Expressive middleware for node.js using generators via co to make writing web applications and REST APIs more enjoyable to write.

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

alias node='node --harmony-generators'

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

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

// logger

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

app.listen(3000);

$ make test

Average latency with one middleware:

0.628ms

Average latency with 400 noop middleware:

1.5ms

If you like silly benchmarks, here's the requests per second:

1 middleware
8083.45

5 middleware
7449.95

10 middleware
7166.66

15 middleware
6992.18

20 middleware
6650.28

30 middleware
6113.57

50 middleware
5117.46

With 50 middleware (likely much more than you'll need), that's 307,020 requests per minute, and 18,421,200 per hour, 442 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
• Julian Gruber
• Jonathan Ong

MIT