I actually really love the MDN and use it as my main go-to reference.
It's definitely not a set of tutorials or hand holding for beginners. Nor is it meant to be. It's a comprehensive reference (like a dictionary or encyclopedia). And as such, it should not - with every definition and article - seek to re-define the most basic of terms. That would cause massive bloat in every article. Instead, it is up to the user to educate themselves to the level that the tool is useful.
The good news is that MDN also provide beginner materials:
(I haven't used those so I can't comment on their quality).
A dictionary does not aspire to teach you to write a novel. Nor does an encyclopedia. They provide knowledge in a structured, searchable/discoverable format. The modern versions on the web allow for easy linking and deep dives via links like the paper versions never could.
There is tons of material for beginning coders. The MDN is meant to simply set out to define terms and capabilities. It is then up to the developer to combine those into constructs (a program!) which does what they want them to do.
@P1xt's response from June '16 is right on point. Use it as a tool to explore. I can't count the number of times I've run across a method/API while browsing the MDN that I hadn't heard of. Even if I don't explore it in full right then, I now know it exists and can go back to it later when I want to use it.
It comes down to using the proper tool to help solve the problem you are faced with.
index?, etc.). If you don't understand what you are reading there that simply means you're missing some foundational information - not a bad thing at all - but further study is required to get up to the point where reading one of those articles is comfortable and easy.
Once you reach that point, I think you'll be thankful that every article doesn't re-define basic terms before getting to the point.