从附近的蓝牙设备读取电池电量（readValue）

function onButtonClick() {

  navigator.bluetooth.requestDevice({filters: [{services: ['battery_service']}]})
  .then(device => {
    // Connecting to GATT Server...
    return device.gatt.connect();
  })
  .then(server => {
    // Getting Battery Service...
    return server.getPrimaryService('battery_service');
  })
  .then(service => {
    // Getting Battery Level Characteristic...
    return service.getCharacteristic('battery_level');
  })
  .then(characteristic => {
    // Reading Battery Level...
    return characteristic.readValue();
  })
  .then(value => {
    let batteryLevel = value.getUint8(0);
    console.log('> Battery Level is ' + batteryLevel + '%');
  })
  .catch(error => {
    console.log('Argh! ' + error);
  });
}