There is a section in the Kotlin documentation on how to configure a Maven project to use Java and Kotlin, and still I needed help from this Slack forum when I had to do just that with a work project which used Maven modules and Spring Boot. I still needed over an 1h the second time I had to do this recently even though I had the first project as a template, so I wrote a blog post about what I learned: How to add Kotlin as a second language to a SpringBoot app using Maven multimodule

Working in a huge nodejs project I see having a place for documentation and spending one hour at setting up something trivial as a utopia 😅

spending 1-2h hours happened only on my second project, when I had the first as the blue print. The first time with only the documentation I failed the first day. Luckily another user of this forum agree to help me! I shared my screen and together with his help we got my project configured to also allow Kotlin 🙇‍♂️ But since I'm going to hold a presentation on this topic in two weeks time, I thought, I better double check if I now understand the steps correctly so I wrote a dummy project with only java, java/kotlin and only kotlin -config. and a blog post to come with it 🙂 hard things in programming: cache invalidation, naming things, off-by-1 error and configuration!