app.js

/**
 * Each `app.use()` only accepts a single generator function.
 * If you want to combine multiple generator functions into a single one,
 * you can use `koa-compose` to do so.
 * This allows you to use `app.use()` only once.
 * Your code will end up looking something like:
 *
 *   app.use(compose([
 *     function *(){},
 *     function *(){},
 *     function *(){}
 *   ]))
*/


var compose = require('koa-compose');
var koa = require('koa');
var app = module.exports = koa();

// x-response-time

function *responseTime(next){
  var start = new Date;
  yield next;
  var ms = new Date - start;
  this.set('X-Response-Time', ms + 'ms');
}

// logger

function* logger(next){
  var start = new Date;
  yield next;
  var ms = new Date - start;
  if ('test' != process.env.NODE_ENV) {
    console.log('%s %s - %s', this.method, this.url, ms);
  }
}

// response

function* respond(next){
  yield next;
  if ('/' != this.url) return;
  this.body = 'Hello World';
}

// composed middleware

var all = compose([
  responseTime,
  logger,
  respond
]);

app.use(all);

if (!module.parent) app.listen(3000);