toJSON.js

class MyClass {
  #somethingCool = 5;
  get somethingCool() {
    return this.#somethingCool; 
  }

  set somethingCool(value) {
    this.#somethingCool = value; 
  }
  toJSON() {
    return {
      somethingCool: this.somethingCool, 
    }
  }
}

const x = new MyClass();
JSON.stringify(x); // '{"somethingCool": 5}'

x.somethingCool = 10;
JSON.stringify(x); // '{"somethingCool": 10}'