Upload progress indicators for fetch?

I'm struggling to find documentation or examples of implementing an upload progress indicator using fetch.

This is the only reference I've found so far, which states:

Progress events are a high level feature that won't arrive in fetch for now. You can create your own by looking at the Content-Length header and using a pass-through stream to monitor the bytes received.

This means you can explicitly handle responses without a Content-Length differently. And of course, even if Content-Length is there it can be a lie. With streams you can handle these lies however you want.

How would I write "a pass-through stream to monitor the bytes" sent? If it makes any sort of difference, I'm trying to do this to power image uploads from the browser to Cloudinary.

NOTE: I am not interested in the Cloudinary JS library, 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 fetch polyfill.

https://fetch.spec.whatwg.org/#fetch-api

Streams are starting to land in the web platform (https://jakearchibald.com/2016/streams-ftw/) but it's still early days.

Soon you'll be able to provide a stream as the body of a request, but the open question is whether the consumption of that stream relates to bytes uploaded.

Particular redirects can result in data being retransmitted to the new location, but streams cannot "restart". We can fix this by turning the body into a callback which can be called multiple times, but we need to be sure that exposing the number of redirects isn't a security leak, since it'd be the first time on the platform JS could detect that.

Some are questioning whether it even makes sense to link stream consumption to bytes uploaded.

Long story short: this isn't possible yet, but in future this will be handled either by streams, or some kind of higher-level callback passed into fetch().

My solution is to use axios, which supports this pretty well:

axios.request({
    method: "post", 
    url: "/aaa", 
    data: myData, 
    onUploadProgress: (p) => {
      console.log(p); 
      //this.setState({
          //fileprogress: p.loaded / p.total
      //})
    }
}).then (data => {
    //this.setState({
      //fileprogress: 1.0,
    //})
})

I have example for using this in react on github.