- Strong type system thanks to the type inference which will save you many headaches. Since types are inferred, you aren't forced to type out the types the whole time, so a sleek syntax + type safety.
- everything that's available in the Java world you can immediately use in Kotlin without the Java boilerplate - want to generate PDFs, simply throw in Apache FOP, want to generate Excel Spreadsheets, simply throw in Apache POI, etc; the ecosystem is huge and very well maintained
- proper multithreading
- Co-Routines which IMO is much easier and cleaner to use than ES6's Promises
- The language is very well designed and I haven't encountered any gotchas unlike JS where you have to tread carefully the whole time
- and if you really want to, you can compile the Kotlin to JS that you can run on NodeJS or just use KotlinJS in the browser.
- Kotlin Native will in the future allow you to run Kotlin without the JVM
- While you're at it, you can do Android native in the same language and there are plans for better iOS support via Kotlin Native.
- Kotlin might replace your bash scripts in the future - one language that does it all!
- The Kotlin standard lib has almost everything you need for general dev, so no need for external helper libraries to do String padding, and so forth.
- You get both Object Orientated and Functional benefits in Kotlin since they took the best features from a large number of languages and simply copied plus improved it instead of re-inventing the wheel.
- null safety is probably a huge advantage over JS.
- In terms of churn, I get a lot of work done since I switched to Kotlin a year ago and it's high quality code that just works.
If you already know JS quite well and you don't have Java experience, by all means, use NodeJS. If you're open for new things, especially coming from a Java background, give Kotlin a try, it's the best thing since sliced Kake.