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 aSymbol
. - 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
Feature | Chrome | Edge | Firefox | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 49 | 12 | 181 | No | 36 | 10 |
Feature | Android webview | Chrome for Android | Edge mobile | Firefox for Android | IE mobile | Opera Android | iOS Safari |
---|---|---|---|---|---|---|---|
Basic support | 49 | 49 | Yes | 181 | No | 36 | 10 |
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.