index.js

var noble = require('noble-mac'); //change to require('noble') for non-macOS 

noble.on('stateChange', function(state) {
  if (state === 'poweredOn') {
    // Seek for peripherals broadcasting the heart rate service
    // This will pick up a Polar H7 and should pick up other ble heart rate bands
    // Will use whichever the first one discovered is if more than one are in range
    noble.startScanning(["180d"]);
  } else {
    noble.stopScanning();
  }
});

noble.on('discover', function(peripheral) {
  // Once peripheral is discovered, stop scanning
  noble.stopScanning();

  // connect to the heart rate sensor
  peripheral.connect(function(error){
    // 180d is the bluetooth service for heart rate:
    // https://developer.bluetooth.org/gatt/services/Pages/ServiceViewer.aspx?u=org.bluetooth.service.heart_rate.xml
    var serviceUUID = ["180d"];
    // 2a37 is the characteristic for heart rate measurement
    // https://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.heart_rate_measurement.xml
    var characteristicUUID = ["2a37"];

    // use noble's discoverSomeServicesAndCharacteristics
    // scoped to the heart rate service and measurement characteristic
    peripheral.discoverSomeServicesAndCharacteristics(serviceUUID, characteristicUUID, function(error, services, characteristics){
      characteristics[0].notify(true, function(error){
        characteristics[0].on('data', function(data, isNotification){
          // Upon receiving data (array of bytes), decode the HR (in bpm) and the RRIs included
          //console.log('length: ' + data.length)
          //console.log('flags: ' + data[0]); //(conv this to binary) see: https://developer.bluetooth.org/gatt/characteristics/Pages/CharacteristicViewer.aspx?u=org.bluetooth.characteristic.heart_rate_measurement.xml
          //console.log('bpm: ' + data[1]);
          var i; RRITxt = ''//'RRI: '
		  for (i = 2; i < data.length; i+=2) { //loop over RRI entries (2 indicies per val)
	          RRITxt += ' ' + (data[i] + 256*data[i+1]); //2nd bit is most significant
	      }
	      console.log(RRITxt)
        });
      });
    });
  });
});