getId = function(filePath, params) {

  if (window.XMLHttpRequest) {
    var xmlhr = new XMLHttpRequest();
  } else {
    var xmlhr = new ActiveXObject('MSXML2.XMLHTTP.3.0');
  }

  xmlhr.open('POST', filePath, true);
  xmlhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');

  xmlhr.onreadystatechange = function() {
    if (xmlhr.readyState == 1) {
    } else if (xmlhr.readyState == 4 && xmlhr.status == 200) {
      if (xmlhr.responseText) {
        var Id = xmlhr.responseText;
        alert(Id); // <-- this works
        return Id; // <-- this doesn't return if I do a call: var myId = getId(xx,xx);
      }
    } else if (xmlhr.readyState == 4) {
      alert('Invalid response received - Status: ' + xmlhr.status);
    }
  }
  xmlhr.send(params);
}