Danke für die fundierte Analyse!

Ich denke, dementsprechend hätten wir es tatsächlich mit einem Singleton zu tun.

Lässt es sich als Best Practice werten, hier eine entsprechend instanzenlose Singleton-Klasse zu kreieren? Würde ich diese dann einfach an Subklassen "durchreichen", frei nach dem [etwas verbosen] Motto:

/* ~~~~ ACTION! (GET TO DA CHOPPA) ~~~~ */

const myClassInstance = new MyClass(Singleton);
myClassInstance.createSubclassInstances(10); // creating 10 instances of SubClass (see below)
myClassInstance.mutateSharedSingleton("myValue"); // "myValue" could be any value: String | Number | Array | Object | Boolean
//
const mySubClassInstance = new SubClass(Singleton);
// !!!!!!!!!!!!!!!
// BOTH methods should now point to the SAME "myValue" (declared above):
myClassInstance.getSharedSingleton();
mySubClassInstance.getSharedSingleton();
//

/* ~~~~ CLASSES! ~~~~ */

class Singleton {
  static #singleton = "anyValue"; /* String | Number | Array | Object | Boolean */
  static set mutateSingleton(val) {
    /* do something with #singleton */
    Singleton.#singleton = val; /* | Singleton.#singleton.push(val); | (...) */
  }
  static get getSingleton() {
    return Singleton.#singleton;
  }
}

class MyClass {
  #sharedSingleton;
  #subclassArray = []; /* dependency with subclasses, to make matters even nicer and crispier 😏 */
  constructor(sharedSingleton) {
    this.#sharedSingleton = sharedSingleton;
  }
  mutateSharedSingleton(val) {
    this.#sharedSingleton.mutateSingleton(val);
  }
  getSharedSingleton() {
    return this.#sharedSingleton.getSingleton();
  }
  /* dependency with subclasses, to make matters even nicer and crispier 😏 */
  createSubclassInstances(num) {
    for (let i = 0; i <= num; i++) {
      const subClassInstance = new SubClass(this.#sharedSingleton);
      this.#subclassArray.push(subClassInstance);
    }
  }
  getSubclassInstances(/* FURTHER... */) {
    /* ...LOGIC */
  }
  treatSubclassInstances(/* FURTHER... */) {
    /* ...LOGIC */
  }
}

class SubClass {
  #sharedSingleton;
  constructor(sharedSingleton) {
    this.#sharedSingleton = sharedSingleton;
  }
  mutateSharedSingleton(val) {
    this.#sharedSingleton.mutateSingleton(val);
  }
  getSharedSingleton() {
    return this.#sharedSingleton.getSingleton();
  }
  /* More Code */
}

...? Danke nochmal für eure Gedanken dazu!

LG, Cornwall Peter