Fortunately, by this point in my studies there is less mystery about proper syntax, and I decided to pop into Stack Overflow, and it didn't take long for me to realize...
...that what I was trying to iterate over LOOKED like an array, but it wasn't. It was an HTMLCollection. But you can turn that into an array with the equally convenient spread operator, and then use forEach().
const myArray = [...document.getElementsByClassName('myClass')]
const myArray = document.getElementsByClassName('myClass')
BEFORE trying forEach() is the code that fixed my problem.
I'm trying to cram a 4-6 month Udacity Fullstack nanodegree program into 1 free month. Saving money is a great kick-in-the-ass motivator, and I've never spent so many hours coding in a week. I can see that up until now I've underestimated my ability to soak in a lot of new information and debug dozens of new problems - my head is spinning a bit, but instead of feeling overwhelmed I actually feel more competent as a coder than ever before.