I wish there was a way to define an external interface that conforms to a normal interface. It feel...

# javascript

[Seth Madison]

I wish there was a way to define an external interface that conforms to a normal interface.

It feels like it could be done with an annotation or something that you put on a normal interface that tells the compiler not to expect getters.

[turansky]

Normal - your custom interface?

[Seth Madison]

Imagine:

interface FooCommon {
    val bar: String
}

I want to be able to do:

external interface FooJs : FooCommon {
    val bar: String
}

fun parseJs(): FooCommon {
    val myDynamic = js("""{bar: "baz"}""")
    return myDynamic.unsafeCast<FooJs>()
}

Instead I have to do something like;

external interface FooJs {
    val bar: String
}


class FooCommonFromJs(private val fooJs: FooJs) : FooCommon {
    override val bar: String
        get() = fooJs.bar
}

fun parseJs(): FooCommon {
    val myDynamic = js("""{bar: "baz"}""")
    return FooCommonFromJs(myDynamic.unsafeCast<FooJs>())
}

[turansky]

We have jso for such cases

[turansky]

Expected implementation:

fun parseJs(): FooCommon =
    jso {
       bar = "baz"
    }

[turansky]

Example

[Seth Madison]

Ah, interesting I did not know about jso.

It looks like it just casts whatever you pass as the type param, which I think is the equivalent of:

fun parseJs(): FooCommon {
    val myDynamic = js("""{bar: "baz"}""")
    return myDynamic.unsafeCast<FooCommon>()
}

Originally I tried this:

fun parseJs() {
    val myDynamic = js("""{bar: "baz"}""")
    val a = myDynamic.unsafeCast<FooCommon>()
    console.log(a.bar)
}

And it didn’t work because FooCommon was not @JsExported!!! It turns out, if you @JsExport a “normal” class, it forces the compiler to not generate JS properties that mirror the kotlin property names, and so an unsafeCast() will “just work”.

[Seth Madison]

So, for posterity, I think the solution here is:

@JsExport
interface FooCommon {
    val bar: String
}


fun parseJs(): FooCommon {
    val myDynamic = js("""{bar: "baz"}""")
    return myDynamic.unsafeCast<FooCommon>()
}

[turansky]

jso recommended in any case

[turansky]

in future Kotlin is see following variant:

@JsExport
@ExternalInJsPleas
interface FooCommon {
    val bar: String
}


fun parseJs(): FooCommon {
   FooCommon(bar = "baz")
}