Registering event handlers with jQuery

We are following jQuery best practice and using the jQuery document ready function (that is, $(document).ready(...)) to register the event handlers for our web page's slider control and initialize our web page elements:

    $(document).ready(function() {
        // Event listener for Slider value changes.
        $("input[type=range].brightnessLevel")
            .on('input', function() {                    // (7)
               brightness_level = $(this).val();         // (8)
               payload = { "level": brightness_level }   // (9)
               postUpdate(payload);
            });

        // Initialize slider value form state on server.
        getState()                                       // (10)
    });
    </script>
</head>

On line (7), we register an event handler for the slider controls input event. This handler function will be called when a user interacts with the slider on...