XMLHttpRequest receiving undefined

I'm making a widget on iphone but I can't get data from the url. On IE, I can get data. However, on chrome and on iphone I can't get the data but it only shows undefined instead of data.

function a() {
    var url="www.xxx.xxx";
    var request = new XMLHttpRequest();
    request.open('GET', url, false); 

    request.send();     
    xmlDoc = request.responseXML;
}

please help me!! I'm really appreciated for any answers.