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.

Installation

$ 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'

Community

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

Example

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);

Running tests

$ make test

Benchmarks

Since requests per seconds is a bad metric, here's the average response time with 400 noop middleware on my macbook pro (you would never use this many haha).

1.5ms

Here's the average latency with only a single hello world middleware:

0.628ms

License

MIT