Presentation API

<button id="presentBtn" class="hidden">Present</button>

.hidden {
  display: none;
}

// 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) => {
  if (available) {
    presentBtn.classList.remove("hidden");
  } else {
    presentBtn.classList.add("hidden");
  }
};

// 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);
  });

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.
};

<button id="reconnectBtn" class="hidden">Reconnect</button>

const reconnect = () => {
  const presId = localStorage.getItem("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;

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

<button id="disconnectBtn" class="hidden">Disconnect</button>
<button id="stopBtn" class="hidden">Stop</button>
<button id="reconnectBtn" class="hidden">Reconnect</button>

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.setItem("presId", connection.id);

  function showConnectedUI() {
    // Allow the user to disconnect from or terminate the presentation
    stopBtn.classList.remove("hidden");
    disconnectBtn.classList.remove("hidden");
    reconnectBtn.classList.add("hidden");
  }

  function showDisconnectedUI() {
    disconnectBtn.classList.add("hidden");
    stopBtn.classList.add("hidden");
    if (localStorage.getItem("presId")) {
      // If there is a presId in localStorage, allow the user to reconnect
      reconnectBtn.classList.remove("hidden");
    } else {
      reconnectBtn.classList.add("hidden");
    }
  }

  // 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 = () => {
    localStorage.removeItem("presId");
    connection = null;
    showDisconnectedUI();
  };
}

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);
  };
});

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"}');

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);
};