Another beginners’ question about `Long` and `Numb...

# javascript

[grahamborland]

Another beginners’ question about Long and Number. I have this Kotlin code:

@JsExport
fun isNegative(amount: Long) : Boolean = amount < 0L

which fails at runtime in Typescript because

TypeError: amount.compareTo_n4fqi2_k$ is not a function

Can someone gently guide me towards what kind of solutions I should be investigating? Should I just not use Long in the Kotlin code, or should I be adding some conversion logic somewhere?

[grahamborland]

I know that numbers are floating-point in JS, and I’m okay with that, so I’m looking for the least painful way to consume this existing Kotlin code in a JS environment.

[ephemient]

you should avoid using Long that interoperates with JS/TS code

[Big Chungus]

Long type is not external, annotated with @JsExport nor mapped to js native type by the compiler. As such its usage in other @JsExport declarations is unsafe.

[grahamborland]

OK, thanks. Looks like I have some pain ahead of me.

[Big Chungus]

Just using Int should be safe enough in JS

[grahamborland]

Yeah, Int isn’t big enough on Android/iOS though. I’d love to find a compromise where I can use a Long implementation on platforms which support it, and floating-point on JS. I’ll see if I can get anywhere with `expect`/`actual`

[Big Chungus]

Hmm, declare your own MyLong value class with boxed Number value, then override it to Long in your actual implementation on android/ios, and Int on js

[Big Chungus]

Or just have a separate function on js that calls you mpp function and converts the result to int before returning

[ephemient]

yeah, instead of @JsExport'ing your function, @JsExport a wrapper that converts to Int (or Double for some more range) if you need to use it as a Number in JS

[ephemient]

(and if you don't need it as a Number, I'd stringify it but I don't think everybody agrees on that)

[grahamborland]

So this seems to do kinda what I need.
common:

expect class YNum : Number, Comparable<YNum>

Android/iOS:

actual typealias YNum = Long

JS:

actual typealias YNum = Double

[ephemient]

that feels like a bad idea; x / y * y will give different results on different platforms

[ephemient]

use Int or Long everywhere as appropriate in Kotlin-only code and add JS wrappers as necessasry