Trying to send data cross domain on page exit with javascript

Question

I have a script on multiple domains that are not mine which sends data to an AWS SQS queue on page exit. When I compare the number of times I receive data to the number of times that the script is loaded on the page I show that 57% of time I do not receive data. I'm expecting to not receive data maybe 10 - 20% of the time which would be acceptable. The data is capturing user actions so I can not send any earlier than leaving the page. I'm wondering if I am doing something obviously wrong in my code. Any help is greatly appreciated.

function addEvent(){
    var myEvent = window.attachEvent || window.addEventListener;
    var chkevent = window.attachEvent ? 'onbeforeunload' : 'beforeunload'; /// make IE7, IE8 compitable
    myEvent(chkevent, function(e) { // For >=IE7, Chrome, Firefox
        if(e || window.event){
            pageUnload();
        }
    });
}
addEvent();

function pageUnload(){
    var host = window.location.host;
    var path = window.location.pathname;
    window.parent.data = '';
    for (var i = 0; i < arrayCount; i++){
        for (var j = 0; j < dataArray[i].length; j++){
            window.parent.data += dataArray[i][j];
        }
    }
    var urlString = 'h='+host+'&p='+path+'&'+window.parent.data;
    var url = 'http://www.AWS_SQS_QUEUE.com?Action=SendMessage&MessageBody='+encodeURIComponent(urlString);
    var method = 'HEAD';//Changed from GET to HEAD to avoid Cross Domain blocking in FF
    if (navigator.sendBeacon) {
        navigator.sendBeacon(url, null);    
    }
    else{
        var xhr = new XMLHttpRequest();
        if("withCredentials" in xhr) {
            xhr.open(method, url, true);
        }
        else if (typeof XDomainRequest != "undefined"){
            xhr = new XDomainRequest();
            xhr.open(method, url);
        }
        else {
            xhr = null;
        }
        if (!xhr){
            return;
        }
        xhr.send();
    }
}

Answer 1

Try setting the third parameter on xhr.open() to false. This should cause the call to be blocking, preventing the page from closing too fast which stops the request from being made.