Introduction to the Koa.js Framework

In my previous article, we went over an introduction to hapi.js framework, and I briefly mentioned Koa.js, another major Node.js framework. This article will us give an introduction to the Koa.js framework, its features, and a tutorial on how to create a simple server using this framework. <!--more-->

What is Koa.js? #

Koa.js is an open source Node.js web framework that was designed by the team behind express. As their official website says, the framework aims to be a smaller, more expressive, and more robust foundation for web applications and APIs. Koa uses asynchronous functions to help eliminate the need for callbacks and significantly improves error handling. Koa does not bundle middlewares within its core. It provides a refined suite of methods that fasten the process of making servers and makes it more exciting.

Koa.js features #

As with all available frameworks, Koa.js also has its own unique features that make it more expressive and developer-friendly. Here is a few highlights of features in Koa.

1. Koa.js is modern and future-proof - Unlike other Node.js frameworks, Koa.js build is based on ES6 which makes the development of complex applications simpler by providing a bunch of new classes and modules. This helps developers in creating maintainable applications.
2. Koa.js uses ES6 generators to simplify synchronous programming and facilitate the flow of controls. These generators can also be used as functions to control code execution on the same stack.
3. Koa.js has a small footprint compared to other Node.js frameworks. This helps developers write thinner middlewares.
4. Koa.js has a built-in catchall for errors that help prevent website crashes.
5. Koa.js uses a context object which is an encapsulation of request and response objects.

Who uses Koa.js #

Many companies use Koa.js framework for their websites and web APIs. Below is a shortlist of five globally recognized companies that use Koa.js framework.

1. Paralect
2. Pubu
3. Bulb
4. GAPO
5. Clovis

Creating a server using Koa.js framework #

Since we now have an idea of what Koa.js is and have seen some of its features, let's get practical and see how to create a simple server with this new framework.

First, create a new directory for your application, then, using the terminal, navigate to the directory of your app and run:

npm init

To create a Node.js package.

Then run to install Koa.js

npm i koa

Afterward, navigate to the index file on your app's directory using your favorite code editor and write the code below to create the server.

const koa = require('koa')
const app = new koa()

app.listen(2400, () => {console.log('Server running at PORT 2400')})

The code used to create the server is pretty straight forward, just import the koa module and use its listen method. To start the server, run node 'name of your index file' on the terminal.

Creating routes using Koa.js #

Unlike its predecessor, Express, Koa.js does not handle routing by default. Instead, it uses a middleware library Known as Koa Router. So, to implement routes in our server, we will first need to run the snippet below to install Koa router library.

npm install koa-router

Then import the Koa router module onto your index file and add your desired routes. Below is a code example to demonstrate route creation using Koa.js.

const koa = require('koa')
const koaRouter = require('koa-router')// importing Koa-Router

const app = new koa()
const router = new koaRouter()

router.get('home', '/', (context) => {
  context.body = "Welcome to my Koa.js Server"
})

app.use(router.routes())
  .use(router.allowedMethods())// registering routes to the application

app.listen(2400, () => console.log('Server running at PORT 2400'))

Start the server again and test the route by sending a request from the browser.

Handling Responses in Koa.js #

As we mentioned earlier, Koa response objects are embedded in its context object. This means that we access the response object from the context object. Let's use a route definition like the one above to demonstrate handling responses.

router.get('home', '/', (context) => {
  context.status = 200 //This is the response status
  context.body   = "Welcome to my Koa.js Server" // This is the response body
})

Handling errors in Koa.js #

To handle errors in Koa, add an error middleware early in your index file. It must be defined early because only errors defined after the middleware can be caught. The code below includes the error middleware in our server.

const koa = require('koa')
const koaRouter = require('koa-router')// importing Koa-Router

const app = new koa()
const router = new koaRouter()

app.use( async (ctx, next) => {
  try {
    await next()
  } catch(err) {
    console.log(err.status)
    ctx.status = err.status || 500;
    ctx.body = err.message;
  }
})

router.get('home', '/', (context) => {
  context.body = "Welcome to my Koa.js Server"
})

app.use(router.routes())
  .use(router.allowedMethods())// registering routes to the application

app.listen(2400, () => console.log('Server running at PORT 2400'))

To test this let's modify the home route method to throw an error when the route is called.

...

router.get('home', '/', (context) => {
  context.throw('Sample error message', 500)
})

...

Now run the server again and call the endpoint using a browser.

Summary #

In this article, we learned about Koa.js, what it is, its features, and how to create servers with the framework. Through creating a simple server, we have seen its unique syntax and some of its new features in action. Koa.js is a new Node.js framework that has been adopted by some globally recognized companies. Any web developer who feels like trying out a new framework should consider Koa.js.

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Peer Review Contributions by: Nadiv Gold Edelstein