Having lived through the CoffeeScript thing, the main issues were: 1. CoffeeScript required a compile step; JavaScript didn't. It also meant that you needed to

Having lived through the CoffeeScript thing, the main issues were: 1. CoffeeScript required a compile step; JavaScript didn't. It also meant that you needed to do your debugging in JavaScript, and deal with a lot more leaky abstractions. On the other hand, Kotlin and Java both compile directly to JVM bytecode. 2. CoffeeScript had a lot of features that work very unpredictably, like a number becoming an array of numbers if you add more lines at the same indentation level (which also means that your array will surprisingly become not an array if you delete too many entries). A lot of people only used CoffeeScript because there was no alternative, rather than actually liking it, and they were the first to jump ship when ES6 gave them an alternative. Kotlin doesn't have any of these issues. It really is an improved Java.