Connect 4 is a turn-based two-player game, where each player would drop a chip down a column, with the objective to get four chip of the same color in a row. It can be vertical, horizontal, or diagonal.

In this chapter, we will build Connect4-as-a-Service. An API that allows you to build a game of Connect 4 on any client, be it a website, mobile app, or just play it from the command line; why not?

In Chapter 1, Building a Basic Express Site, and Chapter 2, MMO Word Game, we covered the most generic use cases for an Express backed API, which is to serve and persist data to and from a database. In this chapter, we'll cover something more fun. We'll build a multiplayer game API!

Some topics that will be covered include authentication, game state modeling, and validation middleware. Also, we will build an app using test-driven development with maximum code coverage.

For your reference, this is the folder structure of our app, which we will build throughout...