const target = { name: 'John', age: 30 };
const handler = {
  get(obj, prop) {
    if (prop in obj) {
      return `[${obj[prop]}]`;
    }
    return `missing: ${prop}`;
  },
  set(obj, prop, value) {
    obj[prop] = value.toUpperCase();
    return true;
  }
};
const proxy = new Proxy(target, handler);
proxy.city = 'paris';
console.log(proxy.name);
console.log(proxy.city);
console.log(proxy.country);