Bangle & Arduino communication through Bluetooth?

Not a problem, I learned something while searching for info about MAC-addresses.

Especially for other Bluetooth- and JS-newbies like me, below skeleton JS-code can either show the value received, or a simple graph. Just remove/add comments at the show_data- or graph-lines. And here's the Arduino-code I'm using.

On the JS-side there's for sure room for improvement, one thing that puzzles me is this line g.drawString(" Batt:" + data, 1, g.getHeigth()/2); which should display Batt:nnn where nnn is the value without leading zeros. On the Bangle 2 display is though instead shown b:nnn, so instead of " Batt:" is " b:" shown (lowercase b). The value itself is shown correctly.

var gatt;
var history = new Float32Array(64);


g.clear();

// Load fonts
require("Font7x11Numeric7Seg").add(Graphics);

function graph(data) {
  // quickly move all elements of history back one
  history.set(new Float32Array(history.buffer,4));

  history[history.length-1] = data;
  //
  g.clear();
  // Draw Graph
  var r = require("graph").drawLine(g, history, {
    miny: 26,
    axes : true,
    gridy : 10,
    title: "Battery"
  });
  // Label last reading
  g.setFontAlign(1,-1);
  g.drawString(Math.round(data), r.x+r.w, r.gety(data)+2);
  // Update the screen
  g.flip();
 
}

function show_data(data) {
  var on = true;
  g.clear();
  g.setFont("7x11Numeric7Seg",3);
  g.drawString(" Batt:" + data, 1, g.getHeigth()/2); 

}

NRF.connect("MA:C-:AD:DR:ES:S! public").then(function(g) {
  gatt = g;
  return gatt.getPrimaryService(0x180F);
}).then(function(service) {
  return service.getCharacteristic(0x2A19);
}).then(function(characteristic) {
  characteristic.on('characteristicvaluechanged', function(event) {
    console.log("battery: "+JSON.stringify(event.target.value.buffer));
    
    show_data(event.target.value.buffer);
    //graph(event.target.value.buffer);
  });
  return characteristic.startNotifications();
    
  }).then(function() {
  console.log("Done!");
});