handler.defineProperty()

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

Syntax

const p = new Proxy(target, {
  defineProperty: function(target, property, descriptor) {
  }
});

Parameters

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

target
The target object.
property
The name or Symbol of the property whose description is to be retrieved.
descriptor
The descriptor for the property being defined or modified.

Return value

The defineProperty() method must return a Boolean indicating whether or not the property has been successfully defined.

Description

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

Interceptions

This trap can intercept these operations:

Invariants

If the following invariants are violated, the proxy will throw a TypeError:

  • A property cannot be added, if the target object is not extensible.
  • A property cannot be added as or modified to be non-configurable, if it does not exists as a non-configurable own property of the target object.
  • A property may not be non-configurable, if a corresponding configurable property of the target object exists.
  • If a property has a corresponding target object property then Object.defineProperty(target, prop, descriptor) will not throw an exception.
  • In strict mode, a false return value from the defineProperty() handler will throw a TypeError exception.

Examples

Trapping of defineProperty

The following code traps Object.defineProperty().

const p = new Proxy({}, {
  defineProperty: function(target, prop, descriptor) {
    console.log('called: ' + prop);
    return true;
  }
});

const desc = { configurable: true, enumerable: true, value: 10 };
Object.defineProperty(p, 'a', desc); // "called: a"

When calling Object.defineProperty() or Reflect.defineProperty(), the descriptor passed to defineProperty() trap has one restriction—only following properties are usable (non-standard properties will be ignored):

  • enumerable
  • configurable
  • writable
  • value
  • get
  • set
const p = new Proxy({}, {
  defineProperty(target, prop, descriptor) {
    console.log(descriptor);
    return Reflect.defineProperty(target, prop, descriptor);
  }
});

Object.defineProperty(p, 'name', {
  value: 'proxy',
  type: 'custom'
});  // { value: 'proxy' }

Specifications

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

Browser compatibility

DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung InternetNode.js
definePropertyChrome Full support 49Edge Full support 12Firefox Full support 18IE No support NoOpera Full support 36Safari Full support 10WebView Android Full support 49Chrome Android Full support 49Firefox Android Full support 18Opera Android Full support 36Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0

Legend

Full support
Full support
No support
No support

See also