how koajs works @cc http://code.tutsplus.com/tutorials/introduction-to-generators-koajs-part-1--cms-21615

koajs-hello.js

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

koa.use(function* (next) {
  //do something before yielding/passing to next generator function in line which will be 1st event in downstream
  console.log("===1===");
  console.log("A");
  yield next;

  // do something when the execution returns upstream, this will be last event in upstream
  console.log("B");
  console.log("===/1==");
});

koa.use(function* (next) {
  // do something before yielding/passing to the next generator function in line, this shall be 2nd event downstream
  console.log("====2==");
  console.log("C");

  yield next;

  // do something when the execution returns upstream and this would be 2nd event upstream
  console.log("D");
  console.log("====/2==");
});

koa.use(function* () { // do something before yielding/passing to next generator function in line. Here it would be last function downstream
  console.log("====3==");
  console.log("E");
  this.body = "hey guys";
  console.log("F"); // First event of upstream (from the last to first)
  console.log("====/3==");
});

koa.listen(3000);

// ===1===
// A
// ====2==
// C
// ====3==
// E
// F
// ====/3==
// D
// ====/2==
// B
// ===/1==