JS: Reflect.isExtensible

(new in JS: ECMAScript 2015)

Reflect.isExtensible(obj)

Return true if obj is extensible. Else, false.

If obj is not object, throw a TypeError exception.

const jj = {};
console.log(Reflect.isExtensible(jj));
Reflect.preventExtensions(jj);
console.log(Reflect.isExtensible(jj) === false);