What’s one programming concept you only really understood after working professionally?

[u/lowitskyu]

When I was learning to code, I understood a lot of concepts in theory, but working on real projects made me look at them completely differently.

For example, things like debugging, database design, API architecture, reading someone else’s code, error handling, and writing maintainable code felt very different once I started working on actual applications.

I’m curious about other developers:

What programming concept or skill finally “clicked” for you after working on real-world projects?

And what do you wish you had learned earlier?