If I have projects which were written without tests because I was an idiot and didn't know "running it and seeing that it works" counted as a test, should I go back and write tests for what I already have or start testing on new code only?
1. Don’t ship any new features without writing unit tests for them first.
2. These new features most likely rely on parts of existing code so write a few tests for that existing code with each new feature.
3. If something breaks in production write a test for it at the time that you fix it.
Little by little you’ll increase your code coverage without grinding development to a halt.