handler.preventExtensions()

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.preventExtensions() method is a trap for the [[PreventExtensions]] object internal method, which is used by operations such as Object.preventExtensions().

Try it

const monster1 = {
  canEvolve: true,
};

const handler1 = {
  preventExtensions(target) {
    target.canEvolve = false;
    Object.preventExtensions(target);
    return true;
  },
};

const proxy1 = new Proxy(monster1, handler1);

console.log(monster1.canEvolve);
// Expected output: true

Object.preventExtensions(proxy1);

console.log(monster1.canEvolve);
// Expected output: false

Syntax

new Proxy(target, {
  preventExtensions(target) {
  }
})

Parameters

The following parameter is passed to the preventExtensions() method. this is bound to the handler.

target: The target object.

Return value

The preventExtensions() method must return a Boolean indicating whether or not the operation was successful. Other values are coerced to booleans.

Many operations, including Object.preventExtensions(), throw a TypeError if the [[PreventExtensions]] internal method returns false.

Description

Interceptions

This trap can intercept these operations:

Or any other operation that invokes the [[PreventExtensions]] internal method.

Invariants

The proxy's [[PreventExtensions]] internal method throws a TypeError if the handler definition violates one of the following invariants:

The result is only true if Reflect.isExtensible() on the target object returns false after calling handler.preventExtensions().

Examples

Trapping of preventExtensions

The following code traps Object.preventExtensions().

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

console.log(Object.preventExtensions(p));
// "called"
// false

The following code violates the invariant.

const p = new Proxy(
  {},
  {
    preventExtensions(target) {
      return true;
    },
  },
);

Object.preventExtensions(p); // TypeError is thrown

Specifications

Specification
ECMAScript® 2026 Language Specification # sec-proxy-object-internal-methods-and-internal-slots-preventextensions