handler.ownKeys()

const monster = {
  _age: 111,
  [Symbol("secret")]: "I am scared!",
  eyeCount: 4,
};
const handler = {
  ownKeys(target) {
    return Reflect.ownKeys(target);
  },
};
const proxy = new Proxy(monster, handler);
for (const key of Object.keys(proxy)) {
  console.log(key);
  // 予想される結果: "_age"
  // 予想される結果: "eyeCount"
}

new Proxy(target, {
  ownKeys(target) {
  }
})

const p = new Proxy(
  {},
  {
    ownKeys(target) {
      console.log("called");
      return ["a", "b", "c"];
    },
  },
);
console.log(Object.getOwnPropertyNames(p));
// "called"
// [ 'a', 'b', 'c' ]

const obj = {};
Object.defineProperty(obj, "a", {
  configurable: false,
  enumerable: true,
  value: 10,
});
const p = new Proxy(obj, {
  ownKeys(target) {
    return [123, 12.5, true, false, undefined, null, {}, []];
  },
});
console.log(Object.getOwnPropertyNames(p));
// TypeError: proxy [[OwnPropertyKeys]] must return an array
// with only string and symbol elements