const base = { greet() { return 'hi'; } };
const obj = Object.create(base, {
  x: { value: 1, enumerable: true },
  y: { value: 2, enumerable: false }
});
const spread = { ...obj };
const assigned = Object.assign({}, obj);
console.log(
  spread.x,
  spread.y,
  spread.greet,
  assigned.y,
  Object.keys(spread).length
);

const wm = new WeakMap();
class Counter {
  #id;
  static #nextId = 0;
  constructor() {
    this.#id = Counter.#nextId++;
    wm.set(this, { count: 0, id: this.#id });
  }
  inc() {
    const data = wm.get(this);
    data.count++;
    return data.count;
  }
}
const a = new Counter();
const b = new Counter();
a.inc(); a.inc();
b.inc();
console.log(`${a.inc()} ${b.inc()} ${wm.has(a)} ${wm.get(a).id} ${wm.get(b).id}`);