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

js
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:

Examples

Trapping of isExtensible

The following code traps Object.isExtensible().

js
const p = new Proxy(
  {},
  {
    isExtensible(target) {
      console.log("called");
      return true;
    },
  },
);

console.log(Object.isExtensible(p));
// "called"
// true

The following code violates the invariant.

js
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

Browser compatibility

See also