Object.isExtensible()

Object.isExtensible(obj)

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

// ...but that can be changed.
Object.preventExtensions(empty);
Object.isExtensible(empty); // === false

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

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

Object.isExtensible(1);
// TypeError: 1 is not an object (ES5 code)

Object.isExtensible(1);
// false                         (ES2015 code)