How to use the google-auth-library.auth.getApplicationDefault function in google-auth-library

const _setAuthCredential = async () => {
  const res = await auth.getApplicationDefault();

  let client = res.credential;
  if (client.hasScopes && !client.hasScopes()) {
    client = client.createScoped([
      "https://www.googleapis.com/auth/cloud-billing",
      "https://www.googleapis.com/auth/cloud-platform",
    ]);
  }

  // Set credential globally for all requests
  google.options({
    auth: client,
  });
};