Tested Application Step 1: Decide What Matters

Take the application you carried through the earlier courses. List every behavior somebody depends on, and sort it: security properties, expensive-to-get-wrong operations, awkward edge cases, happy paths. Then, for each item in the top two groups, write one sentence describing the test you would write, without writing it yet. Finally, look at any tests you already have and ask the refactor question of each. Delete the ones that would break on a rewrite that kept the behavior.