viewer.html

<video id="preloadoa" style="visibility: hidden;" preload="auto"></video>
<div id="oa">
<!--ここにオンエア要素が動的生成される-->
</div>
<script>
      const addConnection = (connection) => {};

navigator.presentation.receiver.connectionList.then((list) => {
  list.connections.forEach((connection) => {
    addConnection(connection);
  });
  list.onconnectionavailable = (evt) => {
    addConnection(evt.connection);
  };
});
    var oav = document.getElementById("oa");
    var poav = document.getElementById("preload");
    function prd(ur1){
        poav.src=ur1;
    }
    function change(){
        function change() {
  var oac = document.createElement('video');
  oac.src = poav.src;
  oac.id = 'mplayer';

  navigator.presentation.requestSession().then((session) => {
    session.send(oac);
    oac.play();
    oac.addEventListener('ended', function() {
      endach();
    }, false);
  }).catch((error) => {
    console.error(error.name + ': ' + error.message);
  });
}

    }
    function endach(){
        alert("終了");
    }
</script>