Allow other modules to "hook" the JS init and add functionality

Thanks for raising this idea.
As this module has been put into bug fixing only mode, it won't deliver any new feature.

You can try Fullcalendar Block module instead. https://www.drupal.org/project/fullcalendar_block →

Hey, I'm attaching the code that helped me to hook it in case others need something similar.

Additionally, the code below shows how to use the Observer pattern:

  // Alter the Fullcalendar Options before rendering the calendar.
  const fullcalendarViewObserver = {
    update: (calendarOptions) => {
      // Add custom buttons.
      calendarOptions.customButtons = {
        previousMonth: {
          icon: "chevron-left",
          text: "Prev",
          click() {
            // Get some stuff done when users click on Prev button. 
          },
        },
        nextMonth: {
          icon: "chevron-right",
          text: "Next",
          click() {
            // Get some stuff done when users click on Next button. 
          },
        },
      };

      return calendarOptions;
    },
  };

  // Add a custom observer to Fullcalendar view Subject.
  Drupal.FullcalendarViewSubject.addObserver(fullcalendarViewObserver);