Proxy.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:

• Object.preventExtensions()
• Reflect.preventExtensions()

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

See also

• Proxy
• handler
• Object.preventExtensions()
• Reflect.preventExtensions()