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 2017 Draft (ECMA-262)
The definition of '[[PreventExtensions]]' in that specification.
Draft  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support ? 22 (22) ? ? ?
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support ? ? 22.0 (22) ? ? ?

See also

Document Tags and Contributors

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