Great help, thank you. So I moved everything into ...

# multiplatform

[Robert]

Great help, thank you. So I moved everything into the companion in the expected classs

companion object {
        val ZERO: BigInteger
        val ONE: BigInteger
        val TEN: BigInteger
        val NEGATIVE_ONE: BigInteger
    }

And then I implemented it in the JS actual class. That seems to work.
Now the final step seems to add it to the type-alias in JVM. It already has ZERO, ONE, TEN but NEGATIVE_ONE has to be added. How do I do this with a typealias?
actual typealias BigInteger = java.math.BigInteger