MIDIConnectionEvent

The MIDIConnectionEvent interface of the Web MIDI API is the event passed to the onstatechange event handler of the MIDIAccess interface and the onstatechange event of the MIDIPort interface. This occurs any time a new port becomes available, or when a previously available port becomes unavailable. For example, this event is fired whenever a MIDI device is either plugged in to or unplugged from a computer.  

MIDIConnectionEvent.MIDIConnectionEvent

Creates a new MIDIConnectionEvent object.

MIDIConnectionEvent.portRead only

Returns a reference to a MIDIPort instance for a port that has been connected or disconnected."

The Navigator.requestMIDIAccess() method returns a promise that resolves with a MIDIAccess object. When a port changes state, a MIDIConnectionEvent is passed to MIDIAccess.onstatechange, information about the port can then be printed to the console.

navigator.requestMIDIAccess()
  .then(function(access) {

     access.onstatechange = function(e) {
       console.log(e.port.name, e.port.manufacturer, e.port.state);
     };
  });