Separate a list in two lists using JavaScript

Question

I have a list of events. They are html elements. The premium events have a data attribute in their parent element: data-premium-event.

I need to separate the events in two lists:

1. a list of premium events
2. a list of regular (non-premium) events

I then show oder hide them using the logic in handleVisibleEvents. The method works correctly but I would like to refactor it while it has a high cognitive complexity.

     this.nrOfRegularEventsDisplayed = 0;
     this.nrOfPremiumEventsDisplayed = 0;
     this.regularEventsToShow = [];
     this.premiumEventsToShow = [];
     this.refreshActiveCategoryTags();
     const events = toArray(this.elements['event']);
     if (events) {
         if (this.loadEventsButton) {
             show(this.loadEventsButton);
         }
         events.forEach((event) => {
             if (event.dataset.eventTags.includes(this.activeCategory.dataset.categoryTag)) {
                 if (event.parentElement.getAttribute('data-premium-event') !== null) {
                     this.premiumEventsToShow.push(event);
                 } else {
                     this.regularEventsToShow.push(event);
                 }
             } else {
                 hide(event.parentElement);
             }
         });
         this.handleVisibleEvents(TagsFilteringOverview.VISIBLE_EVENTS, false);
     }

Answer 1

A common name for such operation is partition. An implementation of it might look something like

const fold = to => fun => iterable =>
  Array.prototype.reduce.call(iterable, fun, to)

const partition = pred => fold ([[], []]) ((acc, x) =>
  (acc[pred(x) ? 0 : 1].push(x), acc))

so that given a predicate

const hasPremium = el => el.parentElement.dataset?.premiumEvent

and example data

const data = [
  {parentElement: {dataset: {premiumEvent: true}}}
  , {parentElement: {}}
  , {parentElement: {}}
  , {parentElement: {}}
  , {parentElement: {dataset: {premiumEvent: true}}}
  , {parentElement: {}}
  , {parentElement: {}}
  , {parentElement: {dataset: {premiumEvent: true}}}
  , {parentElement: {dataset: {premiumEvent: true}}}
]

the usage becomes

console.log(partition (hasPremium) (data))