Is it possible to use an XHR response as a return value?

Question

I would like to assign an XMLHttpRequest response to a variable using a return statement, but I am getting an undefined result. I am guessing that the assignment is trying to execute before the response is ready.

var x = req('param=val&param2=val2'); // x results in 'undefined'
function req(request) {
  var xhr=new XMLHttpRequest();
  xhr.open("post","parser.php");
  xhr.responseType="text";
  xhr.setRequestHeader("content-type","application/x-www-form-urlencoded");
  xhr.send(request);
  xhr.onreadystatechange=function(){
    if(this.readyState==4&&this.status==200) return this.response;};
}

I was previously using setInterval() for every assignment, but it's more verbose than I'd like:

req('param=val&param2=val2');
var wait=setInterval(function(){if(php!=null){
   x = php;
   clearInterval(wait);}
},10);
function req(request) {
  var xhr=new XMLHttpRequest();
  xhr.open("post","parser.php");
  xhr.responseType="text";
  xhr.setRequestHeader("content-type","application/x-www-form-urlencoded");
  xhr.send(request);
  xhr.onreadystatechange=function(){
    if(this.readyState==4&&this.status==200) php = this.response;};
}