Reflect.isExtensible

Reflect.isExtensible(target)

// New objects are extensible. 
var empty = {};
Reflect.isExtensible(empty // === true 

// ...but that can be changed. 
Reflect.preventExtensions(empty 
Reflect.isExtensible(empty // === false 

// Sealed objects are by definition non-extensible. 
var sealed = Object.seal{} 
Reflect.isExtensible(sealed // === false 

// Frozen objects are also by definition non-extensible. 
var frozen = Object.freeze{} 
Reflect.isExtensible(frozen // === false

Reflect.isExtensible(1
// TypeError: 1 is not an object

Object.isExtensible(1
// false