Java script click counter: problems with origin policy?

Is it possible to create a Javascript file that act as "click counter"?

What I want to do is that I put that Javascript file in my domain

mydomain.com/js/clickcounter.js

And anyone can include it to start counting clicks..

Example for THEIRdomain.com/index.html

 <script src="mydomain.com/js/clickcounter.js"></script> 

And a possible script:

function myDomain_TrackClicks(){
    xhttp.open("POST", "mydomain.com/trackClicks.php", true);
    xhttp.send( "pagevisited=" + encodeURIComponent( window.location.href));
}

if(window.attachEvent) {
    window.attachEvent('onload', myDomain_TrackClicks);
} else {
    if(window.onload) {
        var curronload = window.onload;
        var newonload = function(evt) {
            curronload(evt);
            myDomain_TrackClicks(evt);
        };
        window.onload = newonload;
    } else {
        window.onload = myDomain_TrackClicks;
    }
}

Or is that limited by some origin policy? Origin policy really confuses me.