Account.jsx

// rendered at http://localhost:8080/account

export const Account = (props) => {
  const { user } = props;
  return (
    <div>
    { user.username }
    </div>
  );
};

App.jsx

import { SignIn } from './SignIn.jsx';
import { Account } from './Account.jsx';

export const App = () => {
  const [user, set_user] = useState(null);
  return (
    <div>
      <SignIn set_user={set_user} />
      <Account user={user} />
    </div>
  );
};

SignIn.jsx

// rendered at http://localhost:8080/sign-in

export const SignIn = (props) => {
  const { set_user } = props;
  const sign_in = async () => {
    // request
    assert(response.user instanceof Object);
    const user = response.user;
    set_user(user);
  };
  return (
    <div>
    </div>
  );
};