How to import the service account keys config into your code, especially from environment variables

Google (Firestore) credentials service account keys importing

const admin = require('firebase-admin');

const firestoreConfig = process.env.FIRESTORE_CONFIG; // **String** of firestore service account keys https://cloud.google.com/iam/docs/creating-managing-service-account-keys
// `{
//   "type": "service_account",
//   "project_id": "project-id",
//   "private_key_id": "key-id",
//   "private_key": "-----BEGIN PRIVATE KEY-----\nprivate-key\n-----END PRIVATE KEY-----\n",
//   "client_email": "service-account-email",
//   "client_id": "client-id",
//   "auth_uri": "https://accounts.google.com/o/oauth2/auth",
//   "token_uri": "https://accounts.google.com/o/oauth2/token",
//   "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
//   "client_x509_cert_url": "https://www.googleapis.com/robot/v1/metadata/x509/service-account-email"
// }`

const credentialsObject = JSON.parse(firestoreConfig);

admin.initializeApp({
  credential: admin.credential.cert(credentialsObject)
});

const db = admin.firestore();

module.exports = db;