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I have not down voted it myself but there is the assumption that unit tests will always catch bad refactorings, that human error doesn't apply to writing unit test, that every developer is perfectly conscientious, competent, is not under tight deadlines etc., that your legacy code base has comprehensive unit tests..

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The author here: I agree with @Gupie, having unit tests ensures that you have correctly refactored the code and have not introduced new bugs. Checking the unit tests can also help you better understand what problems the code is solving and the edge cases you may be missing.

Normally, if unit tests were missing, I would add them before starting refactoring. In this case, to be honest, I'm not sure how I could have written a unit test to catch the issue. An E2E test is the only one that comes to mind. Please let me know if I am missing an easy way to test for a 404 image when offline. Thank you!

The danger is in the combination of all 3: non-obvious assumption (code should work offline), lack of tests, lack of documentation of the assumption.

Flip ANY of the three, and there is no problem. Had I been the original author I would have gone for the comment, it’s clearly the simplest one in this case.

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