Oh, worse than you'd imagine. Suffice it to say, Scala is a long, long ways from 1-1. Indeed, you have to explicitly export symbols that should be visible to Javascript, because by default they're quite munged.

And that's in the "nice" view of the generated code. Of course, when you run it through the full optimizer, which drops the code size by 80-90%, it turns into truly exceptional gobbledygook. But it runs quite well: when fully optimized, the benchmarks are currently saying that it's typically better than half the speed of hand-coded Javascript, and they're pushing to achieve parity on average...