Is this usecase recommended or discouraged?

[franciscop]

I created some modules for normalizing URLs in different ways (+ there are some others out there), so I would like to bundle them into a single module. This works right now:

app.use((req, res, next) => {
  req.app.use((req, res, next) => {
    console.log("I am called!");
    next();
  });
  req.app.use((req, res, next) => {
    console.log("Me too!");
    next();
  });
  console.log("Great");
  next();
});

Which correctly outputs:

Great
I am called!
Me too!

However I worry that this is abusing express functionality/original intent. So I'd like to ask if this is frowned upon or perfectly valid within express' API.

[blakeembrey]

This is not a good way to do it, you're creating a memory leak by mounting applications every single request loop which means that for each request, you have one extra set of middleware mounted (and a new function for each middleware instance too == more memory overhead).

It sounds like what you want to do is to look up how Express routers work. It allows you to do this:

const router = express.Router();

router.use((req, res, next) => {
  console.log("I am called!");
  next();
});

router.use((req, res, next) => {
  console.log("Me too!");
  next();
});

app.use(router);

Basically, you can create any instance you'd like and accept (req, res, next) - inside that you can do whatever you'd like - be that routing, request management, etc. I wouldn't mess with the application instance itself as you will create a memory leak adding too many instances of middleware.

[dougwilson]

And to build on what @blakeembrey said, from a strict sense of the question "So I'd like to ask if this is frowned upon or perfectly valid within express' API.": all of that is valid use of the Express API that is public & supported, though it does seem odd.

[franciscop]

@blakeembrey that makes total sense, of course the "base" use one is also a middleware so it'll be called each time. A flag or counter could be added to avoid leaking, but then your solution with the Router seems way better so I'll use it that way. Thanks!