Unit testing focuses on testing a single function or module in an automated fashion. Unit tests often assert that a certain function returns an expected value when past certain parameters. Unit tests mock out external dependencies like APIs, database calls, and file system interactions, so the results are fast and deterministic.
Two other styles that are worth looking into that we won't cover in Ollie-UI are integration testing, which focuses on testing the interactions between multiple modules and automated UI testing, which tests the application by automating clicks and key strokes within the actual UI and asserting that it interacts in expected ways.
Tools like Selenium, which automate browser interactions are popular in this space. There are various other testing approaches as well, but in Ollie-UI, the focus is on automated unit testing.