code: function (e) {
          var delta = e.loaded - progressBefore;
          bar.value += delta;
  
          progressBefore = e.loaded;
        }
docs: /*
  it sends (while sending one chunk) for each event of the process such an event
  (put on throttle in chrome and look at it)
  it tells you how many bytes it has .loaded from the .total of the bytes of the chunk