This example demonstrates a pure function called `testFunction` that takes a state number and an action object, returning an updated number for increment, decrement, or unchanged states. The code uses the Expect library to assert five specific input-output pairs, validating correct logic for both counter operations and an unknown action type. After the assertions, it logs "tests passed!" to the console. The reducer pattern mirrors Redux’s action dispatch, though no UI or visualization is rendered—the focus is solely on unit-testing the state transition logic.