var request;
/*
 * doRequest
 * param servletName - Javascript String, Servlet name (as implemented in your config)
 * param servletArguments - Javascript String, any parameters
 */
function doSomeRequest(servletName, servletArguments){
    var servlet = servletName;
    var arg = servletArguments;
    var req = servlet + "?" + arg;

    addRequest(req);
    request.onreadystatechange = function(){
        //do something with the response
    }
}

function addRequest(req) {
    try {   //create a request for netscape, mozilla, opera, etc.
        request = new XMLHttpRequest();
    }catch (e) {

        try {    //create a request for internet explorer
            request = new ActiveXObject("Microsoft.XMLHTTP");
        }catch (e) {   //do some error-handling
            alert("XMLHttpRequest error: " + e);
        }
    }

    request.open("GET", req, true);   //prepare the request
    request.send(null);   //send it
    return request;   //return the request
}