Jest (http://facebook.github.io/jest/) is the flavour of this tut. An introduction to javascript testing and some of the features that Jest has to offer.
When Jest is run (npm test) it automatically detects tests in the tests directory and executes them.
The tutorials here are contained into a seperate, numbered directory. Each directory has its own Readme to explain the purpose and tasks of the tutorial.
There is nothing required to do here. A basic example of a test for the sum function.
This is an intro to the automatic mocking feature that Jest provides.