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Proxy.proxy.preventExtensions

The handler.preventExtensions() method is a trap for Object.preventExtensions().

Syntax

const p = new Proxy(target, {
  preventExtensions: function(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 value.

Description

The handler.preventExtensions() method is a trap for Object.preventExtensions().

Interceptions

This trap can intercept these operations:

Invariants

If the following invariants are violated, the proxy will throw a TypeError:

  • Object.preventExtensions(proxy) only returns true if Object.isExtensible(proxy) is false.

Examples

Trapping of preventExtensions

The following code traps Object.preventExtensions().

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

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

Specifications

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
preventExtensions 49 12 22 No 36 10
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
preventExtensions 49 49 22 36 10 5.0
Server
Node.js
preventExtensions 6.0.0

See also

© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy/Proxy/preventExtensions