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:

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 ({ posts: posts }). This can then be picked up by the partial view (list.html) and looped over with {% for post in posts %} 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 hence is used to manipulate the status of the Response object.

You can get hold of some useful things from the Context object

  • is the application instance for your application.
  • ctx.Request is the Koa Request object. Note the difference to the ctx.req which is the Node request object.
  • ctx.Response and ctx.res is the Response and Node response

This makes for very succinct and terse code, since almost everything you need comes from the context and can be found on the this object. For example, take a look at lines 62 and 67 of the example above. Parse the incoming request? parse(this) of course. Redirect the response? this.redirect(‘/’);

More examples

Let’s continue with some more examples that I think can be useful for most of us.


Error handling is nicely baked into Koa. If you do nothing everything is logged to standard output (stderr) unless the NODE_ENV is test. But you can easily create a global error hook to do your own error handling. Just do app.on(‘err’, function(err){}); and you can do your stuff there.

Another thing that happens automatically is that a HTTP status 500 is returned to the client, of course. Let’s look at an example that does this:

In this example we can also see error propagation in action.

Line 29-33 we have the global catch all error handler that logs the error. Note the “advanced” (not really huh?) logging that has been made for when NODE_ENV isn’t test. In this particular case that would be the exact same behaviour as the default behaviour.

Line 5-21 sets up an interesting try..catch block around … Yes, around what? Well next, of course. All the other middleware and request handling. Powerful stuff!

  • In this case we create a custom response, setting the body, status and type of the response.
  • We also emit the error event so that our console log takes place. It wouldn’t have done that otherwise since we handled the error manually.  

Line 24-26 is a function that responds to all the call to this application.

The order of the calls had me thinking hard. And long. And then hard again. But here it is:

  1. The application set’s up a try..catch around all the middlewares and requests.
  2. Request comes in and is handled by the “catch all routes”-function on line 24-26
  3. This function throws an error
  4. That is caught by the try..catch function (line 5-21).
  5. In this function we handle the error manually.
  6. The final line of code in the try..catch function emits an error event
  7. Which triggers the app.on(‘error’) subscription. That logs the error.

I hope I got that right…

What about testing?

We break again from the example to mention a few words about testing. Like all Javascript frameworks worth its salt the support for testing is excellent (still in awe of this Javascripters!) and Koa Js is no exception.

Let’s take the testing of the error handling example as an … well example. It uses supertest since it tests the entire stack. And mocha of course. Here’s the test code:

Line 1 is important, this is where we get a reference to the application. Simple stuff; just require the application. Ha!

Line 2 sets up a supertest request object by creating a supertest agent and listening to the application.

The first test (line 4-10) is plain old supertest code, and outside the scope of this article.

The second test is more interesting. Remember that the code we’re testing both returned a nicely formatted error but also emitted an error event?

  • Since we have a reference of the app (from line 1) we can now setup an subscription for the “error” event. But it should happens only once.
  • Once it happens we check the .message of the passed in err-parameter.

  • We also get the context (ctx) passed in and can check the status of that.

There’s of course more to say about testing, but I’m safe to say that the support in Koa got you well covered.

And still more examples

I’ll try to do some more examples. Starting with one that not yet is in the examples repository. Not. Yet. :)

Basic Authentication

There’s a nice little middleware to do basic authentication (koa-basic-auth) which makes basic authentication this easy:

  • Line 1 requires the koa-basic-auth middleware. Note that we call this variable auth
  • Line 22 is the secret sauce. Here’s we simply tell the app to use the auth function, for the listed name - pass objects.  
  • Lines 7-18 sees us use an similar try..catch for all the request

It’s not pretty but it’s quick and good enough for administration sites etc.

Content negotiation

Content negotiation simply means that you can have one route that answers for request for several content types. If you ask for HTML you’ll get HTML back and JSON when you ask for JSON etc.

Koa got you covered. Here’s how you do it:

Arguably this example is a bit overlong… Here are the important parts:

  • Line 36 set’s up the content types we accepts, by using the app.accepts() function.
  • Note that we store the response in a type variable that we then can use to filter the responses with.
  • Line 38 checks that we got something we can use
  • Line 43 is one example on how to check the type and then return the correct version.

I think this is not super-elegant, compared to other frameworks I’ve use, but at least it’s simple to understand.

Store stuff

(in Mongo I should have added maybe…). For this we use monk and co-monk in combination, and I talked about this shortly in the previous blog post. This is not included in the example repository either. You can find the code here. Let’s do CRUD for an imaginary user object (the Koa-guys seems to like that :):

Let’s go through the production code first:

First stuff that’s interesting is lines 9 and 11. Here’s we require the co-monk library and store that in a variable called wrap. That can then be used to create a collection-like variable for easy access, as we do on line 11, we call is users.

And exposes the users to our tests on line 14, module.exports.users = users;

We use the co-body library to parse the requests into JSON objects. Look at line 28 for example.

The monk interface is exposed through the co-monk library. This is awesome, since that means that we can have interaction with our user collection like this:

  • Insert : yield users.insert(user);
  • Find all: var res = yield users.find({});
  • Find one: var res = yield users.findOne({_id:id});
  • Update via id: yield users.updateById(id, user);
  • Update via other property: yield users.update({name: ‘Marcus’}, user);
  • Delete: yield users.remove({_id:id});

See?! It’s almost ridiculously simple! Very nice, short and sweet!

