I was a hobbyist programmer for a long, long time before I ever got paid for anything related to programming. For me, the biggest leap was learning about testin

I was a hobbyist programmer for a long, long time before I ever got paid for anything related to programming. For me, the biggest leap was learning about testing. I had heard about tests for a long time, but they were always presented as tedious and dry. In my experience, that's completely inaccurate. Even as a solo programmer, tests are amazing. They let you stay in the "if it works, it's good enough" camp as long as you need. "Okay, it works. Write a test that proves that it works. Now write some more specific tests that prove it works in specific ways, under specific questions." Having tests in place lets you start to experiment with your code in provable ways. The only other thing that comes close to the impact of testing is learning to properly profile a codebase. But if I had to choose one, I'd choose testing to focus on.