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 or undefined.
  • 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 using Object.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

Document Tags and Contributors

 Contributors to this page: kdex, fscholz, arai
 Last updated by: kdex,