Koa: mounting into an express app

[bttmly]

I'm exploring migrating a large production Express app to koa v2 (so all of this applies to the promise API of v2). The koajs.com docs say for app.callback():

"You may also use this callback function to mount your koa app in a Connect/Express app."

However, it seems like you must mount a koa app at the "bottom" of your Express app since koa will never call next() to yield control back to express (like in the 404 case). I can't see an obvious work around for this currently. If the function returned by callback returned the promise, I think something like the following would work:

const koaCallback = koaApp.callback();
expressApp.use(function (req, res, next) {
  koaCallback(req, res)
    .then(function () {
      // not sure exactly how to detect this but seems doable
      if (!koaSentResponse(res)) {
        next() // let express keep processing this request
      }
   });
});

Perhaps somewhat related -- what's the use case for setting ctx.respond to false?

[PlasmaPower]

You probably want res.finished.

I'm not quite sure what ctx.respond = false would be used for, as a custom responder at the beginning of the chain should do the same job. It shouldn't be used, it's not officially supported and it's bound to break stuff.

[PlasmaPower]

I think the docs for you're looking at would assume that Koa would be used to handle a subdirectory like /api/... or similar, as opposed Koa being the default and Express filling in the gaps.

[bttmly]

Huh. The ctx.respond = false line even has a comment saying "allow bypassing koa". Just for some background, the use case here is I want to move routes in an existing application over to koa incrementally, without affecting clients. I could, I suppose, have a list of routes that should go to koa and take care of getting them there manually via an express/connect middleware, but that ends up duplicating a lot of the routing logic.

[PlasmaPower]

@nickb1080 Yep, I figured so. I don't think you want ctx.respond = false here though, I think you're initial solution is good with if (!res.finished) (instead of if (!koaSentResponse(res))). You could also manually assign routes to Koa of course, but that might be a pain depending on how many routes you have.

[PlasmaPower]

Wait, I just realized you're going to get a 404 response from Koa of course. I'll see how that can be prevented.

[PlasmaPower]

It looks like ctx.respond = false might be useful after all. Try making your first middleware return next().then(() => ctx.respond = (ctx.body == null)); or similar (double equal sign as undefined == null but '' != null).

[bttmly]

Yeah I that'll work for avoiding the automatic 404 but I still see no way of calling the next provided by express/connect that would give control back to the enclosing app. In particular, callback returns a function of signature (req, res) -- the (possible) next provided by express/connect isn't even referenced. If it returned the promise I think we'd be in business though.

I'm experimenting with a hacky workaround that subclasses the Koa application class. It's sort of a pain because the respond function is private to that module so I need to inline the whole thing into the subclass.

[PlasmaPower]

@nickb1080 Ooh, that's a tricky one, I thought handleRequest returned the promise. I'll make a PR for that now.

[PlasmaPower]

In case you didn't get a notification, I opened #847 for this.

[jonathanong]

@PlasmaPower has a PR open. if anyone else would like to put in their 2 cents, that would be great!

/cc @nickb1080

[PEM--]

Wow, that would solve a lot of integration issues and smoothen transitions to Koa.

[bttmly]

The test case for PR #847 appears to address all my concerns!

[PlasmaPower]

I'm glad! @jonathanong anything holding it up? The name of the variable might not be great, but it's fine IMO. I have tests for it too.

[fl0w]

Maybe just app.middleware()? IMO it's kind of related to current naming scheme of app.callback() and app.listen(). Highly subjective of course.

[PlasmaPower]

@fl0w Hmm, maybe. The difference of course is that it doesn't accept ctx, next, it accepts req, res.

[bttmly]

This is resolved in https://github.com/koajs/koa/commit/e6539e1cf28e5cb5418d9dd3c364c03198610cf1