code: function setupEvent(element, eType, handler, capture) {
    notifyOfNextSubs[eType] = [];
    subsIDs[eType] = {};
    element.addEventListener(
      eType,
      handler,
      passiveEventSupport
        ? {
            capture,
            // don't set click listener as passive because syntheticClick may call preventDefault
            passive: eType !== 'click',
          }
        : capture,
    );
  }
docs: setup event listeners and notification system for events