const handler = {
  get(target, prop, receiver) {
    if (prop in target) {
      return Reflect.get(target, prop, receiver) * 2;
    }
    return `Missing: ${prop}`;
  },
  set(target, prop, value) {
    if (typeof value !== "number") {
      throw new TypeError("Only numbers allowed");
    }
    return Reflect.set(target, prop, Math.abs(value));
  },
};

const store = new Proxy({ gold: 10, silver: 5 }, handler);

store.bronze = -42;

console.log(store.gold);
console.log(store.bronze);
console.log(store.platinum);