Expressive middleware for node.js using generators via co to make web applications and APIs more enjoyable to write. Koa's middleware flow in a stack-like manner allowing you to perform actions downstream, then filter and manipulate the response upstream. Koa's use of generators also greatly increases the readability and robustness of your application.

Only methods that are common to nearly all HTTP servers are integrated directly into Koa's small ~400 SLOC codebase. This includes things like content-negotiation, normalization of node inconsistencies, redirection, and a few others.

No middleware are bundled with koa. If you prefer to only define a single dependency for common middleware, much like Connect, you may use koa-common.

$ npm install koa

To use Koa you must be running node 0.11.9 or higher for generator support, and must run node(1) with the --harmony flag. If you don't like typing this, add an alias to your shell profile:

alias node='node --harmony'

Another option, if you would like to use koa with node 0.10.x (the current stable branch), or are tired of typing the --harmony flag, is to use gnode to spawn your node instance. However note that performance degrades quickly compared to 0.11.x.

• API documentation
• Examples
• Middleware list
• Wiki
• G+ Community
• Mailing list
• Guide
• FAQ
• #koajs on freenode

var koa = require('koa');
var app = koa();

// logger

app.use(function *(next){
  var start = new Date;
  yield next;
  var ms = new Date - start;
  console.log('%s %s - %s', this.method, this.url, ms);
});

// response

app.use(function *(){
  this.body = 'Hello World';
});

app.listen(3000);

$ make test

If you like silly benchmarks, here's the requests per second using wrk 3.x on my MBP.

1 middleware
8367.03

5 middleware
8074.10

10 middleware
7526.55

15 middleware
7399.92

20 middleware
7055.33

30 middleware
6460.17

50 middleware
5671.98

100 middleware
4349.37

With 50 middleware (likely much more than you'll need), that's 340,260 requests per minute, and 20,415,600 per hour, and over 440 million per day, so unless you're a Facebook and can't manage to spin up more than one process to scale horizontally you'll be fine ;)

• TJ Holowaychuk
• Jonathan Ong
• Julian Gruber

MIT