navigator.bluetooth.requestDevice({
  filters: [{
    services: [TOGGLE_LIGHT],
  }]
})
.then(device => 
  device.gatt.connect())
.then(server => 
  server.getPrimaryService(TOGGLE_LIGHT))
.then(service => 
  service.getCharacteristic(TOGGLE_LIGHT_CH))
.then(characteristic =>
  characteristic.readValue()
.then(value => 
  console.log((new DataView(value)).getUint8(0) 
    ? 'On' 
    : 'Off'
  )
);

// get characteristic using the standard flow
characteristic.startNotifications().then(_ => {
  characteristic.addEventListener(
    'characteristicvaluechanged',
    handleNewValue // function
  );
});

// to stop
characteristic.stopNotifications().then(_ => {
  characteristic.removeEventListener(
    'characteristicvaluechanged',
    handleNewValue // function
  );
});

// this time service and characteristic are known
var phone = new window.GATT.Device();
var battery = phone.register(
  'battery_service', 'battery_level'
); 

// subscribe to the event
button.addEventListener('click', function() {
  battery.getCharacteristic().then(function(ch) {
    ch.startNotifications().then(function(){
      ch.addEventListener(
        'characteristicvaluechanged', printValue
      );
    });
  });
});

// update the progress bar
function printValue(event) {
  updateProgessBar(event.target.value.getUint8(0));
}

function directions() {
  move(new Uint8Array(MOVE_SEQUENCE(
    0,
    key.isPressed('W') ? 20 : (key.isPressed('S') ? -20 : 0),
    key.isPressed('D') ? 20 : (key.isPressed('A') ? -20 : 0),
    0
  ))).then(function() {
    if(!isLanding) {
      setTimeout(directions, 50);
    }
  });
}

takeoff.addEventListener('click', function() {
  performAuthenticationAndTakeOff()    
  .then(function() {
    setTimeout(directions, 50);
  });
});

land.addEventListener('click', function() {
  isLanding = true;
  handshake(new Uint8Array(LAND_SEQUENCE()));
});

navigator.usb.getDevices().then(devices => {
  devices.map(device => {
    console.log(device.productName);  
    console.log(device.manufacturerName);
  });
})