Putting the "Java" in "JavaScript"

sigh.js

/* @flow */

class User {
  n: number;
  constructor(n) {
    this.n = n;
  }
}

class UserDAOIFace {
  getUserById(n): User {
    throw new Error("Call me maybe (never).");
  }
}

class UserDAOImpl extends UserDAOIFace {
  getUserById(n): User {
    return new User(n);
  }
}

class UserDAOTestImpl extends UserDAOIFace {
  getUserById(n): User {
    return new User(420);
  }
}

class UserService {
  dao: UserDAOIFace;
  constructor(dao) {
    this.dao = dao;
  }
  getUserById(x: number): User {
    return this.dao.getUserById(x);
  }
}

var u = new UserService(new UserDAOImpl());