{
-
  viewof zoomLevel.addEventListener('input', updateDuration('zoomLevel'));
  viewof ageGroup.addEventListener('input', updateDuration('ageGroup'));
+
  viewof zoomLevel.addEventListener('input', e => updateDuration('zoomLevel'));
  viewof ageGroup.addEventListener('input', e => updateDuration('ageGroup'));
}

updateDuration = function(s) {
  if (s == 'zoomLevel') {
-
    duration = 0;
+
    mutable duration = 0;
  }
-
  duration = 600;
+
 else mutable duration = 600;
}

-
duration = 0
+
mutable duration = 0