Structure and Intepretation of Computer Programs vs Professor Frisby's Mostly Adequate Guide to Functional Programming

This isn't a book you'll glean direct practical tidbits out of, an introduction to not only functional programming but how to think in a paradigm outside the usual way C*/Python/Ruby/Java/etc... are coded.

Even if you wind up never working in Scheme or any other primarily functional language, the tactics and thought processes you'll learn here will apply to any currently-evolving language to a greater or lesser degree. You'll be able to map your thought process into the paradigm that works best for your current situation and not just be forced into a limited set of idioms that causes unnecessary boilerplate and clunky code mangling.

Like closures and encapsulation without language support of objects.

This book gives a great insight about immutable and mutable state (with pros and cons), typisation, FP, OOP, and many other things in a very beginner-friendly manner

It's available free on Github.

You can submit issues to suggest improvements, or submit pull requests.
Thanks to it being open source, it's constantly being improved and updated.

JavaScript is popular, ubiquitous, and has the necessary power to teach core concepts.