[Storment33]

It is called snapshot testing, very valid technique. Maybe not best suited to a mathematical function like they have here, but I have found it useful for stuff like compilers asserting on the AST, where it would be a pain to write out and assert on the output and may also change shape.

[xenophonf]

TIL. That looks like a nice way to add tests to legacy code without having to re-create what TDD would have had the developers started that way.

[globular-toast]

It is indeed a good way to add regression testing to code with no tests. But it's no substitute for TDD. It can't tell you why something is the way it is, nor can it distinguish between intentional and incidental (although maybe some would argue you shouldn't, given Hyrum's law and all). But it will at least guide you as you try to figure that out and stop you breaking stuff constantly.