gistfile1.js

function log(s) { console.log(s) }
var myDescriptor;

function onReadButtonClick() {
  let serviceUuid = 0xff40;
  let characteristicUuid = 0xff42;

  log('Requesting any Bluetooth Device...');
  navigator.bluetooth.requestDevice({
   // filters: [...] <- Prefer filters to save energy & show relevant devices.
      acceptAllDevices: true,
      optionalServices: [serviceUuid]})
  .then(device => {
    log('Connecting to GATT Server...');
    return device.gatt.connect();
  })
  .then(server => {
    log('Getting Service...');
    return server.getPrimaryService(serviceUuid);
  })
  .then(service => {
    log('Getting Characteristic...');
    return service.getCharacteristic(characteristicUuid);
  })
  .then(characteristic => {
    log('Getting Descriptor...');
    var buffer = new ArrayBuffer(2);
    var view = new DataView(buffer);
    view.setInt8(0, 21);
    console.log(view.getInt8(0));
    setTimeout(() => {
      characteristic.writeValue(buffer);
      console.log("Writing...");
    }, 4000);
  })
  .catch(error => {
    log('Argh! ' + error);
  });
}

onReadButtonClick()