The absolute bare minimum should be for you to write those informal tests down and commit them to the repo. That's some golden knowledge I've gotten over time
It just requires the overhead of setting up the test runner
It's not really any different than in dynamic/interpreted/weakly-typed languages. "Writing the test" for a function sometimes just includes writing a method/function with the appropriate type signature that does nothing (maybe returns a dummy value).
Forcing you to view the code you're implementing from the viewpoint of someone calling that code from the very beginning is one of the advantages/goals of TDD. If you find that it's difficult to set up the objects/data you need to write a test, eg, your code has a bunch of implicit dependencies on other components being in a particular state or it takes a ton of arguments that all have to be constructed, that's usually a strong indicator that you should rethink the design. You're getting early feedback on your API design before you waste time implementing it.