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Proxy.proxy.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

Browser compatibilityUpdate compatibility data on GitHub

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

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/defineProperty