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    This is a much better answer than the accepted one. The imbalance in votes is dismaying. Commented Jan 29, 2019 at 20:24
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    @Lee A unit test should test a unit of functionality, which may or may not correspond to a class. A unit of functionality should be tested at its interface (which may be the API in this case). Testing may highlight design smells and the need to apply some different/more levelisation. Build your systems from small composable pieces, they will be easier to reason about and test. Commented Jan 30, 2019 at 3:44
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    @KonradRudolph: I guess missed the point where the OP added that this question was about designing new code, not changing existing one. So there is nothing to break, which makes most of this answer not applicable. Commented Jan 30, 2019 at 12:10
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    I strongly disagree with the statement that writing unit tests is always a good thing. Unit tests are good only in some cases. It's silly to use unit tests to test frontend (UI) code, they are made to test business logic. Also, it's good to write unit tests to replace missing compilation checks (e.g. in Javascript). Most frontend-only code should write end-to-end test exclusively, not unit tests. Commented Jan 31, 2019 at 7:32
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    Designs can definitely suffer from "Test induced damage". Usually testability improves design: You notice when writing tests that something can't be fetched but must be passed in, making for clearer interfaces and so on. But occasionally you'll stumble across something that requires an uncomfortable design only for testing. An example could be a test-only constructor required in your new code due to existing third party code that uses a singleton for example. When that happens: take a step back and make an integration test only, instead of damaging your own design in the name of testability. Commented Feb 1, 2019 at 12:15