JQuery document ready function

Finally, we encounter the document ready function at line (1o) where we initialize our web page content and register the event listener for the slider:

$(document).ready(function() {                                   // (10)
    $("#clientId").html(CLIENT_ID);

    // Event listener for Slider value changes.
    $("input[type=range].brightnessLevel").on('input', function() {
        level = $(this).val();

        payload = {
            "level": level
         };

        // Publish LED brightness.
        var message = new Paho.Message(                         // (11)
           JSON.stringify(payload)
        );

        message.destinationName = TOPIC;                        // (12)
        message.qos = 2;
        message.retained = true;                                // (13)
        client.send(message);
    });
});

Within the sliders event handler at line (11), we see how to create an MQTT message. Notice the use of JSON.stringify...