HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>addtrack addTrack</title>
</head>
<body>
  <button id="run">Start camera</button>
  <pre id="out">Click Start (allow camera).</pre>
  <script>
    document.getElementById("run").addEventListener("click", function () {
      var out = document.getElementById("out");
      if (!window.isSecureContext || !navigator.mediaDevices) {
        out.textContent = "Need secure context + mediaDevices.";
        return;
      }
      var composed = new MediaStream();
      var log = [];
      composed.addEventListener("addtrack", function (event) {
        log.push(event.track.kind);
      });
      navigator.mediaDevices
        .getUserMedia({ video: true })
        .then(function (source) {
          composed.addTrack(source.getVideoTracks()[0]);
          out.textContent = JSON.stringify(log);
          source.getTracks().forEach(function (t) { t.stop(); });
        })
        .catch(function (err) {
          out.textContent = "Error: " + err.message;
        });
    });
  </script>
</body>
</html>

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addtrack — with addTrack (Try It)