Refactoring code with TDD, continuous testing, and linting

A little demo of agile software development — one aspect. I play around in some Ruby code on the right, breaking things and fixing them. Finally, I refactor this code to remove the unnecessary mutable variable. Meanwhile the console on the left re-tests that code every time I save a change. And if those tests pass, it auto-tests the _entire_ app. Finally, the editor on the right is continuously "linting" my code with Rubocop, showing me errors and style warnings as I go. One of the big benefits to this setup: it's safe and actually fun to refactor (clean up) my code.