How do kotlin-js stacktraces work?

What do you mean?

As in can you transform them into kotlin stacktraces?

No, you can't extract stack traces from JS exceptions. Actually, there's no standart stacktrace except for

stack

property, which is a string.

in fact the answer is not always "no", some tools (for example karma test runner) could extract js stacktace and convert it to kotlin stacktrace using source maps