Chrome: to play a video that is being downloaded via fetch/XHR

Question

What I'm trying to achieve is to make Chrome load a video file as data (via the Fetch API, XHR, whatever) and to play it using <video> while it's still being downloaded without issuing two separate requests for the same URL and without waiting until the file is completely downloaded.

It's easy to get a ReadableStream from the Fetch API (response.body), yet I can't find a way to feed it into the video element. I've figured out I need a blob URL for this, which can be created using a MediaSource object. However, the SourceBuffer#appendStream method, which sounds like just what is needed, isn't implemented in Chrome, so I can't connect the stream directly to the MediaSource object.

I can probably read the stream in chunks, create Uint8Arrays out of them, and use SourceBuffer#appendBuffer, but this means playback won't start immediately unless the chunk size is really small. Also it feels like manually doing something that all these APIs should be able to do out of the box. If there is no other solutions, and I go this way, what caveats should I expect?

Are there probably other ways to create a blob URL for a ReadableStream? Or is there a way to make fetch and <video> share a request? There are so many new APIs that I could easily miss something.

Answer 1

After hours of experimenting, found a half-working solution:

const video = document.getElementById('audio');

const mediaSource = new MediaSource();

video.src = window.URL.createObjectURL(mediaSource);

mediaSource.addEventListener('sourceopen', async () => {

  const sourceBuffer = mediaSource.addSourceBuffer('audio/webm; codecs="opus"');

  const response = await fetch(audioSRC);

  const body = response.body

  const reader = body.getReader()

  let streamNotDone = true;

  while (streamNotDone) {

    const {value, done} = await reader.read();
    
    if (done) {streamNotDone = false; break;}
    
    await new Promise((resolve, reject) => {
      sourceBuffer.appendBuffer(value)

      sourceBuffer.onupdateend = (() => {
        resolve(true);
      })
    }) 

  }
});

It works with https://developer.mozilla.org/en-US/docs/Web/API/MediaSource

Also, I tested this only with webm/opus format but I believe it should work with other formats as well as long as you specify it.