Proxy.handler.defineProperty

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

Syntax

var 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 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:

• Object.defineProperty()
• Reflect.defineProperty()

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

The following code traps Object.defineProperty().

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

var 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, nonstandard properties will be ignored:

• enumerable
• configurable
• writable
• value
• get
• set

var 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	Status	Comment
ECMAScript 2015 (6th Edition, ECMA-262) The definition of '[[DefineOwnProperty]]' in that specification.	Standard	Initial definition.
ECMAScript 2017 Draft (ECMA-262) The definition of '[[DefineOwnProperty]]' in that specification.	Draft

Browser compatibility

See also

• Proxy
• handler
• Object.defineProperty()
• Reflect.defineProperty()