One thing that jumps to the eye in functional code sources is longer-than-usual argument lists.

process(Tree, Env, Plugins, Config, ...).

It's easy to see that as a consequence of functions using arguments as sole input channel. (Global -define()'s don't qualify as input channels—they can't vary at runtime, arguments can.)

Usually it's no big hassle.

Ben from Simile points me to Fresno, a tool that connects to a MozRepl-equipped Firefox and drives it from the command line.