In the book Working Effectively with Legacy Code, Michael Feathers defines legacy code as, simply, code without tests. He elaborates that with tests one can easily modify the behavior of the code quickly and verifiably. In the absence of tests, it is impossible to gauge if the change made the code better or worse.

Often, we do not know enough about legacy code to confidently write a test. Michael recommends writing tests that preserve and document the existing behavior, which are called characterization tests.

Unlike the usual approach of writing tests, while writing a characterization test, you will first write a failing test with a dummy output, say X, because you don't know what to expect. When the test harness fails with an error, such as "Expected output X but got Y", then you will change your test to expect Y. So, now the test will pass, and it becomes a record of the code's existing behavior.

Note that we might record buggy behavior as well. After...