http://blog.juliencrouzet.fr/2014/02/xmlhttprequest-et-le-streaming-etat-des.html

gistfile1.js

(function(xhr) {
    var request = new xhr();

    request.open('GET', 'http://www.urldemonflux.com/blah', true);
    request.onreadystatechange = function () {
        if (request.readyState === xhr.LOADING) {
            if(request.status === 200) {
                window.console.log(request.responseText);
            }
        }
    };
    request.send(null);
})(window.XMLHttpRequest);