JS: Object.isExtensible

Object.isExtensible(obj)

Return true if a object obj is an object and is extensible , else false.

💡 TIP: Better is Reflect.isExtensible . It throws error if argument is not object.

Object.isExtensible({})

// warning: false may mean the argument is not an object
console.log(Object.isExtensible(3) === false);