handler.getOwnPropertyDescriptor()
The handler.getOwnPropertyDescriptor()
method is a trap for the [[GetOwnProperty]]
object internal method, which is used by operations such as Object.getOwnPropertyDescriptor()
.
Try it
Syntax
js
new Proxy(target, {
getOwnPropertyDescriptor(target, prop) {
}
});
Parameters
The following parameters are passed to the getOwnPropertyDescriptor()
method. this
is bound to the handler.
target
-
The target object.
prop
-
The name of the property whose description should be retrieved.
Return value
The getOwnPropertyDescriptor()
method must return an object or undefined
.
Description
Interceptions
This trap can intercept these operations:
Or any other operation that invokes the [[GetOwnProperty]]
internal method.
Invariants
If the following invariants are violated, the trap throws a TypeError
when invoked.
getOwnPropertyDescriptor()
must return an object orundefined
.- A property cannot be reported as non-existent, if it exists as a non-configurable own property of the target object.
- A property cannot be reported as non-existent, if it exists as an own property of the target object and the target object is not extensible.
- A property cannot be reported as existent, if it does not exists as an own property of the target object and the target object is not extensible.
- A property cannot be reported as non-configurable, if it does not exists as an own property of the target object or if it exists as a configurable own property of the target object.
- The result of
Object.getOwnPropertyDescriptor(target)
can be applied to the target object usingObject.defineProperty()
and will not throw an exception.
Examples
Trapping of getOwnPropertyDescriptor
The following code traps Object.getOwnPropertyDescriptor()
.
js
const p = new Proxy(
{ a: 20 },
{
getOwnPropertyDescriptor(target, prop) {
console.log(`called: ${prop}`);
return { configurable: true, enumerable: true, value: 10 };
},
},
);
console.log(Object.getOwnPropertyDescriptor(p, "a").value);
// "called: a"
// 10
The following code violates an invariant.
js
const obj = { a: 10 };
Object.preventExtensions(obj);
const p = new Proxy(obj, {
getOwnPropertyDescriptor(target, prop) {
return undefined;
},
});
Object.getOwnPropertyDescriptor(p, "a"); // TypeError is thrown
Specifications
Specification |
---|
ECMAScript Language Specification # sec-proxy-object-internal-methods-and-internal-slots-getownproperty-p |
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