Dotty contains preliminary support for Scala.js under the flag -scalajs.
Or rather, it contains the infrastructure for preliminary support.
It is far from being actually usable, but this is where you can help!
This small tutorial walks you through a few steps that you can do to further the support of Scala.js in Dotty. Even if you do not typically contribute to a compiler, this can be your chance. It is not very difficult, given that there already exists an extensive test suite, as well as a working implementation for scalac.