Koa is the next generation web framework for Node.js brought to us by the wonderful team behind Express.

Hello World Comparison

Express

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

app.use(function(req, res, next){
  res.send('Hello World');
});

app.listen(3000);

### Koa
javascript
var koa = require('koa');
var app = koa();

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

app.listen(3000);

If you’re wondering what function *(){} is - they are generators. Use node --harmony flag to run.

Understanding Context

A Koa Context encapsulates node’s request and response objects into a single object:

app.use(function *(){
  this;          // is the Context
  this.request;  // is a koa Request
  this.response; // is a koa Response
  this.req;      // nodejs request
  this.res;      // nodejs response
});

## With Routing
javascript
var koa = require('koa');
var app = koa();
var route = require('koa-route');

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

Middleware Flow

With generators, each yield next makes control flow “downstream” and then back “upstream”:

mw1 -> mw2 -> mw3 -> mw3 -> mw2 -> mw1

The generator suspends, passes flow downstream, then resumes on the way back.