assigning ajax.responseText to variable

Question

I understand what is going on here, i'm trying to assign the return value of a function to a variable, this is pretty straightforward, until you start working with asynchronous requests, i guess this is more a call for ideas rather than understanding what's going on

this will return undefined, i want a="ok";

Javascript:

var ajax = {
    touch: function(url) {
        var ajax = new XMLHttpRequest();
        ajax.open('GET',url);
        ajax.send();

        ajax.onreadystatechange=function(){
            if(this.readyState == 4 && this.status == 200) {
                var response = this.responseText.toString();
                return response;
            }
        };      
    },
}
var a = ajax.touch('session.php');
console.log(a);

session.php

<?php
echo "ok";
?>

already tried to set ajax to FALSE so it goes synchronous but that's deprecated