const target = { a: 1, b: 2 };
const handler = {
  get(obj, prop) {
    return prop in obj ? obj[prop] * 2 : 'not found';
  },
  set(obj, prop, value) {
    if (typeof value !== 'number') {
      return false;
    }
    obj[prop] = value + 10;
    return true;
  }
};

const proxy = new Proxy(target, handler);
proxy.c = '5';
proxy.d = 5;

console.log(JSON.stringify({
  a: proxy.a,
  b: proxy.b,
  c: proxy.c,
  d: target.d,
  hasC: Reflect.has(target, 'c')
}));