Whats the point of composing middleware in Koa?

Question

I am diving into Koa2 and I see koa-compose. I get that I give it middlewares and it returns one, but why? What is the benefit of having multiple middleware wrapped as one instead of just adding them separately?

app.use(compose(m1, m2))

vs:

app.use(m1)
app.use(m2)

Answer 1

KoaJS uses koa-compose underneath (here), so app.use(compoase([m1,m2])); and app.use(m1); app.use(m2); are the same. Using koa-compose explicitly can give more power for customization. Following is one such case:

Adding middlewares through app.use(middleware), will cause all of the middlewares to be executed upon each request in the specified order. But if you want to selectively run different set of middlewares for each route (or in a different order), you can use explicitly use koa-compose to create specialized middleware stacks for each route.

var app = require('koa')();
var router = require('koa-router')();
var compose = require('koa-compose');

var allMiddlewares = compose([m1,m2,m3]);

router.get('/', allMiddlewares);
// selectively enable logging middleware for this route
router.get('/test', compose(logger, allMiddlewares));

app
  .use(router.routes())
  .use(router.allowedMethods());