What does `type DeepReadonly = { readonly [K in keyof T]: T[K] extends object ? DeepReadonly : T[K] }` accomplish?

Question

TypeScript Professional — Hard

What does `type DeepReadonly = { readonly [K in keyof T]: T[K] extends object ? DeepReadonly : T[K] }` accomplish?

Accepted Answer

The `type DeepReadonly` in TypeScript recursively makes all properties and nested properties readonly. This means that not only the top-level properties are made readonly, but also any nested properties within objects.