DeepReadonly: Type instantiation is excessively deep and possibly infinite

[budiirawanrapid]

Setup

ts-essentials 9.0.0
typescript 4.5.4

Problem

I have this TS definition

export type JSONObject = { readonly [key in string]?: JSONValue };
export type JSONArray = JSONValue[];
export type JSONValue =
  | string
  | number
  | boolean
  | null
  | JSONArray
  | JSONObject;

Then I tried to make the typing immutable

export type MyObject = DeepReadonly<JSONObject>;

const test: MyObject = {
  id: 'a88bd472-a0c7-4e98-86d6-467d8ff1932f',
  data: {
    field1: 2,
    field2: 3,
  },
};

But I got error in Typescript

Type instantiation is excessively deep and possibly infinite

I'm sure this is because of JSONObject that could have JSONObject as the value, so it'll become infinite. Is there a better way for this scenario?

Here is the link to replicate the issue https://stackblitz.com/edit/typescript-6dunp8?file=index.ts

[Beraliv]

Hey Budi @budiirawanrapid!

Thank you for the report!

I also could reproduce it here – https://tsplay.dev/WvYnrm

Looks like recursive calls are not properly handled with this declaration of types
It tries to go further and further until gets to the recursive limit

I will try to come up with a workaround or a solution and will let you know

[Beraliv]

I found interesting answer on Stackoverflow about unbinding keys from original type, making it non-homomorphic – https://stackoverflow.com/a/67040809/3745332

e.g. https://tsplay.dev/W4yZKw

It helps but we lose the info about keys: they aren't optional, readonly or required anymore

Looking for other ideas

[Beraliv]

That I can suggest at the moment: you can add readonly for the definition of JSONArray and then the whole object will be readonly (as every key in JSONObject is already readonly)

From our side it's quite difficult to identify infinite objects

That's why we don't support it at the moment

I will mention it in our README, but thank you anyway for your finding! 🚀

[budiirawanrapid]

Hi @Beraliv, thank you so much for your response. 😄

From our side it's quite difficult to identify infinite objects

Yes, I noticed the difficulty. For JSONObject, there's no base scenario that could stop the recursion.

That I can suggest at the moment: you can add readonly for the definition of JSONArray and then the whole object will be readonly (as every key in JSONObject is already readonly)

👍 definitely will apply the readonly for JSONArray

Feel free to close this PR if no longer valid. Thank you.