Mastering TypeScript 3 - Third Edition

Unit tests

A unit test is typically a white-box test where all of the external interfaces to a block of code are mocked or stubbed out. If we are testing some code that does an asynchronous call to load a block of JSON, for example, unit testing this code would require mocking out the returned JSON. This technique ensures that the object under test is always given a known set of data. When new requirements come along, this known set of data can grow and expand, of course. Objects under test should be designed to interact with interfaces, so that those interfaces can be easily mocked or stubbed in a unit test scenario.