Ok, let’s take a look that the test code:

First thing to notice here is the removeAll-function that we use in the beforeEach and after-hooks:

  • We’re using the co-library here to wrap the functionality of our call in a generator (or “Generator based flow-control goodness for nodejs (and soon the browser), using thunks or promises, letting you write non-blocking code in a nice-ish way.” from the npm-site).
  • This is great because that means that we can yield stuff and use co-monk again. For example look at line 11. One line. Remove everything and yield to get the “async” behaviour we want.
  • Note that the co-function takes the done-variable as parameter; co(…)(done); This is a bit of trickery that first tripped me up. I’m not sure I understand it fully but this works at least. And it makes sense in a way to; first to this (inside of the co-function) and then call done. A bit like promises, maybe.

Then on line 24-30 we do a normal supertest test, posting a user object to our API and validating the result

Testing for a existing user is a bit trickier. We first need to insert the user and then get it from our API. An easy way to do that is to wrap it in a co-generator-function again. We wrap the whole it-block in one co-function that:

  • Inserts the user into the mongo db (line 34)
  • Calls our API via supertest (lines 36-49)
  • Validates the response (lines 40-42)

We use the same trick for the update test (lines 47-56) and the delete test (58-66)

This testing was a bit hard to get going first but once I wrapped my head around (well not completely maybe…) the co-function I think it came out pretty nice.

Upload files

The final example is not something that you’ll do everyday but something that I’m needing in a little app I’m building; uploading files (and streaming them back).

Here’s the way to do that with Koa:

There’s number of interesting things here, not all of them have to do with file uploading:

Check lines 7 and 27; that’s how you serve static files from a directory. Looks a lot like Express right? Simple but useful

This examples doesn’t use routing… at least not the koa-routes we’ve come to love. Instead the single route handling (lines 31-46) checks the this.method for posts and just ignores everything that isn’t.  

  • This means that for the public-directory (that serves static files, if you remember) we get our index.html served

After our post-handling we redirect back to the root of the application.

There’s also little stab at a custom 404-page. On line 21 we check if the request has a body and that it’s not idempotent (?!), if not it serves up the 404.html.

Ok, that was a lot of good stuff that didn’t even remotely had to do with uploads. Let’s look at the uploading parts:

First, line 8 sets up a parse variable, by requiring the co-busboy library (man, for some of those name, I guess that you had to be in the room when they were named…). This is a little handy library that helps us to parse multi-part requests. File uploads are always multipart (I think… at least mostly) and we are legio that have forgotten ‘enctype=”multipart/form-data”’ on our forms.

With the parse-functionality that co-busboy gives us we can do “var parts = parse(this);”. This gives us the different part of the the multipart request and we can then loop and yield through each of them “while (part = yield parts)”

co-busboy is a convoluted in that the parts that you parse out can be one of the two types (read more on the co-busboy npm page):

  • Either it is an array, and then it holds values in key-value pairs (key in part[0], value in part[1])
  • If it’s not an array (aka plain old form values), well then it is a stream. This comes handy just now.

With co-busboy we can now easily store the stream to disk:

  • We create a writeStream (var stream = fs.createWriteStream(‘/tmp/’ + Math.random() + part.filename);)
  • pipe the part to the stream.  Since the part is a stream we just pipe it onto the writeStream. Yeah, you probably need to grok streams to fully understand this (and Node I’ve heard). I absolutely hate them and never got to good terms with them. The article I pointed to is a great start and you can lab with them in your browser here. A writeStream is a “stream” (whatever that is…) for writing stuff. We pipe the part to the writeStream and it’s written… Got that, Marcus?
  • This writes the file to disk (to the path you provided when you created the writeStream).

That was not too hard, now was it?

Stream files back to the client

Ok, streaming files to the client was so super-easy that I just had to include that here too:

By using the path Node module we can easily get the extension name of a file as of line 12.

Streaming now becomes very easy and can reduced to a one-line (line 13). Let’s break it down:

  • fs.createReadStream(path) simply creates a readStream (a stream for reading, Marcus…)
  • this readstream is then assigned to the this.body property. Meaning that we stream the file back to the body.
  • finally we save a reference to the stream to be used in the clean up (see next bullet)

Line 2 sees us require the finished-library (that “Execute a callback when a request closes, finishes, or errors”) to a onFinished variable. I told you these things were tiny.

  • This is then used on the last line of our request to ensure we close everything off in a nice, non-leaking fashion. Without callbacks of course :P

Short and sweet!


The goal of this blog post was to walk your through a bunch of Koa Js examples, that could help you get going and understand the Koa capabilities faster. Some are from the Koa-people and some are from me. I learned a lot by doing this very carefully and slowly. I hope you have picked up somethings by reading this.

I’m planning to build a little application using most of the things above. I will probably blog about that when it’s done. With Koa that should total to 29 lines of code, right? :)

Thanks for reading this!

I like Koa Js. I’ve written a couple of post about it. Here they will be when they are published:

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