handler.defineProperty()
The handler.defineProperty()
method is a trap for the [[DefineOwnProperty]]
object internal method, which is used by operations such as Object.defineProperty()
.
Try it
Syntax
js
new Proxy(target, {
defineProperty(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
Interceptions
This trap can intercept these operations:
Or any other operation that invokes the [[DefineOwnProperty]]
internal method.
Invariants
If the following invariants are violated, the trap throws a TypeError
when invoked.
- 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()
.
js
const p = new Proxy(
{},
{
defineProperty(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
js
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
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