Functional Programming in JavaScript

I was really into functional programming for a while. I also used to teach a functional programming in JavaScript workshop back in the day. In my current job, our entire backend is written in Elixir. So it's a fully functional programming backend. If it works at runtime, or if it works at compile time and when you're building it, it will work in prod. It will work for your users because that's just by nature,.functional programming doesn't have side effects. But anyway, that is high level. What functional programming is for those who might be wondering?