Mount Koajs app on top of Express

Question

From koajs.com:

app.callback()

Return a callback function suitable for the http.createServer() method to handle a request. You may also use this callback function to mount your koa app in a Connect/Express app.

Now I have an Express app that already starts its own http server. How can I mount a koa app on top of this existing server, so that it shares the same port?

Would I include the koa app as an Express middlware? Do I still use app.callback() for that?

Answer 1

expressapp.use(koaapp.callback()) is fine. but remember, koaapp.callback() does not have a next, so there's no passing errors to the express app or skipping the koaapp once you use it.

it's better to keep them completely separate since their APIs are incompatible

var koaapp = koa()
var expressapp = express()
http.createServer(req, res) {
  if (true) koaapp(req, res);
  else expressapp(req, res);
})

Answer 2

Since you need a server instance in order to mount a middleware on a specific /prefix, it would be something like

var http = require('http');
var expressApp = require('express');
var koaApp = require('koa');

// ...

expressApp.use('/prefix', http.createServer(koaApp.callback()));