Example adding a private field to an existing object

add-private-field.js

class Super {
  constructor(obj) {
    return obj;
  }
}

class Adder extends Super {
  #field;
  constructor(obj) { super(obj); }
  static set(obj, val) { obj.#field = val; }
  static get(obj) { return obj.#field; }
}

let x = {};
Adder.get(x); // TypeError
Adder.set(x, 1); // TypeError
new Adder(x);
Adder.get(x); // undefined
Adder.set(x, 1); // undefined
Adder.get(x); // 1