NRF.on('connect', function(addr) not called ?

Yes, many thanks, well done:

This program displays a red square when Bluetooth connects, and a blue square on disconnection. This works as expected, good.

NRF.on('connect', function(addr) {
  // Red square.
  g.setColor(255,0,0);
  g.fillRect(0,0,20,20);
});

NRF.on('disconnect', function(reason) {
  // Blue square.
  g.setColor(0,0,255);
  g.fillRect(20,0,40,20);
});