Useful TypeScript Snippets

DeepReadOnly.ts

/**
 * Recursively convert any data structure to readonly (with handling of privimitives):
 */

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;

/**
 * Example Usage:
 */
interface IRecipient {
  email: string;
  name: string;
}

interface IEmail {
  body: string;
  recipients: IRecipient[];
}

type IReadonlyEmail = DeepReadonly<IEmail>;

const myEmail: IReadonlyEmail = {
  body: "Hello",
  recipients: [
    {
      email: "jhon.doe@example.net",
      name: "John Doe"
    }
  ]
};

/**
 * The following line throws a compilation error:
 */
myEmail.body = "Hello John!";