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Showing content from http://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/ownKeys below:

handler.ownKeys() - JavaScript | MDN

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The handler.ownKeys() method is a trap for the [[OwnPropertyKeys]] object internal method, which is used by operations such as Object.keys(), Reflect.ownKeys(), etc.

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);
  // Expected output: "_age"
  // Expected output: "eyeCount"
}

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

The following parameter is passed to the ownKeys() method. this is bound to the handler.

target

The target object.

The ownKeys() method must return an array-like object where each element is either a String or a Symbol containing no duplicate items.

This trap can intercept these operations:

• Object.getOwnPropertyNames()
• Object.getOwnPropertySymbols()
• Object.keys()
• Reflect.ownKeys()

Or any other operation that invokes the [[OwnPropertyKeys]] internal method.

The proxy's [[OwnPropertyKeys]] internal method throws a TypeError if the handler definition violates one of the following invariants:

• The result is an Object.
• The list of keys contains no duplicate values.
• The type of each key is either a String or a Symbol.
• The result list must contain the keys of all non-configurable own properties of the target object. That is, for all keys returned by Reflect.ownKeys() on the target object, if the key reports configurable: false by Reflect.getOwnPropertyDescriptor(), then the key must be included in the result List.
• If the target object is not extensible, then the result list must contain all the keys of the own properties of the target object and no other values. That is, if Reflect.isExtensible() returns false on target, then the result list must contain the same values as the result of Reflect.ownKeys() on target.

The following code traps Object.getOwnPropertyNames().

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

console.log(Object.getOwnPropertyNames(p));
// "called"
// [ 'a', 'b', 'c' ]

The following code violates an invariant.

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

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