handler.getOwnPropertyDescriptor()

The handler.getOwnPropertyDescriptor() method is a trap for Object.getOwnPropertyDescriptor().

Syntax

const 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

Trapping of getOwnPropertyDescriptor

The following code traps Object.getOwnPropertyDescriptor().

const 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.

const obj = { a: 10 };
Object.preventExtensions(obj);
const p = new Proxy(obj, {
  getOwnPropertyDescriptor: function(target, prop) {
    return undefined;
  }
});

Object.getOwnPropertyDescriptor(p, 'a'); // TypeError is thrown

Specifications

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
getOwnPropertyDescriptor
49
12
18
No
36
10
49
49
18
36
10
5.0

See also

© 2005–2021 MDN 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/Proxy/getOwnPropertyDescriptor