handler.preventExtensions()

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.
Living Standard  

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support491222 No3610
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support4949 Yes22 No3610

See also

Document Tags and Contributors

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