What should and should not be unit tested when prototyping javascript/NodeJS application?

I know that ideally I should unit test everything. That was my attitude so far. However, as a one-man startup I consider it to be totally wrong approach, especially for prototyping. Mainly, because majority of code is going to be changed or modified before reaching production.

I found this answer but it still seems to be an overkill for prototyping.

I understand that not testing is not an option, but how to find a balance of what should and should not be tested?

If you answering this question I would greatly appreciate links to resources as well, if you know any good write-ups covering that topic please.