XmlHTTP Request Error: "Failed"

If the server your contacting doesn't have CORS set in the header (e.g. access-control-allow-origin: *), you will not be able to make the request. If you don't have access to the server to set a CORS header, you'll need to contact the server from a server you do control and then pass that to the browser (either with a CORS header or not if it's served from the same domain)

But your code works fine. The problem is with your not returning

var xmlHTTP = new XMLHttpRequest();
xmlHTTP.open('GET',');
xmlHTTP.send();

xmlHTTP.onreadystatechange = function() {
    if (this.readyState == 4 && this.status == 200) {
      document.body.innerHTML =
      this.responseText;
    }
  };

For server status errors, add this to onreadystatechange

if( this.status > 299 && this.readyState == 4) {
    console.log('Server Error: ' + xmlHTTP.statusText);
}

For xmlHTTP code errors...

xmlHTTP.onerror = function() {
    console.log('xmlHTTP Error', xmlHTTP.responseText)
}