Testing is definitely required! Not just for the complex parts, but for the simple ones as well. You never know when the simple part becomes complex :)
I agree, but if testing is not part of your routine starting with "test absolutely everything" will result overwhelming and you may just give up entirely.
A good approach is to start where it hurts. Then add tests for every bug you find. Then if you still have time and budget feel free to go all in!
It also depends if you are building a library (I'd test as close to 100% as possible in that case) or a Gatsby website (I'd maybe only test a few UI components)
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