The document ready function

The jQuery document ready callback is where we set up the event handler for the HTML slider:

        $(document).ready(function(){
            // Event listener for Slider value changes.
            $("input[type=range].brightnessLevel")
              .on('input', function(){
                  level = $(this).val();
                  payload = {"level": level};
                  socket.emit('led', payload);         // (6)
            });
        });
    </script>
</head>

On line (6), we see how to emit a message in JavaScript. The call to socket.emit('led', payload) emits a message to the Python server with the brightness level we want to apply to our LED.

It's the Python @socketio.on('led') handler that receives this message and changes the LED's brightness.

If you review this Python handler, you will notice the line: emit("led", state, broadcast=True). This...