Well, for me, there also once was a time, when I did not quite understand [].map() and [].forEach(). They looked strange to me, and when I received a script to review, which was full of those expressions, I needed a lot of time and had the person explain the code to me. I just could not cope with it. Since then, I expanded my experience and trained those expressions, so they come natural, now.
I think, your boss has the very same problem. She might be unfamiliar with the look of it and her brain just cannot make sense of it. Maybe, she doesn't even understand the advantages, yet.
So, from my experience, what you should do is prepare a presentation. Compare (by 1-on-1 example) a for-loop to those iteration-methods. Explain, why they are better for the product (e.g. better performance?) and why they are a good fit for your code architecture (e.g. keep complexity-level low). At this point, you could throw a comparison of a full source with and without iteration-methods at her and show her how they improve overview. Then, try to add a compromise at the end. It is hard for your boss to read and understand everything at once, so you might want to introduce new methods gradually in code she has to work with a lot. For example, you might decide to start with forEach() and reduce()and implement the others using loops. After a while, when you think your boss got the grip on them, add the next ones, like map() and filter() .
You should send that presentation to your boss or show it to her in a 1-on-1 session and ask her for her opinion. Tell her that this presentation is the reason why you wrote the code that particular way. Offer her to introduce her and the team to "advanced" and "new" JS functionalities and FP principles, which improve the overall code quality and lead to a better product with less time spent reading code bulk.
I guess, I was missing such a private course :D . So, try to always offer help and advice. But keep in mind: If your boss says, that even then she does not want you to use those methods, you will have to listen to her. She is your boss after all, it is her decision what to do with your brains, and you are hired to get the job done, not to have an argument about some features.