It's very easy to write JS in a functional style, since as you point out, it already supports first-class functions, which is the most important building block.

To really go whole hog, try setting one constraint for yourself: only use immutable data. Make all your variables `const`s, and only use immutable data structures (Immutable.js is a useful library for this).

You'll discover that it's basically impossible to write code in the familiar imperative style – everything needs to be a pure function, that just takes an input and returns a consistent output, without mutating state. This really is the "secret sauce" of FP.