I'd say it'd be the notion of variables and function (AKA methods). It seems like most, if not all, programming languages—not markup languages—have some sort of way to define variables and functions. Master these concepts and you're pretty much set to learn every programming language out there.
Thank you for the precise and clear answer. Does "variables and function (AKA methods)" have a defined way to learn them I mean what they call them in the programming world? There must be a term for that.
I think they're just called variables and functions in general. In functional programming, you would have functions everywhere, as its name suggests. In object-oriented programming, you encounter object methods (in classes) rather than functions. They're still pretty much the same thing, though.
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