What does unit testing assess in software development?

Unit testing is focused on assessing individual units of source code, typically at the function or method level. The primary purpose of unit testing is to validate that each unit of the software performs as intended, often checking for specific conditions and outputs based on given inputs. By testing smaller components in isolation, developers can ensure that the logic within is correct before combining them with other parts of the system. This type of testing helps in identifying bugs early in the development process, making it easier to trace and fix issues as they arise.

In contrast, assessing overall system performance, the integration of multiple modules, or user interface design involves different testing methodologies. Overall system performance might involve load testing or performance testing, while integration testing focuses on how various modules work together. User interface testing, on the other hand, examines the usability and design aspects rather than the individual functions. Therefore, unit testing's specific goal is to ensure the correctness of individual pieces of code, which is fundamental to building reliable software.