Getting data over Bluetooth and saving it to a file

I have been trying to connect my Thunderboards to my raspberry pi. I have successfully done this with a nodejs nodual but I can't get to save to the same file sequentially.

The final goal is to connect to 3 Thunderboards of known ID sequentially, get the data from them and save it to a text file. (connect to 1 save data, connect to 2 save data, connect to 3 save data and repeat). I know the full GATT database of uuid's for the services and characteristics.

Here is my code. This finds a thuderboard and then opens a file and saves to it. However, it overwrites it every time. How can I get it to append the file every time?

 var noble = require('noble');

    var ThunderboardReact = require('node-thunderboard-react');
    var thunder = new ThunderboardReact({'noble': noble});
    var fs = require('fs');
    var path = "test.txt";
    var position= 0;


    thunder.init((error) => {
    thunder.startDiscovery((device) => {
     console.log('- Found ' + device.localName);
     thunder.stopDiscovery();
     device.connect((error) => {
      console.log('- Connected ' + device.localName);
      startMonitorOrientation(device);
     });
     });
    });

    // Monitor the sensored data
    function startMonitorOrientation(device) 
    {
    // Start to monitor the orientation of the device
    device.startMonitorOrientation((error) => {
    if(error) {
      console.log(error.toString());
      process.exit();
    }
});

        // Set a listener for orientation events fired on the ThunderboardReactDevice object

        device.on('orientation', (res) => 
        {
        fs.open(path, 'r+', function(err, fd) 
        {
            if (err) 
            {
                return console.error(err);
            }
            // Show the event data
            console.log('- Orientation:');
            console.log('  - alpha :' + res.alpha + '°');
            console.log('  - beta  :' + res.beta + '°');
            console.log('  - gamma :' + res.gamma + '°');


            fs.writeSync(fd, 'alpha: ');
            fs.writeSync(fd, (res.alpha).toString()); 
            fs.writeSync(fd, '°  beta: ');
            fs.writeSync(fd, (res.beta).toString()); 
            fs.writeSync(fd, '°  gamma: ');
            fs.writeSync(fd, (res.gamma).toString()); 
            fs.writeSync(fd, '° ');

            fs.close(fd, function(err)
            {
                if (err)
                {
                    console.log(err);
                }
            }); 
        });
    });
}

I have read all the relevant sections nodejs.org including all of the fs section.

I would rather do this in C but I have had no luck when searching through people.csail.mit.edu/albert/bluez-intro. All the examples and tutorials I don't think are relevant.

via Kieron