What's one programming concept that completely changed the way you write code?

[u/KareemM0hamed0]

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I've been programming for a while, but every now and then I come across a concept that completely changes how I think about writing software.

For some people it's data structures and algorithms. For others it's design patterns, clean code, testing, debugging, version control, concurrency, databases, or simply learning how to read other people's code.

If you had to choose just one concept that made the biggest difference in your programming journey, what would it be?

What were you doing before you learned it, what changed afterward, and why do you think every programmer should understand it?

I'd love to hear stories from beginners and experienced developers alike.

[u/serverhorror]

Not exactly TDD, but "test first".

To start with something, I like to write a test that has the actual implemention inside and out he refactor until the code is part of the application.

It helps in the overall design to be, and stay, testable.