Unit tests

A unit test might call a calculateTotal function with a cart of two items and a discount code, then assert that the result is exactly what it should be. Because it touches no network, database or browser, hundreds of them run in a second or two, so developers run them constantly while working and CI runs them on every change.

Anything that would make a test slow or unpredictable, such as an API call or the current time, is swapped for a mock or a fake. Unit tests form the base of the testing pyramid: many cheap tests at the bottom, fewer integration tests above them and a handful of end-to-end tests at the top. They are good at pinning down logic and edge cases, and weak at proving the whole app works.