DeepReadonly Object Typescript

Question

It is possible to create a DeepReadonly type like this:

type DeepReadonly = {
  readonly [P in keyof T]: DeepReadonly;
};

         


        

Answer 1

As of TypeScript 2.8, this is now possible and actually an example in the PR for Conditional Types: https://github.com/Microsoft/TypeScript/pull/21316

Also see the notes on type inference for Conditional Types: https://github.com/Microsoft/TypeScript/pull/21496

I modified the example slightly to use the type inference for the readonly array value type because I find (infer R)[] clearer than Array but both syntaxes work. I also removed the example NonFunctionPropertyNames bit as I want to preserve functions in my output.

type DeepReadonly =
    T extends (infer R)[] ? DeepReadonlyArray :
    T extends Function ? T :
    T extends object ? DeepReadonlyObject :
    T;

interface DeepReadonlyArray extends ReadonlyArray> {}

type DeepReadonlyObject = {
    readonly [P in keyof T]: DeepReadonly;
};

Doing DeepReadonly this way also preserves optional fields (thanks to Mariusz for letting me know), e.g.:

interface A {
    x?: number;
    y: number;
}

type RA = DeepReadonly;

// RA is effectively typed as such:
interface RA {
    readonly x?: number;
    readonly y: number;
}

While TS still has some easy ways to lose "readonly-ness" in certain scenarios, this is as close to a C/C++ style const value as you will get.