W3cubDocs

/JavaScript

Proxy.proxy.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 compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
getOwnPropertyDescriptor 49 12 18 No 36 10
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
getOwnPropertyDescriptor 49 49 18 36 10 5.0
Server
Node.js
getOwnPropertyDescriptor 6.0.0

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/getOwnPropertyDescriptor