Alexander Shvets,First, let me say your site and PDF are fantastic. Like others on this forum, I would also pay top dollar for a printed hard copy to add to my book collection. I sport your site every chance I get with new developers. The concepts are solid and based on good programming theory. The animations are the icing on the cake. They resonate with an old guy like me down to my young kids. Your work transcends. Concerning the unit testing, I think you should add the theory to your topics because part of having good code is the ability to refactor it without breaking it. As you mentioned in your "Refactoring" section, a good refactor must have tests. Perhaps you would consider starting key concepts that every unit test should have. Below are just a few suggestions for Unit Test Code Smells. =)
If you need help, I can volunteer my time.
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