I disagree. There is no code that can't be tested. There is certainly no code that has to be changed so it can be tested.
The only reason you would decide code is untestable without touching it is because it doesn't seem worth the effort, so you test the thing that calls that code instead and mock it out.
At the highest level it is all inputs and outputs after all. Your spaghetti app can still be tested end-to-end, which will cost you the most but will work.
We're talking about SaaS and web apps and shit here of course, because we're on HN where downtime on Cloudflare equals a day off work. In other industries you'd be running full blown simulations and proofs and maybe even inventing new mechanisms of testing so you can get a hard project off the ground. Maybe even fabricating materials.
Sure, you can test more or less anything given sufficient time and force, but a massive fragile harness wrapped around the system under test is not what I mean by 'testing well'.
Also there are other things in software besides web; unit testing code that is entangled with desktop UI can be difficult or impossible without changing the original code. Try testing a WPF app that talks directly to the Dispatcher when there is no UI thread, for example.
The only reason you would decide code is untestable without touching it is because it doesn't seem worth the effort, so you test the thing that calls that code instead and mock it out.
At the highest level it is all inputs and outputs after all. Your spaghetti app can still be tested end-to-end, which will cost you the most but will work.
We're talking about SaaS and web apps and shit here of course, because we're on HN where downtime on Cloudflare equals a day off work. In other industries you'd be running full blown simulations and proofs and maybe even inventing new mechanisms of testing so you can get a hard project off the ground. Maybe even fabricating materials.