Ive had a hard time adjusting to arrow functions for some reason. If you remember input => output, that should make it click for you. Just remember: Why dont you check out .map .filter and .foreach next?
Javascript module patterns
I’ve come across a good blog post about javascript module patterns. You want to start using these to make managing larger projects easier and not polluting your global with tons of functions. Basic Module Example Locally Scoped Object Literal Revealing Module Pattern