@stoprocent/bleno/examples/battery-service/battery-level-characteristic.js

A Node.js module for implementing BLE (Bluetooth Low Energy) peripherals

const os = require('os');
const bleno = require('../..');
const { execSync } = require('child_process');
const Descriptor = bleno.Descriptor;
const Characteristic = bleno.Characteristic;

class BatteryLevelCharacteristic extends Characteristic {
  constructor () {
    super({
      uuid: '2A19',
      properties: ['read'],
      descriptors: [
        new Descriptor({
          uuid: '2901',
          value: 'Battery level between 0 and 100 percent'
        }),
        new Descriptor({
          uuid: '2904',
          value: Buffer.from([0x04, 0x01, 0x27, 0xAD, 0x01, 0x00, 0x00]) // maybe 12 0xC unsigned 8 bit
        })
      ]
    });
  }

  onReadRequest (offset, callback) {
    if (os.platform() === 'darwin') {
      try {
        const { stdout } = execSync('pmset -g batt');
        const data = stdout.toString();
        // data - 'Now drawing from \'Battery Power\'\n -InternalBattery-0\t95%; discharging; 4:11 remaining\n'
        let percent = data.split('\t')[1].split(';')[0];
        console.log(percent);
        percent = parseInt(percent, 10);
        callback(this.RESULT_SUCCESS, Buffer.from([percent]));
      } catch (error) {
        callback(this.RESULT_UNLIKELY_ERROR);
      }
    } else {
      // return hardcoded value
      callback(this.RESULT_SUCCESS, Buffer.from([98]));
    }
  }
}

module.exports = BatteryLevelCharacteristic;