This is installed into the client and server directories as an actual
dependency. This is accomplished by symlinking this directory into their
respective other directories, and then they each install it as a file located
in their ./other/shared directories. This happens in the install step of
the setup script.
This is done mostly for the server to be able to be deployed on its own.
There's not a lot of code shared here (and mostly it's just for tests), but I thought it would be fun to show one way to accomplish this kind of code reuse.
One important note is that these files are not transpiled at any point. So it's a good idea to avoid using features not available in Node8 :)