Unable to use custom serializer for Javascript primitive types

[shikharid]

Describe the bug
Have certain data classes which need to be de/serialised and contain JS types (primarily BigInt).
Added a custom serializer but serialisation always throws a ClassCastException.

Happy to contribute a fix, just give me some pointers.

To Reproduce

• Try running following tests written for js bigint

@Serializable
data class ObjWithPrimitiveJsType(@Contextual val value: BigInt)

fun toBigInt(value: Long) = (js("BigInt") as (dynamic) -> BigInt)(value.toString())

class BigIntSerializer : KSerializer<BigInt> {
    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("BigInt.js", PrimitiveKind.INT)
    override fun deserialize(decoder: Decoder): BigInt = toBigInt(decoder.decodeLong())

    override fun serialize(encoder: Encoder, value: BigInt) = encoder.encodeLong(value.toString().toLong())
}

class NativeLongTestsJs {

    fun `js primitive serializer bug`() {
        val data = ObjWithPrimitiveJsType(toBigInt(Random.nextLong()))
        val encoded = json.encodeToString(data)
        println(encoded)
    }
}

Expected behavior

• Value should ser/deser properly, ser throws CCE instead
• The culprit seems to be an overridden serialize method in the generated code which does a type check like:
• Generated code:

  serialize_tl1wd4_k$(encoder, value) {
    return encoder.encodeLong_3didw_k$(toLong_0(toString_1(value)));
  }
  serialize_5ase3y_k$(encoder, value) {
    return this.serialize_tl1wd4_k$(encoder, value instanceof BigInt ? value : THROW_CCE());
  }

• For JS primitive types, instanceof checks fail and one needs to do a "typeof" check instead iirc
• This means kotlinx serialization can never be used to work with JS primitive types
  • Especially never for js types, as their BigInt doesn't even return a boxed value. It's just a constructor call that returns the primitive bigint value

---

Workarounds considered

• Use a wrapper type for all primitive types, kinda dirty but solves the problem. Boilerplate for js devs who consume it.
• Use "new Number" type constructors for creating primitive values, like:

  val jsNewNumber = js(
    "function(x) { return new Number(x); }"
  ) as (dynamic) -> JsNumber

  • This is not a solution as these objects can be passed directly from js apps in my case
  • And there I cannot control how these values are created (via constructor or notation)

Ideal Solution

• Some way to mark a serializer js primitive so that it either avoids the instanceof checks or uses typeof check

Environment

• Kotlin version: 2.1.20
• Library version: 1.8.1
• Kotlin platforms: JS for above issue (actual code is targets js/android/ios)
• Gradle version: 8.7
• IDE version (if bug is related to the IDE) [e.g. IntellijIDEA 2019.1, Android Studio 3.4]
• Other relevant context [e.g. OS version, JRE version, ... ]

[shikharid]

For anyone that stumbles upon this, this is the dirty hack I did for serializer:

// create a serializer that will accept Any object, but ser/des it expecting BigInt
class JsLongSerializer() : KSerializer<Any> {

    override val descriptor: SerialDescriptor =
        PrimitiveSerialDescriptor("bigint.${Platform.Target}", PrimitiveKind.LONG)

    override fun serialize(encoder: Encoder, value: Any) = encoder.encodeLong(value.unsafeCast<BigInt>().toString().toLong())

    override fun deserialize(decoder: Decoder) = BigInt(decoder.decodeLong().toString())
}

// then configure json as so to get around compile-time generic type checks
val JSON = Json {
    serializersModule = SerializersModule {contextual(PlatformLong::class, JsLongSerializer() as KSerializer<PlatformLong>)}
}

The end effect is instanceof type checks aren't added in generated JS code, only a nullability check added in generated code for the CCE case.
So life's good with some dirty hacks.