Thanks for your response. My issue here is not so much that Kotlin borrowed features but that it appears many credit Kotlin now for those feature, when in fact most of it has been already available under Groovy a decade before Kotlin came out.
Grooscript 1.3 was released in Sep 2016, only 8 months ago [1]. It does only have one or two developers working on it part-time as far as I can tell. I can't imagine too many people would want to put time into Grooscript (Groovy for Javascript) when Apache Groovy (Groovy for the JVM) is languishing, though.