Web application programming interfaces (APIs) have become more and more popular, as more people have started to recognize the need to separate their server-side applications from their client-side applications. APIs give developers the opportunity to offer the same web services to multiple clients, while also reducing the coupling between their systems.

In this chapter, we will build a REST API using Koa. We will make use of the different Koa concepts that we covered in the previous chapters in order to build a full-fledged API.

The API that we will build will be for managing contacts. We will be able to perform create, read, update, and delete (CRUD) operations on contacts in our database via the API. We will persist and retrieve data to and from a MongoDB database, using Mongoose. We will also make use of Nodemon to debug our applications, and Postman to test our APIs.

Through building the API, you will learn about the following topics:

• How to build a CRUD service in Koa
• How to send JSON data back as a response
• How to structure an API in Koa
• How to implement logging for all requests that are made
• How to set up routing in a Koa application
• How to retrieve and process request body data in Koa
• How to implement validation in Node APIs
• How to define and implement custom middleware in a real-world application