Proxy.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:

• Object.getOwnPropertyDescriptor()
• Reflect.getOwnPropertyDescriptor()

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

See also

• Proxy
• handler
• Object.getOwnPropertyDescriptor()
• Reflect.getOwnPropertyDescriptor()