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


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


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

The target object.
The name or Symbol of the property whose description is to be retrieved.
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.


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


This trap can intercept these operations:


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.


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) {
    return Reflect.defineProperty(target, prop, descriptor);

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


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

Browser compatibility

Update compatibility data on GitHub
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


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See also