my HTML code:

<select name="noorder" id="combobox" class="form-control">
    //get my vehicle from database
    <?php
        $querycon = mysqli_query($conn, "SELECT * FROM m_order WHERE o_status='1' ORDER BY o_id");
        while($rowcon = mysqli_fetch_array($querycon, MYSQLI_ASSOC))
        {
            $invoice = sprintf("%s%"."04d", $rowcon['o_code'], $rowcon['o_id']);
    ?>
            <option value="<?php echo $rowcon['o_id']; ?>"><?php echo $invoice; ?></option>
    <?php
        }
    ?>
</select>
<select name="kendaraan" class="form-control" id="combobox2" onclick="getVechile();">

</select>

My Ajax works fine, my console log return that the file finished load. But my dropdown list not appended by Jquery. Anyone know where's my mistakes?

You did't request for json obj/data into ajax success callback, then no need for dataType: "json", inside ajax properties. Remove that and change return $xx; into echo $xx;

AJAX request gets data from PHP file only when its printed out on the page.

Returning and data from PHP (AJAX backend) to jQuery/Javascript does not mean anything.

Change

return $xx; 

to

echo $xx;

You use the dataType: "json" in your ajax call, so it get response in JSON. First remove dataType: "json" line from ajax call. Then replace return $xx; with echo $xx; in your php script.

Hope this solution may works for you, Thanks!