When using JS on the web, for me, the units of understanding are:
The Document Object Model (you may have already interacted with this using CSS). Being able to understand that all the elements on a page are arranged in a tree and how to move the branches around.
Callbacks. JS can be a little unintuitive because of its asynchronous nature. Understanding callbacks makes most of this clear.
Eventually this understanding leads to all the shenanigans with promises, but I believe callbacks to be the unit of understanding.
Methods. From elements of the DOM tree, to inbuilt classes, to libraries, most things are interacted with using methods.
I suggest going through things in the MDN documentation to see their methods. E.g. the JS Date class.
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