javascript / latest / global_objects / proxy / proxy / defineproperty.html /

handler.defineProperty()

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

Try it

Syntax

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

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

Browser compatibility

Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet Deno Node.js
defineProperty
49
12
18
No
36
10
49
49
18
36
10
5.0
1.0
6.0.0

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/defineProperty