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

Specification
ECMAScript (ECMA-262)
The definition of '[[GetOwnProperty]]' in that specification.

Browser compatibility

BCD tables only load in the browser

See also