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<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>id — unique</title>
</head>
<body>
  <pre id="out"></pre>
  <script>
    var canvas = document.createElement("canvas");
    var first = canvas.captureStream();
    var second = canvas.captureStream();
    document.getElementById("out").textContent =
      "first: " + first.id + "\n" +
      "second: " + second.id + "\n" +
      "same: " + (first.id === second.id);
  </script>
</body>
</html>

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MediaStream.id — unique per stream (Try It)