types.ts

/**
 * Exposes type functions.
 */

/**
 * [Reference](https://stackoverflow.com/questions/42999983/typescript-removing-readonly-modifier)
 */
export type DeepMutable<T> = {
  -readonly [P in keyof T]: DeepMutable<T[P]>;
};

/**
 * A deeply `Readonly<T>`.
 */
export type DeepReadonly<T> = {
  readonly [P in keyof T]: DeepReadonly<T[P]>;
};

/**
 * A dual of [[DeepMutable]].
 */
export type DeepImmutable<T> = DeepReadonly<T>;

/**
 * A proof to check that [[DeepMutable]] allows assignments.
 */
const _proofDeepMutable = () => {
  const _x: DeepMutable<{
    readonly xx: {
      readonly xxx: number;
    };
  }> = { xx: { xxx: 42 } };

  _x.xx.xxx = 10;
};