Koa middleware ordering

Question

I'm having a problem with koa js and middleware order.

Im' using multiple middlewares, Koa-router, formidable, koa-static-folder and one for setting headers.

Right now, with current order, when I'm uploading a file from the frontend to backend via an API, it will throw a CORS error, which means the headers are not there (so this middleware is not loaded properly).

When I move them around, something else will stop working.

Here is my code:

this.app.use(serve({rootDir: './uploads', rootPath: '/public/uploads/'}));
this.app.use(router.routes());
this.app.use(formidable());
this.app.use((ctx, next) => {

    // Website you wish to allow to connect
    ctx.set('Access-Control-Allow-Origin', '*');

    // Request methods you wish to allow
    ctx.set('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');

    // Request headers you wish to allow
    ctx.set('Access-Control-Allow-Headers', 'X-Requested-With,content-type');

    // Set to true if you need the website to include cookies in the requests sent
    // to the API (e.g. in case you use sessions)
    ctx.set('Access-Control-Allow-Credentials', "true");

    // Pass to next layer of middleware
});

Can someone explain to me how to determine the right order of the middleware?

Are you actually calling `next()` in the final bit that sets the headers? But yes, the order is important. What happens if you do them in reverse order to the current order? — Matt Fletcher, Nov 23 '17 at 18:45
Order the functions like a http response: headers first, then content then footer. — Jonas Wilms, Nov 23 '17 at 18:51
Also I'd recommend using [Helmet](https://github.com/helmetjs/helmet) for anything header related in Koa/Express — Matt Fletcher, Nov 23 '17 at 19:00
@JonasW. And any wildcards for 404s and such should go at the very end. — Matt Fletcher, Nov 23 '17 at 19:01
When headers are on the first place, it will not find routes at all. even when i added next(); to the end of headers middleware — Michal Takáč, Nov 23 '17 at 19:27
Ive been able to fix it by replacing the headers middleware with koa-cors, but obviously the problem is in switching to next middleware, not sure why as i was calling next(). — Michal Takáč, Nov 23 '17 at 19:36

Koa middleware ordering

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