`NaN` problems

[frontsideair]

Hi, I just found out about this project while writing my own Jsonify helper.

I wanted to document a few limitations regarding the unfortunate situation with NaN. Maybe this can be added to the readme.

Unfortunately, NaN is not a TypeScript literal and cannot be detected at the type-level. I'm surprised that you found a way to detect Infinity and -Infinity so I still have some hope. :)

The affected helpers are Jsonify and Finite, while this list may not be exhaustive.

In case of Jsonify, a NaN value is encoded as null a number can be incorrectly encoded as number when it's a NaN.

In case of Finite, since NaN is not included in finite numbers (according to Number.isFinite check), it can also return an incorrect type in case of NaN.

For context, I needed a Jsonify utility to have typed loader data in Remix, after a JSON encode/decode chain. I also opened an issue (microsoft/TypeScript#48697) in TypeScript repo to request a first-class helper type.

I hope this issue will be helpful and I can try to help in any way I can.

[ikeyan]

@fregante While there is no NaN literal type in TS, you can define a NaN enum member, and it behaves like a NaN enum literal.

for example:

https://www.typescriptlang.org/play/?#code/KYOwrgtgBAgiCi5oG8BQUNQHJQLxQAYoB6QgGlQF9VUBjAexAGcAXKR4ALigEY9YESAHRYA3CVIAVAJ4AHYFADkcRJEVQAlkygh6bAIZMmGgOYh9AIwA2ClvSgs5CxT0VC6jVlH1ZIF4ABO3Ej+AfwqwmKojvLYPgDKLGH4AAYAJMgRkCKUKeLEpNL0YFC0+iBQACbALMC0BnE4MQoW0lCsARogJhoAZhplLBqMqEA

enum AnEnum {
    N = 0 / 0,
}

const one: 1 = AnEnum.N; // Type 'AnEnum' is not assignable to type '1'.
const aNumber: number = AnEnum.N;
type NaNStr = `${AnEnum.N}`; // you can detect a NaN type by stringification