The following works fine under both platforms:

val matcher = Pattern.compile("\\d+").matcher("2147483647")

matcher.find()

matcher.groupCount

The following fails in js but not jvm:

val matcher = Pattern.compile("\\d+").matcher("2147483647")

//matcher.find()

matcher.groupCount

There's nothing in the Matcher scaladoc that seems to imply that calling find before groupCount is needed.

It does, however, seem like an unfixable bug. I'm no javascript expert, but it does rather look that regexes can only expose the group count for a given match, not for a matcher.