handler.isExtensible()

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

Syntax

const p = new Proxy(target, {
  isExtensible: function(target) {
  }
});

Parameters

The following parameter is passed to the isExtensible() method. this is bound to the handler.

target
The target object.

Return value

The isExtensible() method must return a boolean value.

Description

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

Interceptions

This trap can intercept these operations:

Invariants

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

  • Object.isExtensible(proxy) must return the same value as Object.isExtensible(target).

Examples

Trapping of isExtensible

The following code traps Object.isExtensible().

const p = new Proxy({}, {
  isExtensible: function(target) {
    console.log('called');
    return true;
  }
});

console.log(Object.isExtensible(p)); // "called"
                                     // true

The following code violates the invariant.

const p = new Proxy({}, {
  isExtensible: function(target) {
    return false;
  }
});

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

Specifications

Specification
ECMAScript (ECMA-262)
The definition of '[[IsExtensible]]' in that specification.

Browser compatibility

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See also