I would like to see whether the header was text/html or text/xml. If it was text/html then there was an error and I would rather catch that before proceeding.
Use the getResponseHeader() method.
Minimal example:
<script>
function hand () {
console.log(this.getResponseHeader('content-type'));
}
var x = new XMLHttpRequest();
x.onreadystatechange = hand;
x.open('GET', 'index.html', true);
x.send();
</script>