How to make a web app for your own Bluetooth Low Energy device? Step 7

main.js

// Characteristic object cache
let characteristicCache = null;

// Connect to the device specified, get service and characteristic
function connectDeviceAndCacheCharacteristic(device) {
  if (device.gatt.connected && characteristicCache) {
    return Promise.resolve(characteristicCache);
  }

  log('Connecting to GATT server...');

  return device.gatt.connect().
      then(server => {
        log('GATT server connected, getting service...');

        return server.getPrimaryService(0xFFE0);
      }).
      then(service => {
        log('Service found, getting characteristic...');

        return service.getCharacteristic(0xFFE1);
      }).
      then(characteristic => {
        log('Characteristic found');
        characteristicCache = characteristic;

        return characteristicCache;
      });
}