coffee :

perfectSquares = ->
  num = 0
  loop
    num += 1
    yield num * num
  return
window.ps or= perfectSquares()

javascript :

var perfectSquares;
perfectSquares = function*() {
  var num;
  num = 0;
  while (true) {
    num += 1;
    (yield num * num);
  }
};
window.ps || (window.ps = perfectSquares());

详情见 http://coffeescript.org/#fat-arrow

我的koa啊 …