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handler.isExtensible()

The handler.isExtensible() method is a trap for the [[IsExtensible]] object internal method, which is used by operations such as Object.isExtensible().

Try it

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 value.

Description

Interceptions

This trap can intercept these operations:

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

Invariants

If the following invariants are violated, the trap throws a TypeError when invoked.

  • Object.isExtensible(proxy) must return the same value as Object.isExtensible(target).

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

Browser compatibility

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

See also

© 2005–2023 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/isExtensible