27. How can you create a deep readonly type using TypeScript?

To create a deep readonly type in TypeScript, you can use recursion combined with the built-in Readonly type. This involves creating a custom recursive type that checks if a given type is an object and, if so, applies Readonly to all its properties, including nested objects.

Example:

type DeepReadonly<T> = T extends Function ? T : T extends object ? { readonly [K in keyof T]: DeepReadonly<T[K]> } : T;

interface User {
  name: string;
  details: {
    age: number;
    address: string;
  };
}

const user: DeepReadonly<User> = {
  name: 'John',
  details: {
    age: 30,
    address: '123 Street'
  }
};

user.details.age = 31; // Error, 'age' is readonly
user.name = 'Alice'; // Error, 'name' is readonly

The DeepReadonly<T> type recursively applies readonly to all properties of an object, including those that are nested, ensuring the entire object is read-only.