Reading brain waves with Espruino

So did you try any other characteristics, or just 039afff8-2c94-11e3-9e06-0002a5d5c51b?

To write data, you'd need to do something like :

var service; 
// ...
}).then(function(g) {
  console.log("Connected");
  gatt = g;
  return gatt.getPrimaryService(
    "039afff0-2c94-11e3-9e06-0002a5d5c51b");
}).then(function(s) {
  service = s;
  return service.getCharacteristic(
    "039afff8-2c94-11e3-9e06-0002a5d5c51b");
}).then(function (c) {
  console.log("Got Characteristic");
  characteristic = c;
  characteristic.on('characteristicvaluechanged', function(event) {
     processBrainData(event.target.value.buffer);
  });
  return characteristic.startNotifications();
}).then(function() {
  return service.getCharacteristic("039affa0-2c94-11e3-9e06-0002a5d5c51b"); //<--- might be a different one
}).then(function() {
  return c.writeValue([0x04]);
}).then(function() {
  console.log("Connected.");
  enableOutput(buf1);
});

But as they have developer docs, I wonder whether you could actually just get in touch with them and ask them if they have an explanation of their Bluetooth protocol?