<button id="presentBtn" style="display: none;">Present</button>
<script>
  / The Present button is visible if at least one presentation display is available
  const presentBtn = document.getElementById("presentBtn");

  / It is also possible to use relative presentation URL e.g. "presentation.html"
  const presUrls = [
    "https://example.com/presentation.html",
    "https://example.net/alternate.html",
  ];

  / Show or hide present button depending on display availability
  const handleAvailabilityChange = (available) => {
    presentBtn.style.display = available ? "inline" : "none";
  };

  / Promise is resolved as soon as the presentation display availability is known.
  const request = new PresentationRequest(presUrls);
  request
    .getAvailability()
    .then((availability) => {
      / availability.value may be kept up-to-date by the controlling UA as long
      / as the availability object is alive. It is advised for the web developers
      / to discard the object as soon as it's not needed.
      handleAvailabilityChange(availability.value);
      availability.onchange = () => {
        handleAvailabilityChange(availability.value);
      };
    })
    .catch(() => {
      / Availability monitoring is not supported by the platform, so discovery of
      / presentation displays will happen only after request.start() is called.
      / Pretend the devices are available for simplicity; or, one could implement
      / a third state for the button.
      handleAvailabilityChange(true);
    });
</script>

<script>
  presentBtn.onclick = () => {
    / Start new presentation.
    request
      .start()
      / The connection to the presentation will be passed to setConnection on success.
      .then(setConnection);
    / Otherwise, the user canceled the selection dialog or no screens were found.
  };
</script>

<button id="reconnectBtn" style="display: none;">Reconnect</button>
<script>
  const reconnect = () => {
    / read presId from localStorage if exists
    const presId = localStorage["presId"];
    / presId is mandatory when reconnecting to a presentation.
    if (presId) {
      request
        .reconnect(presId)
        / The new connection to the presentation will be passed to
        / setConnection on success.
        .then(setConnection);
      / No connection found for presUrl and presId, or an error occurred.
    }
  };
  / On navigation of the controller, reconnect automatically.
  document.addEventListener("DOMContentLoaded", reconnect);
  / Or allow manual reconnection.
  reconnectBtn.onclick = reconnect;
</script>

<script>
  navigator.presentation.defaultRequest = new PresentationRequest(presUrls);
  navigator.presentation.defaultRequest.onconnectionavailable = (evt) => {
    setConnection(evt.connection);
  };
</script>

<button id="disconnectBtn" style="display: none;">Disconnect</button>
<button id="stopBtn" style="display: none;">Stop</button>
<button id="reconnectBtn" style="display: none;">Reconnect</button>
<script>
  let connection;

  / The Disconnect and Stop buttons are visible if there is a connected presentation
  const stopBtn = document.querySelector("#stopBtn");
  const reconnectBtn = document.querySelector("#reconnectBtn");
  const disconnectBtn = document.querySelector("#disconnectBtn");

  stopBtn.onclick = () => {
    connection?.terminate();
  };

  disconnectBtn.onclick = () => {
    connection?.close();
  };

  function setConnection(newConnection) {
    / Disconnect from existing presentation, if not attempting to reconnect
    if (
      connection &&
      connection !== newConnection &&
      connection.state !== "closed"
    ) {
      connection.onclose = undefined;
      connection.close();
    }

    / Set the new connection and save the presentation ID
    connection = newConnection;
    localStorage["presId"] = connection.id;

    function showConnectedUI() {
      / Allow the user to disconnect from or terminate the presentation
      stopBtn.style.display = "inline";
      disconnectBtn.style.display = "inline";
      reconnectBtn.style.display = "none";
    }

    function showDisconnectedUI() {
      disconnectBtn.style.display = "none";
      stopBtn.style.display = "none";
      reconnectBtn.style.display = localStorage["presId"] ? "inline" : "none";
    }

    / Monitor the connection state
    connection.onconnect = () => {
      showConnectedUI();

      / Register message handler
      connection.onmessage = (message) => {
        console.log(`Received message: ${message.data}`);
      };

      / Send initial message to presentation page
      connection.send("Say hello");
    };

    connection.onclose = () => {
      connection = null;
      showDisconnectedUI();
    };

    connection.onterminate = () => {
      / Remove presId from localStorage if exists
      delete localStorage["presId"];
      connection = null;
      showDisconnectedUI();
    };
  }
</script>

const addConnection = (connection) => {
  connection.onmessage = (message) => {
    if (message.data === "Say hello") connection.send("hello");
  };
};

navigator.presentation.receiver.connectionList.then((list) => {
  list.connections.forEach((connection) => {
    addConnection(connection);
  });
  list.onconnectionavailable = (evt) => {
    addConnection(evt.connection);
  };
});

<script>
  connection.send('{"string": "你好，世界!", "lang": "zh-CN"}');
  connection.send('{"string": "こんにちは、世界!", "lang": "ja"}');
  connection.send('{"string": "안녕하세요, 세계!", "lang": "ko"}');
  connection.send('{"string": "Hello, world!", "lang": "en-US"}');
</script>

<script>
  connection.onmessage = (message) => {
    const messageObj = JSON.parse(message.data);
    const spanElt = document.createElement("SPAN");
    spanElt.lang = messageObj.lang;
    spanElt.textContent = messageObj.string;
    document.body.appendChild(spanElt);
  };
</script>