JQuery ajax progress via xhr

ProgressEvent.lengthComputable

The ProgressEvent.lengthComputable read-only property is a Boolean flag indicating if the resource concerned by the ProgressEvent has a length that can be calculated. If not, the ProgressEvent.total property has no significant value.

So in your case if you debug a little , you will find evt.lengthComputable = false; so you can not trace the progress;

    xhr.addEventListener("progress", function (evt) {
        console.log(evt.lengthComputable); // false
        if (evt.lengthComputable) {
            var percentComplete = evt.loaded / evt.total;
            progressElem.html(Math.round(percentComplete * 100) + "%");
        }
    }, false);

DEMO

FYI

If lengthComputable is false within the XMLHttpRequestProgressEvent, that means the server never sent a Content-Length header in the response.