Proxy — Intercepta Operaciones de Objetos

const handler = {
  get(target, prop) {
    return prop in target ? target[prop] : `'${prop}' not found`;
  },
  set(target, prop, value) {
    if (typeof value !== 'number') throw new TypeError('Numbers only!');
    target[prop] = value;
    return true;
  }
};

const stats = new Proxy({}, handler);
stats.hp = 100;
console.log(stats.hp);      // 100
console.log(stats.mana);    // "'mana' not found"
stats.name = 'Josh';        // TypeError: Numbers only!