AJAX将类添加到表单元格

Question

I have 2 select menus on my form: Type and Category. When a user selects something from the Type menu I want it to then perform a PHP script which queries a database and returns a list of matching categories for the selected Type for them to choose from in the Category menu.

This is working well but I would now like to update it so that it adds a class to the categoryGroup ("success" or "error" depending on the AJAX result). Here's my table with the 2 cells:

<div class="form-group">
  <label for="title" class="control-label col-sm-3">Type</label>
  <div class="input-group col-xs-8">
    <select class="form-control" name="type" id="type" onchange="getCategories(this.value)">
      <option value="" selected></option>
      <option value="Business">Business</option>
      <option value="Commercial">Commercial</option>
      <option value="Commercial Land">Commercial Land</option>
      <option value="Land">Land</option>
      <option value="Rental">Rental</option>
      <option value="Residential">Residential</option>
      <option value="Rural">Rural</option>
    </select>
  </div>
</div>

<div class="form-group" id="categoryGroup">
  <label for="title" class="control-label col-sm-3">Category</label>
  <div class="input-group col-xs-8" class="" id="categoryList">
    <select class="form-control" name="category" id="category">
      <option value="" selected></option>
    </select>
  </div>
</div>

and here's my script that's working to replace the Category select menu with the appropriate options from the AJAX call:

<script type="text/javascript">
  function getCategories(str) {
    if (str == "") {
      document.getElementById("categoryList").innerHTML = "";
      return;
    }
    if (window.XMLHttpRequest) { // code for IE7+, Firefox, Chrome, Opera, Safari
      xmlhttp = new XMLHttpRequest();
    } else { // code for IE6, IE5
      xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
    }

    xmlhttp.onreadystatechange = function() {
      if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
        document.getElementById("categoryList").innerHTML = xmlhttp.responseText;
      }
    }

    xmlhttp.open("POST", "getPropertyCategories.php?type=" + str, true);
    xmlhttp.send();
  }
</script>

I can't work out how to update this to also add a class to the categoryGroup id

Answer 1

Quite basic, but I think you get the gist of it:

 xmlhttp.onreadystatechange = function() {
     if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
         document.getElementById("categoryList").innerHTML = xmlhttp.responseText;
     }

     var status = xmlhttp.status == 200 ? 'success' : 'error'
     var group = document.getElementById("categoryGroup")
     group.classList.add(status)
}