Oh, I thought you were using SAPUI5 which contains jQuery library. Here's a plain JavaScript way of doing the same:
var fragment = document.createDocumentFragment();
var container = document.createElement('div');
container.innerHTML = xmlhttp.response;
fragment.appendChild(container);
var targets = fragment.firstChild.getElementsByTagName("h1");
var valueJS = targets[0].innerHTML;
Regards,
Kimmo