Licensed under CC0 (https://creativecommons.org/publicdomain/zero/1.0/deed)

twitch-speakout.js

window.addEventListener('message', function(e) {
  const data = e.data || {};
  if (data.type !== 'item')
    return;
  const message = data.message || {};
  const obj = { detail: { command: message.command, body: message.body, from: message.from }};
  
  const synth = window.speechSynthesis;
  if (obj.detail.command === 'PRIVMSG') {
    var utter = new SpeechSynthesisUtterance(obj.detail.body);
    utter.voice = synth.getVoices().filter(v => v.lang === 'ja-JP')[0];
    utter.rate = 1;
    utter.pitch = 1;
    synth.speak(utter);
  }
});

If it doesn't work use the following instead;
(Released under CC0)

window.addEventListener('message', function(e) {
  const data = e.data || {};
  if (data.type !== 'item')
    return;
  const message = data.message || {};
  const obj = { detail: { command: message.command, body: message.body, from: message.from }};
  
  const synth = window.speechSynthesis;
  if (obj.detail.command === 'PRIVMSG') {
    var utter = new SpeechSynthesisUtterance(obj.detail.body);
    utter.voice = synth.getVoices().filter(v => v.lang === 'ja-JP')[0];
    utter.rate = 1;
    utter.pitch = 1;
    synth.speak(utter);
  }
});

EDIT: Applied above to the gist. Previous version can be seen at https://gist.github.com/mecab/a14a761d76a9136a96466b0fc4c2acc3/e93bca2e683b30ca2ccbb03702972be20734012f

Thank you for releasing this amazing code under CC0.
When I implement this codes into Chat Box in Streamlabs, it reads out each comment three times repeatedly.
Do you know how to fix it?

@yuichitanakadesu Sorry for the very late reply. I got some reports that recent OBS seems to read comments by itself (i.e., the browser inside OBS can use speech API), I haven't confirmed yet though. In this case, you won't need the external browser anymore so please just close it.