Javascript Checksum Builder

I am in the process of building out an http proxy to C-Bus using my CNI and nodejs (nodejs.org). This will enable to enable me to hit a webpage and control my c-bus stuff, plus give me a platform to weave in my other stuff like sonos and whatnot. Contemplating putting it up on github.

In the process I banged my head against the wall for a while on getting the checksums right for the commands. I figured this code may help someone else who comes along looking for a way to build the command strings they need to write to the CNI socket.

(note, "device" is the hex value of your device, like "0D", "command" is either "on" or "off", I'll support more commands later)

Code:

function cmdString(device,command) {
    var turnon = '05380079';
    var turnoff = '05380001';
    
    if(command=='on') {
        message = turnon + device;
    }
    else {
        message = turnoff + device;
    }

    var sum=0;
    var len=message.length;
    
    for(var i=0;i<len;i=i+2) {
        bt=message.substring(i,i+2);
        ibt=parseInt(bt,16);
        sum=sum+ibt;
    }
    
    var m=(sum % 256);
    var check=(256+((~m)+1)).toString(16).toUpperCase();

    while (check.length < 2) {
        check = "0" + check;
    }
    
    return '\\' + message + check + 'g' + '\r';
}