What distinguishes unit tests from tests that do not return a result, such as tests with side effects?

Unit tests differ in that they test a specific piece of code (module) and return a result — usually a boolean value (success/failure) or throw an exception on error. They should be isolated, fast, and deterministic.

Tests with side effects may not return an explicit result but influence external systems (e.g., database writes, message sending). Such tests are more complex to automate and verify because the outcome is not always obvious from the test code.

Key difference:

• Unit tests check logic and return values.
• Tests with side effects check for changes in state or interactions with external components, often requiring additional tools for verification (mocks, stubs, state checks).