I have to pass some form data in a AJAX call to a HTTP Servlet. I noticed that FormData object seems to be empty and debugging with Chrome the console returns error: failed to load resource.

function makeCall(method, url, formElement, cback, reset = true) {
    var req = new XMLHttpRequest();// visible by closure
    req.onreadystatechange = function() {
        cback(req)
    }; // closure
    req.open(method,url);
    if (formElement == null) {
        req.send();
    } else {
        req.send(new FormData(formElement));
    }
    if (formElement !== null && reset === true) {
        formElement.reset();
    }
}