const config = {
  host: "localhost",
  port: 3000,
  db: {
    name: "mydb",
    retries: 5
  }
};

Object.freeze(config);

config.port = 9999;
config.newProp = "injected";
config.db.retries = 99;
delete config.host;

console.log(
  config.port,
  config.newProp,
  config.db.retries,
  config.host
);