How to separate classes that extend same abstract class?

Question

My question: how do I convert a list of abstract classes back into their original classes (the classes that extend the abstract class).

I have a backend that returns a list of classes (List of UserModels) that all extends an abstract class. something like this:

public abstract class UserModel
{
...
}

public abstract class ContactModel: UserModel
{
...
}

public abstract class AdminModel: UserModel
{
...
}

In my front-end (Angular) I have the same setup. In C# I can simpely split those by saying var isAdmin = someModel is AdminModel;.

But when I get this list of users in the front-end I don't know how to convert these models from their abstract class back into the original classes.

I tried:

const isAdmin = Object.keys(AdminModel).every(key => user.hasOwnProperty(key));
const admin = user as AdminModel;

=> returned true with both models

and

const isAdmin = user instanceof AdminModel;
const admin = user as AdminModel;

=> returned false with both models

I hope this is possible. Thanks.