Over the course of my learnings in different other disciplines, I've found (after the fact) that understanding one fundamental thing is what it took for everything else in the entire discipline to start making sense to me.

As an example: I struggled with transitioning from CorelDraw to Photoshop for a long time because I didn't intuitively get the 'layer' system of Photoshop (CorelDraw, at the time - dunno if that's still the case - just gave you one workspace and you moved things about relative to each other using 'Align' settings.

I struggled with CSS until I understood the Box Model (margin vs padding vs border), and now I can theoretically understand the other things and where they fit.

With JavaScript, I'm wondering what that 'smallest unit of understanding is'? It feels like it should be functions, but despite understanding functions, it took me a while to understand callbacks, and after understanding callbacks, it took me a while to understand eventHandlers, and so on.

Curious about the experiences of more advanced JavaScript users: what was that crucial turning point for you?

All responses welcome!