Koa2 is the second version of the Koa framework. Koa is the product of the same team that created Express.js; however, it is rather different from Express.js and not nearly as popular. It is now in the process of heavy development, with version 2 being the stable release of this framework.
Koa2 belongs to the family of Node.js MVC frameworks, where MVC stands for Model-View-Controller. The three components of MVC are the model which is the data of the application, the view which is the data representation via the app UI, and the controller transferring requests and updates between the model and the view.
Comparing Koa2 vs Express, the first one is more modular and lightweight with better customization possibilities. Koa2 allows you to start with an application that is almost empty and add only those features you need, as you go along. Also, it has good error-handling capabilities due to its use of generators that allow doing without callbacks and the issues that may be related to them.
Koa2 uses a number of specific methods which makes it somewhat difficult to learn. Moreover, it requires at least some basic knowledge in promises and generators. In any case, this difficulty should not discourage JavaScript developers appreciating its obvious benefits.

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