JSTests/test262/test/built-ins/Proxy/isExtensible/trap-is-undefined-target-is-proxy.js

// Copyright (C) 2020 Alexey Shvayka. All rights reserved.
// This code is governed by the BSD license found in the LICENSE file.

/*---
esid: sec-proxy-object-internal-methods-and-internal-slots-isextensible
description: >
  If "isExtensible" trap is null or undefined, [[IsExtensible]] call is
  properly forwarded to [[ProxyTarget]] (which is also a Proxy object).
info: |
  [[IsExtensible]] ( )

  [...]
  4. Let target be O.[[ProxyTarget]].
  5. Let trap be ? GetMethod(handler, "isExtensible").
  6. If trap is undefined, then
    a. Return ? IsExtensible(target).

  IsExtensible ( O )

  1. Assert: Type(O) is Object.
  2. Return ? O.[[IsExtensible]]().

  [[IsExtensible]] ( )

  [...]
  7. Let booleanTrapResult be ! ToBoolean(? Call(trap, handler, « target »)).
  [...]
  10. Return booleanTrapResult.
features: [Proxy]
---*/

var array = [];
var arrayExtensible = true;
var arrayTarget = new Proxy(array, {
  isExtensible: function() {
    return arrayExtensible;
  },
});

var arrayProxy = new Proxy(arrayTarget, {
  isExtensible: undefined,
});

assert(Object.isExtensible(arrayProxy));

Object.preventExtensions(array);
arrayExtensible = false;

assert(!Object.isExtensible(arrayProxy));