@maple the main idea is that there's no "state" in functional programming—everything is just inputs and outputs, and the same inputs always make the same outputs. this makes it easier to do analyses of the program (to prove correctness or to disallow bugs or etc), so it's very popular in science. Not really useful for the web and pretty much impossible to use for gamedev. You *can* do it in JS if you want to but there's no real benefit.