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

<head>
    <meta charset="UTF-8">
    <title>Document</title>
    <script src="https://unpkg.com/mqtt/dist/mqtt.min.js"></script>

    <script>
        const client = mqtt.connect("ws://localhost:1884") // you add a ws:// url here
        client.subscribe("mqtt/demo")

        client.on("message", function (topic, payload) {
            console.log("on.message: ", topic, payload)
            client.end()
        })

        client.publish("mqtt/demo", "hello world!")
    </script>
</head>

<body>

</body>

</html>