What is Unit Testing or Component Testing?

I test systems, not code. Why do I need to understand unit or component testing?
Developers usually run unit or component and integration testing themselves. Testers usually test whole systems. If you don't know what developers are testing, then how do you know you're not duplicating what they do? How do you know there are gaps in the testing - what they don't test - and you don't test either?
You need to understand what developers test, so your later testing complements it. You need to have a conversation to find out what they're doing. How do developers do their testing and what do they test, what don't they test and why? And what can you do about it?
This video explores what component testing is, it's goals, difficulties and limitations. Testers need to understand component testing well enough to have meaningful discussions with developers to optimise the test process as a whole. Ultimately, the relationship between testers and developers has to be collaborative not adversarial!
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The Testing Glossary Project playlist: • The Testing Glossary P...