Koa2: how to write chain of middleware?

So in express, we can have a chain of middleware, copies an example:

middleware = function(req, res){
  res.send('GET request to homepage');
});

app.get('/', middleware, function (req, res) {
  res.send('GET request to homepage');
});

What's the equivalent way to write this in koa2 please ? I'm thinking of using it for the route, for each route i want to have a middleware to check if user is already logged in.

Thanks !