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

Syntax

var p = new Proxy(target, {
  ownKeys: function(target) {
  }
});

Parameters

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

target
The target object.

Return value

The ownKeys method must return an enumerable object.

Description

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

Interceptions

This trap can intercept these operations:

Invariants

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.

Examples

The following code traps Object.getOwnPropertyNames().

var 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.

var obj = {};
Object.defineProperty(obj, 'a', { 
  configurable: false, 
  enumerable: true, 
  value: 10 }
);

var 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

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of '[[OwnPropertyKeys]]' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of '[[OwnPropertyKeys]]' in that specification.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support4912181 No3610
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support4949 Yes181 No3610

1. In Firefox 42, the implementation got updated to reflect the final ES2015 specification: The result is now checked if it is an array and if the array elements are either of type string or of type symbol. Enumerating duplicate own property names is not a failure anymore.

See also

Document Tags and Contributors

 Contributors to this page: fscholz, nmve, kdex, amitport, m31271n_, slavede, arai
 Last updated by: fscholz,