Marcus Node Bits - Let us flex Koa Js, shall we?

· April 8, 2014

The first two post of this mini-series, we picked up the basic on getting Koa Js to start as well as understand what it’s build from and the concepts behind it. It’s time to do something for real. Well over time, one might add. This post is all about using Koa to build different websites and web api’s.

By using Koas own examples I will show you how you can use Koa for a lot of common tasks and scenarios. Let’s dive right in.


The first thing to understand is that Koa is very modular. “Ok, got it”, you think. “No”, I answer, “very modular! The bits are tiny.” So a Koa application is to a large extent made up by middleware you include, that is not included per default. The list of middleware is quite staggering and will quite some time to learn and take in. Luckily you don’t have to learn all of them, and they are so tiny that they one-by-one is not a problem.

You should also check out the co-project that provide generator/yield style access to a VAST amount of features and scenarios. Co-monk is just one example, that we saw in the last post and will meet again later in this post.


I wanted to take a look at some of the examples Koa Js provides and make some comments. Clone the repository and follow along. My plan is that these examples will prove useful for me and you as we start to build real things.

Yes, I have copied the example files out to separate gists. This is because I don’t trust linking directly to repositories. And the content of the repository files might change too.

There’s one package.json for all the examples, which is awesome because that means that we can do npm install in the root and get all the examples. All examples must be run with the node --harmony flag, as described earlier.


The first example that I recommend that you look at is the /blog example. Head into that directory and fire the site up (with node --harmony index.js or better yet nodemon --harmony index.js). Most of the action goes on in the single index.js file:

 * Module dependencies.

var logger = require('koa-logger');
var route = require('koa-route');
var koa = require('koa');
var app = module.exports = koa();

// middleware


// route middleware
var routes = require('./routes.js');
app.use(route.get('/', routes.list));
app.use(route.get('/post/new', routes.add));
app.use('/post', routes.create));
app.use('/post/:id', routes.update));
app.use(route.get('/post/:id/edit', routes.edit));
app.use(route.get('/post/:id/delete', routes.remove));

// listen
console.log('listening on port 3000');

This site is very basic of course but shows off a number of features (and hence middleware):

  • On line 7 we’re including koa-logger and make sure to use it on line 20 (app.use(logger())). This produce some very handy and nice output in the console. I use it for most of my sites.
  • Routing we’ve seen before and it’s very simple to use, since it reminds a lot of other frameworks like ExpressJs. It’s of course a package of it’s own, koa-route.
  • With co-body you can easily parse the content of a posted payload to an object. Note, on line 10 that the variable, from the require-statement is called parse. This is then used on line 62 to parse the request (var post = yield parse(this);) The this here might confuse you, shouldn’t it be request? I have a little section on that below.
  • This site uses a view engine to called swig. To use that a little module has been created (/lib/render.js) that is included on line 6. Using this little render-function we can render templated views as simple as shown on line 35. Notice how we’re passing the data, as the second parameter. This can then be picked up by the partial view (list.html) and looped over with a for-loop. Read more about templates and supported engines on the co-views npm-page.

All in all this is a very good entry example I think. A nice little exercise could be to replace the in-memory storage with Mongo. Psst… use co-monk, for that. Let’s deep dive into something else. Let’s look at error handling. UPDATE: I couldn’t hold it in…

Where’s the request and response, dude?

Let’s break shortly from the examples, more are to come, and mention the API of Koa Context object. It’s very well described on their site (follow that link), but I dare to do some comments here for somethings that made me go Huh? a couple of times.

First, the request and response is baked into the Context object. This actually works better than you would think. ctx.header is just a shortcut to the header of the Request object. And ctx.status = 200; is just shorthand for this.response.status = 200;.

The same is true for this in the middleware functions. They are all bound to a new Context object (that you can augment and return for instance). Hence why you can do this.response.body = 'Hello';.

There you go. Be sure to dive deeper into the documentation of Koa, I’ll be waiting here, drinking coffee, when you come back.


I really like Koa. As I said in the first post, I like that it’s a minimal version of something. I like that it’s very modular. I like that it uses generators, which makes the code a lot easier to read. And, as I hope to show, I like that it gives me a lot of power without putting too much onto me.

We have taken a look at some of the examples that Koa provide and learnt about middleware. In the next post, I will do a more in-depth review of one of the examples and show you how to extend it to fit your own needs. See you then.

The source code for the examples on GitHub and my GitHub repository with the files. All examples must be run with the node --harmony flag, as described earlier.

Side note: I stumbled upon a wonderful video introduction to generator/yield in JavaScript from JSConfUS 2013. You should check it out. Or wait… here it is embedded.

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