handler.getOwnPropertyDescriptor()

Try it

const monster = {
  eyeCount: 4,
};

const handler = {
  getOwnPropertyDescriptor(target, prop) {
    console.log(`called: ${prop}`);
    // Expected output: "called: eyeCount"

    return { configurable: true, enumerable: true, value: 5 };
  },
};

const proxy = new Proxy(monster, handler);

console.log(Object.getOwnPropertyDescriptor(proxy, "eyeCount").value);
// Expected output: 5

Syntax

new Proxy(target, {
  getOwnPropertyDescriptor(target, property) {
  }
})

Parameters

The following parameters are passed to the getOwnPropertyDescriptor() method. this is bound to the handler.

target

The target object.

property

A string or Symbol representing the property name.

Return value

The getOwnPropertyDescriptor() method must return an object or undefined, representing the property descriptor. Missing attributes are normalized in the same way as Object.defineProperty().

Description

Interceptions

This trap can intercept these operations:

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

Or any other operation that invokes the [[GetOwnProperty]] internal method.

Invariants

The proxy's [[GetOwnProperty]] internal method throws a TypeError if the handler definition violates one of the following invariants:

• The result must be either 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. That is, if Reflect.getOwnPropertyDescriptor() returns configurable: false for the property on target, then the trap must not return undefined.
• A property cannot be reported as non-existent, if it exists as an own property of a non-extensible target object. That is, if Reflect.isExtensible() returns false for the target object, then the trap must not return undefined.
• A property cannot be reported as existent, if it does not exist as an own property of the target object and the target object is not extensible. That is, if Reflect.isExtensible() returns false for the target object, and Reflect.getOwnPropertyDescriptor() returns undefined for the property on target, then the trap must return undefined.
• A property cannot be reported as non-configurable, unless it exists as a non-configurable own property of the target object. That is, if Reflect.getOwnPropertyDescriptor() returns undefined or configurable: true for the property on target, then the trap must not return configurable: false.
• A property cannot be reported as both non-configurable and non-writable, unless it exists as a non-configurable, non-writable own property of the target object. That is, in addition to the previous invariant, if Reflect.getOwnPropertyDescriptor() returns configurable: false, writable: true for the property on target, then the trap must not return configurable: false, writable: false.
• If a property has a corresponding property on the target object, then the target object property's descriptor must be compatible with descriptor. That is, pretending target is an ordinary object, then Object.defineProperty(target, property, resultObject) must not throw an error. The Object.defineProperty() reference contains more information, but to summarize, when the target property is non-configurable, the following must hold:
  • configurable, enumerable, get, and set must be the same as original. writable must also be the original by virtue of the previous invariant.
  • the property must stay as data or accessor
  • the value attribute can only be changed if writable is true

Examples

Trapping of getOwnPropertyDescriptor

The following code traps Object.getOwnPropertyDescriptor().

const p = new Proxy(
  { a: 20 },
  {
    getOwnPropertyDescriptor(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(target, prop) {
    return undefined;
  },
});

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

Specifications

Browser compatibility

See also

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