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The handler.ownKeys()
method is a trap for Object.getOwnPropertyNames()
.
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 Object.getOwnPropertyNames()
.
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 2017 Draft (ECMA-262) The definition of '[[OwnPropertyKeys]]' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|
Basic support | ? | 18 (18) | ? | ? | ? |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | ? | ? | 18.0 (18) | ? | ? | ? |
Compatibility notes
Firefox
- In Gecko 42 (Firefox 42 / Thunderbird 42 / SeaMonkey 2.39), the
ownKey
implementation got updated to reflect the final ES2015 specification (see bug 1049662):- 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.