DeepReadonly Object Typescript

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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<T[number]> but both syntaxes work. I also removed the example NonFunctionPropertyNames bit as I want to preserve functions in my output.

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

interface DeepReadonlyArray<T> extends ReadonlyArray<DeepReadonly<T>> {}

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

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<A>;

// 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.

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