Koa: Can we mount express app into koa?

[ericfong]

Can we have something like

koaApp.use(expressToKoaMount('/prefix', expressAppOrHttpServer))

Tried implement that with on-finished(https://github.com/jshttp/on-finished)
But got many header is set error

[dougwilson]

Typically you can with the following sugar (Koa 1.x):

koaApp.use(function* (next) {
  yield expressApp.bind(null, this.req, this.res)
  yield next
})

[ericfong]

Thanks for your answer, But I got something like

  TypeError: Cannot read property 'length' of null
      at pathtoRegexp (/.../node_modules/path-to-regexp/index.js:63:48)
      at new Layer (/.../node_modules/express/lib/router/layer.js:45:17)
      at Function.route (/.../node_modules/express/lib/router/index.js:494:15)
      at EventEmitter.app.(anonymous function) [as bind] (/.../node_modules/express/lib/application.js:480:30)

seems express think '.bind' is register a middleware that listen to 'BIND' http method.

I use node 5.2.0 by the way.

[dougwilson]

Hi @ericfong , interesting. Express 4.x doesn't even support Node.js 5.x officially yet, and it sounds like there is some kind of conflict with a new feature added in Node.js 5.1.0. You may need to downgrade to pre-5.1.0 to use Express, wait for a new version of Express that works with 5.1.0, or implement some kind of work-around in that code example, like the following:

koaApp.use(function* (next) {
  yield Function.prototype.bind.call(expressApp, null, this.req, this.res)
  yield next
})

[tj]

I wouldn't advise it personally, Koa and Express have very different semantics, so you might find some strange bugs. If possible I'd just recommend using nginx or similar in front.

[ericfong]

Finally, it works by:
(mounting kue-ui into koa app)

// kue-ui
const expressApp = express()
const prefix = '/kue-ui/'
expressApp.use(prefix, kueUiExpressApp)
koaApp.use(function*(next) {
  // do routing by simple matching, koa-route may also work
  if (this.path.startsWith(prefix)) {
    // direct to express
    if (this.status === 404 || this.status === '404') {
      delete this.res.statusCode
    }
    // stop koa future processing (NOTE not sure it is un-doc feature or not?)
    this.respond = false
    // pass req and res to express
    expressApp(this.req, this.res)
  } else {
    // go to next middleware
    yield next
  }
})

A bit hacky, hope kue-ui can have a koa / promise version

[paul42]

btw @ericfong this helped me a ton. thanks! got it working with koa.js app and agendash

[adipascu]

I used this snippet to use express 4 inside koa 2

  const Koa = require('koa');
  const app = new Koa();
  app.use(async ctx => {
    ctx.status = 200;  // this is required to prevent incorrect codes returned after express runs
    ctx.respond = false;
    express_app(ctx.req, ctx.res);
  });

  app.listen(3000);