Why write unit tests?

Unit tests are written to guarantee the reliability and predictability of the code throughout the whole life of the project. They give the developer confidence that individual functions and modules work correctly and do not break when changes are made.

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#Main reasons why unit tests are needed

1. Early bug detection Unit tests catch bugs at the function level while the system is still small. This is far cheaper and faster than looking for the error later in a large module or, worse, in production.
2. Confident refactoring When code is changed or improved, there is a risk of breaking something. Tests act as "insurance": if something goes wrong, the tests fail and you see the problem immediately.
3. Long-term project stability The project grows, developers change, the logic gets more complex. Unit tests capture the expected behavior of functions, so the system stays stable even years later.
4. Documentation of behavior The tests show how a function is supposed to behave in specific situations. They work like living documentation that does not go stale (unlike text descriptions).
5. Isolation and predictability of logic Tests force you to write modules that are independent, clean and free of hidden side effects. This improves the architecture.

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#Summary

Unit tests are a tool that helps make code safer, more stable and longer-lived, reducing the number of bugs and the stress of development.