[haspok]

I would highly recommend learning FP fundamentals in another language, not Javascript.

For example, there is an excellent introductory FP course on Coursera by Martin Odersky ("Functional Programming Principles in Scala"). It happens to be based on the Scala language syntax, but most of what you learn there will be directly applicable in other FP languages too. For me, taking this course was a game changer ~10 years ago. Highly recommended.

Once you understand the basics, picking up stuff like "arrow functions" (lambdas) will be just a matter of adjusting to the JS syntax.