- How good do IDEs understand Scala these days? Compared to C#/Java, where IDEs instantly know a crazy lot about your code. I am looking at features like IntelliSense, marking wrong code, marking typos, marking unused code, telling me about unhandled Exceptions etc.
IntellIj has a very nice plug-in, it will tell you a lot about your code, including suggestions of best practices. The down side is that the more "functional" your code gets, the slower IDE becomes(that's also true for Eclipse), but chances are that when that happens, you won't care about having an IDE. So IntellIj will be great to get you started, it does help a LOT.
Is there any difference, on the IDE side, between Scala support and Scala.JS support?
Not sure about this, it seems like you can just code in IntelliJ/Eclipse, but the debugging has to happen in the browser, it does work very nicely(you debug the scala code, not the generated javascript)
I owe you the bonus question :-P