download blob

download-blob.js

(async () => {
  const video = document.querySelector('video');
  const stream = video.captureStream();
  const recorder = new MediaRecorder(stream);
  const chunks = [];

  recorder.ondataavailable = e => chunks.push(e.data);
  recorder.onstop = async () => {
    const blob = new Blob(chunks, { type: 'video/webm' });
    const url = URL.createObjectURL(blob);
    const a = document.createElement('a');
    a.href = url;
    a.download = 'video.webm';
    a.click();
  };

  recorder.start();
  video.play();
  setTimeout(() => recorder.stop(), video.duration * 1000);
})();