What does `return function *(){...}` mean? [duplicate]
It means that the function returns an iterator (so it can be repeatedly called with .next() to yield more values.
check out http://wingolog.org/archives/2013/05/08/generators-in-v8 for more info
It's an ES6 construct, so at the moment you see it more in node rather than client side js
Koa makes use of a new JavaScript feature called generators, and the * is the way to identify a generator in V8.