broadcastch

<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

</body>
<script>
    const bc = new BroadcastChannel("bc");

    bc.onmessage = (event) => {
        if (event.data === "Am I the first?") {
            bc.postMessage(`No you're not.`);
            alert("Another tab of this site just got opened");
        }
        if (event.data === `No you're not.`) {
            alert("An instance of this site is already running");
        }
    };

    bc.postMessage('Am I the first?');
</script>

</html>