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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 monster = {
  canEvolve: true,
};

const handler = {
  isExtensible(target) {
    return Reflect.isExtensible(target);
  },
  preventExtensions(target) {
    target.canEvolve = false;
    return Reflect.preventExtensions(target);
  },
};

const proxy = new Proxy(monster, handler);

console.log(Object.isExtensible(proxy));
// Expected output: true

console.log(monster.canEvolve);
// Expected output: true

Object.preventExtensions(proxy);

console.log(Object.isExtensible(proxy));
// Expected output: false

console.log(monster.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® 2026 Language Specification> # sec-proxy-object-internal-methods-and-internal-slots-isextensible>

Browser compatibility

	Desktop					Mobile							Server
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	WebView on iOS	Bun	Deno	Node.js
`isExtensible`	49	12	31	36	10	49	31	36	10	5.0	49	10	1.0.0	1.0	6.0.0