PermissionStatus.onchange = function() { ... }
PermissionStatus.addEventListener('change', function() { ... })

navigator.permissions.query({name:'geolocation'}).then(function(permissionStatus) {
  console.log('geolocation permission state is ', permissionStatus.state);
  permissionStatus.onchange = function() {
    console.log('geolocation permission state has changed to ', this.state);
  };
});