To install: 
npm install

To test:
npm test

To run:
npm start

Purpose:
This API takes historic viewership data and provides a list of recommended subtopics for a given subtopic.
The recommendation is based on the conditional propability that a given viewer watched each subtopic given that they watched the provided subtopic.

Try it out:
After starting the server, navigate to
localhost:3000/recommendations?subtopic=57d80b25e04b0603003bffa3&limit=3&pretty=true
And see the recommendations!

API reference:
The server has two routes

/recommendations route which supports the following options in the query string
subtopic (required): the UUID of the subtopic that the recommendations should be based on.
limit: an integer specifying the number of results desired
pretty: a boolean specifying whether the result should be rendered

/listen route which receives post requests for new user listens in the following format:
{
    subtopic: 'uuid of the subtopic the user listened to',
    user: 'the uuid of the user'
}