Having a suite of automated tests for your code helps improve software quality and maintainability in several dimensions. Writing unit tests for your software is likely to cause you to incorporate several design aspects that make it easier for other developers to use your and which have the side-effect of dramatically increasing the maintainability of your code overall. Testable code typically has fewer tight couplings between components, dependencies that are injectable, and encourages SOLID design principles in a naturalistic way because SOLID code makes writing tests (and therefor all usage) of your code easier. Automated unit tests also help you write a verifiable usage contract between your components that enables you to find and isolate bugs or perform major refactoring on your code without fear of breaking existing features. All of this leads to higher quality software.
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