How to implement this C++ architecture in Javascript?

A method from an inherited class should return the object type from who has inherited. In C++ this behaviour is easy to accomplish. But I don't know how to do it in javascript. I know it is wrong but I wrote like this.

class A {
 someMethod() {
  return new A();
 }
}
class B extends A {
}
var b = new B();
b.someMethod() // should return an object of type B not A

in C++ this is easy to do

template <typename c>
struct a
{
    a() : _value(NULL) {}
    a(std::string v) : _v(v) {}
    static c* from_string(std::string &v)
    {
        return new c(v);
    }
private:
    std::string _value;
};

struct b : public a<b>
{
    b() : b<a>() {}
    b(std::string &v) : node<b>(a) {}
};

How should this be implemented using javascript?