Why do we need unit tests?

Unit tests are used to verify the correctness of individual parts of the code — functions or methods — in isolation from the rest of the system.

They help to:

• Quickly detect errors at early stages of development.
• Ensure stability when making changes and refactoring.
• Document the behavior of the code through usage examples.
• Improve the quality and reliability of the software product.

For example, if you have a function that calculates the sum of numbers, a unit test will check that it returns the correct result for different input data.