[TypeScript] Deeply mark all the properties of a type as read-only in TypeScript

We will look at how we can use mapped types, conditional types, self-referencing types and the “infer” keyword to create a reusable generic type that traverses down the properties of an object and marks of all them as read-only. This is especially useful when used with Redux, to ensure our whole state tree is marked as read-only and immutable.

For example we have complex interface:

interface IRootState {
  userId: string;
  showCompletedOnly: boolean;
  todoTypes: string[];
  todos: ITodo[];
  iconGrid: string[][];
}

interface IEmail {
  from: string;
  to: string[];
  body: string;
}

interface ITodo {
  isCompleted: boolean;
  text: string;
  linkedEmail: IEmail;
}

If we want to add readonly to all the props of IRootState. We want to do it automaticlly.

type DeepReadonlyObject<T> = { readonly [K in keyof T]: DeepReadonly<T[K]> };

type DeepReadonly<T> = T extends (infer E)[] ?
  ReadonlyArray<DeepReadonlyObject<E>> :
  T extends object ? DeepReadonlyObject<T> :
    T;

type IReadonlyRootState = DeepReadonly<IRootState>;