The handler.defineProperty()
method is a trap for Object.defineProperty()
.
handler.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
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 thedefineProperty()
handler will throw aTypeError
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 Language Specification # sec-proxy-object-internal-methods-and-internal-slots-defineownproperty-p-desc |
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