const handler = {
  get(target, prop, receiver) {
    if (prop === 'fullName') {
      return `${Reflect.get(target, 'firstName', receiver)} ${Reflect.get(target, 'lastName', receiver)}`;
    }
    return Reflect.get(target, prop, receiver);
  },
  set(target, prop, value, receiver) {
    if (typeof value !== 'string') {
      return false;
    }
    return Reflect.set(target, prop, value.trim(), receiver);
  },
  has(target, prop) {
    return prop.startsWith('_') ? false : Reflect.has(target, prop);
  }
};

const person = new Proxy({ firstName: '  Clara', lastName: 'Oswald  ', _secret: 'hidden' }, handler);
person.firstName = '  Clara';
person.lastName = '  Oswald';

console.log(person.fullName);
console.log('_secret' in person);
console.log(Reflect.ownKeys(person).length);