Function is_extensible 

pub fn is_extensible<T>(target: &Object<T>) -> Result<bool, JsValue>
where
    Object<T>: ErasableGenericBorrow<Object<JsValue>>,

The static Reflect.isExtensible() method determines if an object is extensible (whether it can have new properties added to it). It is similar to Object.isExtensible(), but with some differences.

MDN documentation