We build a product in Clojure that runs on the JVM and in the browser (https://kpow.io).

Clojure/Clojurescript gives us a language/toolset that works in both the front and back ends. In some cases we have we have code in a single source file (.cljc) that works in both the browser and on the JVM with no modification.

The feature that cross-environment functionality implements in a single source file is complex in some cases (e.g. a grammar / parser / interpreter for a subset of JQ).

Both the front and back end use the same core functions and same datastructures to implement the product. Data is moved between front/back via Transit (and encoding format for Clojure datastructures).

In my experience the reduction in complexity through the stack is staggering. Once you've learned Clojure that is..