Authentication Firebase

I'm trying to do a simple user registration login on Firebase, but i'm receiving this error: Uncaught ReferenceError: FirebaseAuthClient is not defined

Here my code:

var authClient = new FirebaseAuthClient(rootRef, function(error, user) {
  if (error) {
    alert(error);
    retu;
  }
  if (user) {
    // User is already logged in.
    doLogin(user);
  } else {
    // User is logged out.
    showLoginBox();
  }
});


function showLoginBox() {

  document.getElementById("#registerButton").addEventListener("click", function() {
    var email = document.getElementById("#email").val();
    var password = document.getElementById("#password").val();
    authClient.createUser(email, password, function(error,  user) {
      if (!error) {
        doLogin(user);
      } else {
        alert(error);
      }
    });
  });
}