battery.js

function getBatteryLevel() {
  navigator.bluetooth
    .requestDevice({ filters: [{ services: ["battery_service"] }] })
    .then(device => device.gatt.connect())
    .then(server => server.getPrimaryService("battery_service"))
    .then(service => service.getCharacteristic("battery_level"))
    .then(characteristic => characteristic.readValue())
    .then(value => {
      console.log(`> Battery Level is ${value.getUint8(0)}%`);
    })
    .catch(error => {
      console.log("Something went wrong", error);
    });
}