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


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


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

The target object.

Return value

The ownKeys method must return an enumerable object.


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


This trap can intercept these operations:


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().

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


// TypeError: proxy [[OwnPropertyKeys]] must return an array 
// with only string and symbol elements


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.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support4912181 No3610
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support4949 Yes18136105.0

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: wbamberg, fscholz, nmve, kdex, amitport, m31271n., slavede, arai
Last updated by: wbamberg,