twitch-latency.js

// prints how long in the past the current "end" of the stream is
// this is a lower bound on latency that's viewer-independent

// put in the m3u of a stream
// e.g. just open network tab, refresh, then filter for m3u
// don't use the meta one (the first) but the one that's called repeatedly 
const playlistUrl = "";

async function test() {
  const minNow = new Date().getTime();
  const res = await fetch(playlistUrl, {
    headers: {
      "accept": "application/x-mpegURL, application/vnd.apple.mpegurl, application/json, text/plain"
    }
  });

  const text = await res.text();
  
  const dateLine = text.split('\n').filter(l => l.includes('EXT-X-PROGRAM-DATE-TIME:')).at(-1);
  const lengthLine = text.split('\n').filter(l => l.includes('EXTINF:')).at(-1)

  const maxNow = new Date().getTime();
  const date = new Date(dateLine.substring(dateLine.indexOf(':') + 1));
  const pos = lengthLine.indexOf(':');
  const length = Number(lengthLine.substring(pos + 1, lengthLine.indexOf(','))) * 1000;
  const minEndAge = minNow - date.getTime() - length;
  const maxEndAge = maxNow - date.getTime() - length;
  const timestamp = new Date().toISOString();
  return `${timestamp}: stream end is between ${(minEndAge/1000).toFixed(3)} and ${(maxEndAge/1000).toFixed(3)} s in the past`;
}

// run this a few times:
for (let i = 0; i < 200; i++) { setTimeout(() => { test().then(l => console.log(l))}, i * 20)}