Object.isExtensible(obj)

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

// They can be made un-extensible
Object.preventExtensions(empty);
Object.isExtensible(empty); // false

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

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

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

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