handler.getOwnPropertyDescriptor()
The handler.getOwnPropertyDescriptor()
method is a trap for Object.getOwnPropertyDescriptor()
.
Syntax
var p = new Proxy(target, { getOwnPropertyDescriptor: function(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
The handler.getOwnPropertyDescriptor()
method is a trap for Object.getOwnPropertyDescriptor()
.
Interceptions
This trap can intercept these operations:
Invariants
If the following invariants are violated, the proxy will throw a TypeError
:
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
The following code traps Object.getOwnPropertyDescriptor()
.
var p = new Proxy({ a: 20 }, { getOwnPropertyDescriptor: function(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.
var obj = { a: 10 }; Object.preventExtensions(obj); var p = new Proxy(obj, { getOwnPropertyDescriptor: function(target, prop) { return undefined; } }); Object.getOwnPropertyDescriptor(p, "a"); // TypeError is thrown
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) The definition of '[[GetOwnProperty]]' in that specification. |
Standard | Initial definition. |
ECMAScript 2017 Draft (ECMA-262) The definition of '[[GetOwnProperty]]' 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) | ? | ? | ? |
See also
License
© 2016 Mozilla Contributors
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-us/docs/web/javascript/reference/global_objects/proxy/handler/getownpropertydescriptor