The port read-only property of the MIDIConnectionEvent interface returns the port that has been disconnected or connected.

A MIDIPort object.

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

navigator.requestMIDIAccess().then((access) => {
  access.onstatechange = (event) => {
    console.log(event.port.name, event.port.manufacturer, event.port.state);
  };
});