class CountableSet<K> extends Set<K> {
  _map: Map<K, number>

  constructor(values?: Iterable<K>) {
    super(values)  // `super` calls `add` which initialize `this._map`
    this._map ??= new Map()
  }

  add(key: K) {
    this._map ??= new Map()
    this._map.set(key, (this._map.get(key) ?? 0) + 1)
    return super.add(key)
  }
}

console.log(new CountableSet(["foo"]))
// expected to output
// CountableSet(1) [Set] { 'foo', _map: Map(1) { 'foo' => 1 } }

class CountableSet extends Set {
    constructor(values) {
        super(values);
        this._map ?? (this._map = new Map());
    }
   ...

class CountableSet extends Set {
        constructor(values) {
                super(values);
                this._map = void 0; 
                this._map ?? (this._map = new Map());
        }
...

console.log(new CountableSet(["foo"]))
// rolldown-vite output
// CountableSet(1) [Set] { 'foo', _map: Map(0) {} }