Can someone tell why `Promise<List<Item>>` is expo...

# javascript

[Volodymyr Galandzij]

Can someone tell why Promise<List<Item>> is exported to TypeScript as Promise<any> while Promise<JsArray<Item>> preserves both the container and item types?
Is it expected behavior and we should use `JsArray`/`JsReadonlyArray` to preserve list item type in the Typescript declarations?

// JsItemsRepository.kt file

@OptIn(ExperimentalJsCollectionsApi::class)
class JsItemsRepository() {
    fun getAllArrayAsync(): Promise<JsReadonlyArray<Item>> = scope.promise { … }
    fun getAllListAsync(): Promise<List<Item>> = scope.promise { … }
    fun getFirstAsync(): Promise<Item> = scope.promise { … }
}

// mymodule.d.ts file

class JsItemsRepository {
    getAllArrayAsync(): Promise<ReadonlyArray<com.test.my.Item>>;
    getAllListAsync(): Promise<any/* kotlin.collections.KtList<com.test.my.Item> */>;
    getFirstAsync(): Promise<com.test.my.Item>;
}

[Edoardo Luppi]

Hey! Which version of Kotlin are you using?

[Volodymyr Galandzij]

Hey! The latest and greatest: 2.1.21

[Volodymyr Galandzij]

FWIW my Gradle js block is:

kotlin {
    js {
        browser { }
        binaries.executable()
        generateTypeScriptDefinitions()
    }
}

[Edoardo Luppi]

This looks like the compiler not detecting that the Collection type List must be exported.
To prove it, add an exported dummy function that returns List<Item>, and you'll see the Promise type will be correctly generated.

@Artem Kobzar let me know if you know of an open issue for this one, or if I have to create one.

[Volodymyr Galandzij]

Magic!

class JsItemRepository {
    getItems(): kotlin.collections.KtList<com.test.my.Item>;
    getAllAsync(): Promise<ReadonlyArray<com.test.my.Item>>;
    getAllListAsync(): Promise<kotlin.collections.KtList<com.test.my.Item>>;
    getFirstAsync(): Promise<com.test.my.Item>;
}

[Volodymyr Galandzij]

Thanks @Edoardo Luppi

[Edoardo Luppi]

Obviously that workaround is sub-optimal, but at least you know how it works now!

[Artem Kobzar]

Oh, there is no known issue like this. Volodymyr, could you please create an issue in YouTrack?

[Volodymyr Galandzij]

Sure thing! Do you need a repro project, or the example I provided here is enough?

[Artem Kobzar]

I believe you can just provide in the description a simple top-level function with a Promise<List<Int>> return and the text of the generated d.ts for the function.
So, it would be like this:

fun foo(): Promise<List<Int>> = ...

The generated d.ts:

declare function foo(): Promise<any/* kotlin.collections.KtList<number> */>>

However the expected behavior is:

declare function foo(): Promise<kotlin.collections.KtList<number>>>

[Edoardo Luppi]

A very basic reproducer is:

// main.kt

// List won't be exported to KtList
@JsExport
fun promiseListExport(): Promise<List<Int>> {
    return Promise.resolve(listOf(1))
}

// Uncomment the following function so that the compiler
// picks up the List type and correctly exports it
// @JsExport
// fun listExport(): List<Int> {
//     return listOf(1)
// }

[Volodymyr Galandzij]

@Artem Kobzar done:
https://youtrack.jetbrains.com/issue/KT-78316/List-is-not-exported-to-TypeScript-declaration-if-wrapped-in-Promise