handler.ownKeys()
The handler.ownKeys()
method is a trap for the [[OwnPropertyKeys]]
object internal method, which is used by operations such as Object.keys()
, Reflect.ownKeys()
, etc.
Try it
Syntax
js
new Proxy(target, {
ownKeys(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
Interceptions
This trap can intercept these operations:
Or any other operation that invokes the [[OwnPropertyKeys]]
internal method.
Invariants
If the following invariants are violated, the trap throws a TypeError
when invoked.
- 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
Trapping of getOwnPropertyNames
The following code traps Object.getOwnPropertyNames()
.
js
const p = new Proxy(
{},
{
ownKeys(target) {
console.log("called");
return ["a", "b", "c"];
},
},
);
console.log(Object.getOwnPropertyNames(p));
// "called"
// [ 'a', 'b', 'c' ]
The following code violates an invariant.
js
const obj = {};
Object.defineProperty(obj, "a", {
configurable: false,
enumerable: true,
value: 10,
});
const p = new Proxy(obj, {
ownKeys(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 |
---|
ECMAScript Language Specification # sec-proxy-object-internal-methods-and-internal-slots-ownpropertykeys |
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