const handler = {
  get(target, prop, receiver) {
    if (prop in target) {
      const value = Reflect.get(target, prop, receiver);
      return typeof value === "number" ? value * 2 : value;
    }
    return `Property "${prop}" not found`;
  },
  set(target, prop, value) {
    if (typeof value !== "number") {
      throw new TypeError("Only numbers allowed");
    }
    Reflect.set(target, prop, value ** 2);
    return true;
  },
  has(target, prop) {
    return prop.startsWith("_") ? false : prop in target;
  },
};

const store = new Proxy({ _secret: 99, score: 5 }, handler);

store.level = 3;

console.log(store.score);
console.log(store.level);
console.log("_secret" in store);
console.log("score" in store);
console.log(store._secret);