handler.isExtensible()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.
The handler.isExtensible()
method is a trap for the [[IsExtensible]]
object internal method, which is used by operations such as Object.isExtensible()
.
Try it
const monster1 = {
canEvolve: true,
};
const handler1 = {
isExtensible(target) {
return Reflect.isExtensible(target);
},
preventExtensions(target) {
target.canEvolve = false;
return Reflect.preventExtensions(target);
},
};
const proxy1 = new Proxy(monster1, handler1);
console.log(Object.isExtensible(proxy1));
// Expected output: true
console.log(monster1.canEvolve);
// Expected output: true
Object.preventExtensions(proxy1);
console.log(Object.isExtensible(proxy1));
// Expected output: false
console.log(monster1.canEvolve);
// Expected output: false
Syntax
new Proxy(target, {
isExtensible(target) {
}
})
Parameters
The following parameter is passed to the isExtensible()
method. this
is bound to the handler.
target
-
The target object.
Return value
The isExtensible()
method must return a Boolean
indicating whether or not the target object is extensible. Other values are coerced to booleans.
Description
Interceptions
This trap can intercept these operations:
Or any other operation that invokes the [[IsExtensible]]
internal method.
Invariants
The proxy's [[IsExtensible]]
internal method throws a TypeError
if the handler definition violates one of the following invariants:
- The result must be the same as
Reflect.isExtensible()
on the target object.
Examples
Trapping of isExtensible
The following code traps Object.isExtensible()
.
const p = new Proxy(
{},
{
isExtensible(target) {
console.log("called");
return true;
},
},
);
console.log(Object.isExtensible(p));
// "called"
// true
The following code violates the invariant.
const p = new Proxy(
{},
{
isExtensible(target) {
return false;
},
},
);
Object.isExtensible(p); // TypeError is thrown
Specifications
Specification |
---|
ECMAScript® 2025 Language Specification # sec-proxy-object-internal-methods-and-internal-slots-isextensible |