class Foo {
  #foo = 0;

  test(other) {
    return [
      this.#foo++,
      this.#foo,
      ++this.#foo,
      this.#foo,
      other.obj.#foo++,
      other.obj.#foo,
      ++other.obj.#foo,
      other.obj.#foo,
    ];
  }
}

const f = new Foo;
const results = f.test({ obj: f });
expect(results[0]).toBe(0);
expect(results[1]).toBe(1);
expect(results[2]).toBe(2);
expect(results[3]).toBe(2);
expect(results[4]).toBe(2);
expect(results[5]).toBe(3);
expect(results[6]).toBe(4);
expect(results[7]).toBe(4);