- IDEs for Scala are pretty good, nowadays. They do all of what you mentioned (except unhandled exceptions, because that's a Java-specific thing). - There's no

- IDEs for Scala are pretty good, nowadays. They do all of what you mentioned (except unhandled exceptions, because that's a Java-specific thing). - There's no difference between Scala and Scala.js support. To your IDE, Scala.js is Just Scala. - There is a development cycle, absolutely, and it's pretty fast! With Workbench (https://github.com/lihaoyi/workbench https://github.com/lihaoyi/workbench) you don't even need to reload the page: it does so automatically! - For React, there is essentially scalajs-react (https://github.com/japgolly/scalajs-react https://github.com/japgolly/scalajs-react). It does not support inline-XML per se, because it relies more on ScalaTags instead (https://github.com/lihaoyi/scalatags https://github.com/lihaoyi/scalatags), but at some point there was a proof of concept showing it was possible to implement with macros.