Returning ResponseXML, Set Within An Anonymous Function, Only Once ReadyState = 4

I have several functions with code along the lines of:

var xmlDoc = requestXML("ajax.asp?SP=SelectRelatedTags&tag=" +
array[i]);

The requestXML() function includes the code:

var xmlDoc = null;
http_request.onreadystatechange = function() {
if (http_request.readyState == 4) {
if (http_request.status == 200) {
xmlDoc = http_request.responseXML;
} else {
alert('There was a problem with the request.' +
http_request.status);
}}};
http_request.open('GET', url, true);
http_request.send(null);
return xmlDoc;

However, the last line (the return) executes before the readyState
reaches 4. How do I return the xmlDoc to the functions only once the
xmlDoc has been assigned? I tried putting the return statement in a
while loop with the condition that the readyState must = 4 - this
worked, but makes the browser popup a message saying the script is
slowing down the system.