bluetooth midi reconnection (pairing)

Yes - there are a few options, and one or other of them might work for you...

• You can enable a passkey, which will itself force pairing: http://www.espruino.com/BLE+Security#passkey-pin-pairing
  
• You can make the Puck try and request bonding with NRF.startBonding: http://www.espruino.com/Reference#l_NRF_startBonding
  
• I guess you're using the ble_midi module based on your other posts, but if you're calling NRF.setServices yourself like you do in https://forum.espruino.com/conversations/386071/#comment16960253 you can add a 'security' field to the charactertistic (http://www.espruino.com/Reference#l_NRF_setServices) that allows you to say it cal only be accessed over an encrypted connection, which will in turn force the Mac to pair.