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

Syntax

var 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

The following code traps Object.preventExtensions().

var 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.

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

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

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of '[[PreventExtensions]]' in that specification.
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
The definition of '[[PreventExtensions]]' in that specification.
Draft  

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome Full support 49Edge Full support 12Firefox Full support 22IE No support NoOpera Full support 36Safari Full support 10WebView Android Full support 49Chrome Android Full support 49Edge Mobile Full support YesFirefox Android Full support 22Opera Android Full support 36Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0

Legend

Full support  
Full support
No support  
No support

See also

Document Tags and Contributors

Contributors to this page: wbamberg, fscholz, nmve, kdex, arai
Last updated by: wbamberg,