FYI I, an author of TeaVM, used to work in Kotlin/JS team in JetBrains. Later I came to conclusion that Kotlin is still very Java-ish language, which inherits a lot of design of JVM platform and will always be 2nd class citizen in JS world. In that sense Kotlin/JS is not that different from TeaVM. There were same challenges on designing interop between Kotlin and JS, as with TeaVM and JS. True, in some rare cases having source code makes things easier. True, Kotlin/JS interop is little bit better that TeaVM's. This was not caused by impossibility to make better interop in TeaVM, this was deliberate choice. There's always trade-off between performance and interop quality. My decision was folloing: since it's impossible to make perfect interop and be 1st class citizen, I better make TeaVM perform good its primary job: execute Java.

These thoughts made me quit my job at Kotlin team and join Delightex to work on their product that uses TeaVM heavily: https://edu.cospaces.io/