title: Upload progress indicators for fetch?
body: <p>I'm struggling to find documentation or examples of implementing an upload progress indicator using <a href="https://github.com/github/fetch" rel="noreferrer">fetch</a>.</p>
  <p><a href="https://jakearchibald.com/2015/thats-so-fetch/" rel="noreferrer">This is the only reference I've found so far</a>, which states:</p>
  <blockquote>
  <p>Progress events are a high level feature that won't arrive in fetch for now. You can create your own by looking at the <code>Content-Length</code> header and using a pass-through stream to monitor the bytes received.</p>
  <p>This means you can explicitly handle responses without a <code>Content-Length</code> differently. And of course, even if <code>Content-Length</code> is there it can be a lie. With streams you can handle these lies however you want.</p>
  </blockquote>
  <p>How would I write &quot;a pass-through stream to monitor the bytes&quot; sent? If it makes any sort of difference, I'm trying to do this to power image uploads from the browser to <a href="http://cloudinary.com/documentation/image_upload_api_reference" rel="noreferrer">Cloudinary</a>.</p>
  <p><strong>NOTE</strong>: I am <em><strong>not</strong></em> interested in the <a href="https://github.com/cloudinary/cloudinary_js" rel="noreferrer">Cloudinary JS library</a>, as it depends on jQuery and my app does not. I'm only interested in the stream processing necessary to do this with native javascript and Github's <code>fetch</code> polyfill.</p>
  <hr />
  <p><a href="https://fetch.spec.whatwg.org/#fetch-api" rel="noreferrer">https://fetch.spec.whatwg.org/#fetch-api</a></p>