interface User {
    readonly id: number
    name: string
}
const user: User = { id: 1, name: "Alice" }
user.id = 2

type DeepReadonly<T> = { readonly [K in keyof T]: DeepReadonly<T[K]> }
interface Config {
  db: { host: string; port: number }
}
const config: DeepReadonly<Config> = {
  db: { host: "localhost", port: 5432 }
}
// produces a TypeScript error
config.db.host = "newhost"