AJAX :: Document.onclick The Bar.php Call Does Not Work And XMLHttpRequest

Here's my problem code:

document.onclick=handler;
function handler(e) {
//do stuff
var params = "something";
var request = new XMLHttpRequest();
request.open("GET", "http://www.foo.com/bar.php?" + params, true);
[Code]....

Now is user clicks something else but not a link everything works just fine. But if she clicks a link, the bar.php call does not work (never comes to the server). If I change the call to a synchronuous one request.open("GET", [URL] false) also clicking a link works fine. But I would of course prefer the async way.