One benefit of testing I haven't seen much discussed yet is that testing can make fixing bugs easier. Many times in my career, there was a bug reported in a function I am not familiar with, based on a very particular set of conditions. There is a lot of work required to simply recreate the conditions that will allow me to debug. If I have written a test, where I have created appropriate fixtures and isolated the logic, I can easily recreate the failure condition and find where my logic is broken.
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