handler.ownKeys()

The handler.ownKeys() method is a trap for Reflect.ownKeys().

const p = new Proxy(target, {
  ownKeys: function(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 enumerable object.

The handler.ownKeys() method is a trap for Reflect.ownKeys().

This trap can intercept these operations:

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

If the following invariants are violated, the proxy will throw a TypeError:

• The result of ownKeys() must be an array.
• The type of each array element is either a String or a Symbol.
• The result List must contain the keys of all non-configurable own properties of the target object.
• 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.

The following code traps Object.getOwnPropertyNames().

const p = new Proxy({}, {
  ownKeys: function(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: function(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

• Proxy
• handler
• Object.getOwnPropertyNames()
• Reflect.ownKeys()