https://github.com/koajs/koa/blob/master/lib/application.js#L96 https://github.com/koajs/compose/blob/master/index.js#L27

compose-sample.js

var co = require('co');

var stack = [
  function *first(next) { // second generator
    console.log('1 prev');
    yield next; // second に delegate
    console.log('1 next');
  },
  function *second(next) { // third generator
    console.log('2 prev');
    console.log(next.constructor.name);
    yield next; // third に delegate
    console.log('2 next');
  },
  function *third(next) { // noop generator
    console.log('3 prev');
    yield next; // noop に delegate
    console.log('3 next');
  }
];

co(function *() {
  console.log('start');
  var prev = function *() {}; // noop generator
  var i = stack.length;

  while (i--) {
    prev = stack[i].call(this, prev)
    console.log('#' + (i+1) + '#');
  }

  yield *prev; // first generator に delegate
})();