I am implementing a cloud function for signing up with unique username and password.

In order to throw exceptions, I was previously doing the following:

signUpValidation.js

if (!validateUsername(username)) {
  throw new functions.https.HttpsError(
    "invalid-argument",
    "Invalid username.",
    {
      status: "error",
      code: "auth/invalid-username",
      message: "Username must be between 3 and 30 characters, including numbers, letters, hyphens, periods, or underscores.",
    }
  );
}

signUp.function.js

try {
  await validateSignUpData(
     username,
     email,
     password,
     repeatPassword,
     name,
     birthday,
     clientIp
  );
} catch(err) {
    if (err instanceof functions.https.HttpsError) {
      throw err;
    }

    // An unknown error has occurred
    console.error(err);

    throw new functions.https.HttpsError(
      "unknown",
      "Unexpected error.",
      { 
        status: "error",
        code: err.code ?? "unknown",
        message: err.message ?? "The registration request could not be processed. Please, try again later."
      }
    );
}

But, I don't really like this way of throwing the exceptions in the signUpValidation module... it makes more sense to me to throw "AuthErrors" instead of "HttpsErrors".

So, as it seems not possible to extend the default Firebase Errors, I have decided to create my own util/authErrors module:

class AuthError extends Error {
  constructor(code, message) {
    super(message);
    this.code = code;
    this.name = "AuthError";
  }
}

const authErrors = Object.freeze({
  usernameAlreadyExists(message = "The username is already in use by an existing account") {
    return new AuthError('auth/email-already-exists', message);
  }

   ... more errors
});

module.exports = authErrors;

as you can see, I have created my custom Error and some factory functions for every error type. Then, in my signUpValidation.js, I just do:

if (!(await isUsernameUnique(username))) {
  throw authErrors.usernameAlreadyExists();
}

Is it possible to extend a FirebaseError? If not, why?

Is it considered a bad practice to work this way in order to throw custom exceptions in Cloud Functions? I mean, should I just throw HttpsErrors?