The Koa.js Framework: An Overview

Sandini Pitawala
5 min readApr 13, 2021


What is Koa.js?

Koa.js is an open source Node.js web framework that was designed by the team behind express.

The framework strives to be a smaller, more expressive, and more stable foundation for web applications and APIs, according to their official website.

Koa makes use of asynchronous functions to help eliminate the need for callbacks and significantly improves error handling.

Koa does not bundle middlewares within its core.

It offers a refined set of methods for speeding up the process of making servers and makes it more exciting.

Features of Koa.js

Koa.js has its own unique features that make it more expressive and developer-friendly, much like all other frameworks. Here are a few highlights of features in koa;-

  1. Koa.js is modern and future-proof — Unlike other Node.js frameworks, Koa.js is built using ES6, which simplifies the creation of complex applications by providing a bunch of new classes and modules. This helps developers in creating maintainable applications.
  2. Koa.js makes use of ES6 generators to make synchronous programming easier and the flow of controls more fluid. These generators can also be used to manage the execution of code on the same stack as functions.
  3. In comparison to other Node.js applications, Koa.js has a small footprint. 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?

For their websites and web APIs, many organizations use the Koa.js framework. The following is a list of five well-known organizations that use the Koa.js framework;-

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

Creating a server using Koa.js framework

Let’s get practical and see how to use this new framework to create a simple server.

Create a new directory for your application first, then use the terminal to navigate to that directory of your app and run:

(To create a Node.js package.)

Run to install Koa.js

After that, use your code editor to navigate to the index file in your app’s directory and write the code below to build the server.

Importing the koa module and calling its listen method is all that is needed to build the server. Run node ‘name of your index file’ on the terminal to launch the server.

Creating routes using Koa.js

Koa.js, unlike its predecessor Express, does not have routing as a default feature. Instead, it uses a middleware library Known as Koa Router. So, in order to implement routes in our api, we must first install the Koa router library running the snippet below.

Then, in your index file, import the Koa router module and add your desired routes. A code example of route formation using Koa.js is given below.

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

Handling Responses in Koa.js

Koa response objects are embedded in its context object, as previously mentioned. This implies that the response object is accessed via the context object. To illustrate how to handle responses, let’s use a route definition like the one above.

Handling errors in Koa.js

Add an error middleware early in your index file to manage errors in Koa. Only errors identified after the middleware can be caught, so it must be defined early.

Let’s put this to the test by modifying the home route method throw an error when the route is called.

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

Pros & Cons about Koa


  • Generator support from ground up Using generators (a bleeding edge feature, even for Node.js) would clean up your code from the mess caused by all those callbacks; making your code more manageable.
  • Development team has a proven track record
  • Koa is developed by the team behind a widely used node.js framework (express.js).
  • Extremely lightweight
  • Koa is very lightweight with just 550 lines of code.
  • sync/await keywords are supported and has transcended beyond generator functions
  • Generators functions are of course a huge plus, but at the time Koa team has transcended generations functions and shifted towards async/await style programming. It has made the Koa best framework available in the market.


  • Community is relatively small.
  • Not compatible with express style middleware.
  • Koa uses generators which are not compatible with any other type of Node.js framework middleware.


So from this article, we learned about Koa.js, including what it is, its functionality, how to use it to build servers and it’s pros and cons. We’ve seen its unique syntax and some of its new features in action by building a simple server. Koa.js is a new Node.js framework that has been implemented by a number of well-known organizations around the world. Koa.js is a good choice for any web developer who wants to try out a new framework.