Accessing the responseText in the "outer function"

[janroald]

function foo(callURL) {
    var xmlhttp = getHTTPObject();
    xmlhttp.open('POST', callURL, true);
    xmlhttp.onreadystatechange = function() {
      if (xmlhttp.readyState == && xmlhttp.Status == 200) {
         var bar = xmlhttp.responseText;
      }
    }
    xmlhttp.send(null);
   //How can i at this point in the code return the responseText to where the function "foo" was originally called?
}