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Discussion on: Deep Readonly Generic in Typescript

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grief profile image
Grief •

Your code works incorrectly for tuple types:

const works: DeepReadonly<['one'|'two', 1 | 2]> = [1, 2]; // TS2322: Type 'number' is not assignable to type 'Readonly<"one" | "two">'.
const doesntwork: DeepReadonly<{x: ['one'|'two', 1 | 2], y: string}> = {x: [1, 2], y: 'asd'}; // no error
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bwca profile image
Volodymyr Yepishev •

That's a good observation, I haven't accounted for the tuples when designing it, thanks 👍

Perhaps, I'll revise it 🤓

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Volodymyr Yepishev •

Apparently it's the inferring the array type and passing it back to the Array generic that messes up the case for tuples. Turns out this step can be avoided by passing the property type directly, then the generic gets even smaller:

type DeepReadonly<T> = Readonly<{
  [K in keyof T]: 
    T[K] extends (number | string | symbol) ? Readonly<T[K]>
    : Readonly<DeepReadonly<T[K]>>;
}>
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Here's the playground with the tuple example you provided.

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grief profile image
Grief • • Edited

I don't get, what's the benefit over this simpler variant?

type DeepReadonly<T> = {
  readonly [K in keyof T]: DeepReadonly<T[K]>;
};
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bwca profile image
Volodymyr Yepishev •

I don't see any apparent advantages, this one seems less verbose 🤔