使用ajax发送表单PHP

I have a form to upload files with 2 parameters (type = "file", type = "text") to pass through ajax in PHP. Looking at google I found the script you see below. The script works perfectly and also visualize the progress bar, but I can spend only what concerns the parameter type = "file" but I can add to what I write in the field type = "text".

<script>
function _(el){
    return document.getElementById(el);
}
function uploadFile(){
    var file = _("userfile").files[0];
    var formdata = new FormData();
    formdata.append("userfile", file);
    var ajax = new XMLHttpRequest();
    ajax.upload.addEventListener("progress", progressHandler, false);
    ajax.addEventListener("load", completeHandler, false);
    ajax.addEventListener("error", errorHandler, false);
    ajax.addEventListener("abort", abortHandler, false);
    ajax.open("POST", "TEST.php");
    ajax.send(formdata);
}
function progressHandler(event){
    _("loaded_n_total").innerHTML = "Uploaded "+event.loaded+" bytes of "+event.total;
    var percent = (event.loaded / event.total) * 100;
    _("progressBar").value = Math.round(percent);
    _("status").innerHTML = Math.round(percent)+"% uploaded... please wait";
}
function completeHandler(event){
    _("status").innerHTML = event.target.responseText;
    _("progressBar").value = 0;
    location.href="home.php";
}
function errorHandler(event){
    _("status").innerHTML = "Upload Failed";
}
function abortHandler(event){
    _("status").innerHTML = "Upload Aborted";
}
</script>

<form id="upload_form" enctype="multipart/form-data" method="post">
<input class="formUpDes" type="text" name="description" id="description">
<input class="formUpFile" type="file" name="userfile" id="userfile">
<input class="formUpBt" type="button" value="upload" onclick="uploadFile()">
<progress class="formUpProg" id="progressBar" value="0" max="100"></progress>
<h3 id="status"></h3>
<p id="loaded_n_total"></p>
</form>

I would just add the text that is written in the text field in the function UploadFile() so that it gets passed to the PHP file.

please help me

Add to uploadFile():

formdata.append('description', _('description').value);