How can I use deflated/gzipped content with an XHR onProgress function?

I wasn't able to solve the issue of using onProgress on the compressed content itself, but I came up with this semi-simple workaround. In a nutshell: send a HEAD request to the server at the same time as a GET request, and render the progress bar once there's enough information to do so.

---

function loader(onDone, onProgress, url, data)
{
    // onDone = event handler to run on successful download
    // onProgress = event handler to run during a download
    // url = url to load
    // data = extra parameters to be sent with the AJAX request
    var content_length = null;

    self.meta_xhr = $.ajax({
        url: url,
        data: data,
        dataType: 'json',
        type: 'HEAD',
        success: function(data, status, jqXHR)
        {
            content_length = jqXHR.getResponseHeader("X-Content-Length");
        }
    });

    self.xhr = $.ajax({
        url: url,
        data: data,
        success: onDone,
        dataType: 'json',
        progress: function(jqXHR, evt)
        {
            var pct = 0;
            if (evt.lengthComputable)
            {
                pct = 100 * evt.position / evt.total;
            }
            else if (self.content_length != null)
            {
                pct = 100 * evt.position / self.content_length;
            }

            onProgress(pct);
        }
    });
}

And then to use it:

loader(function(response)
{
    console.log("Content loaded! do stuff now.");
},
function(pct)
{
    console.log("The content is " + pct + "% loaded.");
},
'<url here>', {});

---

On the server side, set the X-Content-Length header on both the GET and the HEAD requests (which should represent the uncompressed content length), and abort sending the content on the HEAD request.

In PHP, setting the header looks like:

header("X-Content-Length: ".strlen($payload));

And then abort sending the content if it's a HEAD request:

if ($_SERVER['REQUEST_METHOD'] == "HEAD")
{
    exit;
}

---

Here's what it looks like in action:

The reason the HEAD takes so long in the below screenshot is because the server still has to parse the file to know how long it is, but that's something I can definitely improve on, and it's definitely an improvement from where it was.

A slightly more elegant variation on your solution would be to set a header like 'x-decompressed-content-length' or whatever in your HTTP response with the full decompressed value of the content in bytes and read it off the xhr object in your onProgress handler.

Your code might look something like:

request.onProgress = function (e) {
  var contentLength;
  if (e.lengthComputable) {
    contentLength = e.total;
  } else {
    contentLength = parseInt(e.target.getResponseHeader('x-decompressed-content-length'), 10);
  }
  progressIndicator.update(e.loaded / contentLength);
};