record screen in firefox.js

function wackRecording() {
  if (!navigator.mediaDevices) {
    console.error('getUserMedia not supported');
    return Promise.reject();
  }

  const possibleCodecs = [
    'video/webm',
    'video/mp4',
  ];

  const constraints = {
    video: {
      mediaSource: 'screen',
    },
  };

  const codec = possibleCodecs.find(c => MediaRecorder.isTypeSupported(c));
  if (!codec) {
    console.error('no codec supported');
    return Promise.reject();
  }
  console.log('using', codec);

  return navigator.mediaDevices.getUserMedia(constraints)
    .then((stream) => {
      console.log('got stream');
      const mediaRecorder = new MediaRecorder(stream, { mimeType: codec });

      const recording = {
        chunks: [],
      };

      recording.stop = () => {
        recording.stop = () => {};
        mediaRecorder.stop();
        console.log('recorder stopped:', mediaRecorder.state);
      };

      mediaRecorder.ondataavailable = (e) => {
        recording.chunks.push(e.data);
      };

      mediaRecorder.onstop = () => {
        const blob = new Blob(recording.chunks, { type: codec });
        const url = URL.createObjectURL(blob);
        const link = document.createElement('a');
        document.body.appendChild(link);
        link.href = url;
        link.download = 'recording.webm';
        link.click();
        URL.revokeObjectURL(link);
      };

      mediaRecorder.start();
      console.log(mediaRecorder.state);
      console.log('recorder started:', mediaRecorder.state);

      return recording;
    })
    .catch((err) => {
      console.log(`The following error occurred: ${err}`);
      return Promise.reject();
    });
}

(async () => {
  window.recording = await wackRecording();
  console.log('use window.recording.stop() to stop and save recording!');
})();