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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>addTrack basic</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var canvas = document.createElement("canvas");
    var source = canvas.captureStream();
    var videoTrack = source.getVideoTracks()[0];
    var stream = new MediaStream();
    stream.addTrack(videoTrack);
    document.getElementById("out").textContent =
      "tracks: " + stream.getTracks().length + "\n" +
      "active: " + stream.active;
  </script>
</body>
</html>

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addTrack() — empty stream (Try It)