const handler = {
  get(target, prop, receiver) {
    if (prop in target) {
      return Reflect.get(target, prop, receiver) * 2;
    }
    return `${prop} not found`;
  },
  set(target, prop, value) {
    if (typeof value !== "number") {
      throw new TypeError("Only numbers allowed");
    }
    target[prop] = value + 10;
    return true;
  },
  has(target, prop) {
    return prop.startsWith("x") ? false : prop in target;
  },
};

const obj = new Proxy({ score: 5, xp: 100 }, handler);

obj.level = 3;
console.log(obj.score);
console.log(obj.level);
console.log("xp" in obj);
console.log("score" in obj);
console.log(obj.rank);