var xmlHttp;
try {
    // Firefox, Opera 8.0+, Safari
    xmlHttp = new XMLHttpRequest();
}
catch (e) {
    // Internet Explorer
    try {
        xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
    }
    catch (e) {
        try {
            xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        catch (e) {
            alert("Your browser does not support AJAX!");
        }
    }
}