Chapter quiz · Middleware Mastery

Test what you learned.

8 questions on Middleware Mastery. Pass 70% to clear the chapter.

Quiz

Middleware — Chapter Quiz

Eight questions on the onion/cascade model, await next(), middleware order, error handling, koa-compose, koa-mount, and third-party mounting.

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Question 1
1
Describe Koa's middleware execution model.
ALinear — each middleware runs once in registration orderBOnion — middleware runs downstream on the way in, then upstream on the way back after await next()CParallel — all middleware run at the same timeDEvent-based — middleware subscribes to request events
Koa uses an "onion" model. Code before await next() runs going into the stack; code after await next() runs as the stack unwinds. This enables clean pre/post logic in a single function.
Question 2
2
What happens if a Koa middleware never calls await next()?
AKoa throws an errorBAll downstream middleware is skippedCThe request hangs foreverDNothing — Koa calls the rest automatically
Not calling next() short-circuits the stack. The remaining downstream middleware never runs. This is intentional for early-exit patterns (auth guard, cache hit), but unintentional omission typically results in a 404.
Question 3
3
Where should a top-level error-handling middleware be placed relative to other middleware?
AAt the very bottom, after all other middlewareBAt the very top (first app.use), wrapping everything in try/catchCOrder does not matterDIn a separate app.on(\"error\") listener only
Because of the onion model, the first middleware registered wraps all others. A try { await next() } catch (err) { ... } block at the top catches errors thrown anywhere downstream.
Question 4
4
What does koa-compose do?
ABundles a Koa app for the browserBTakes an array of middleware functions and returns a single composed middlewareCCompresses HTTP responsesDAdds CSS/HTML template support to Koa
compose([mw1, mw2, mw3]) returns one middleware that executes the array in order. It is what Koa uses internally, and you can use it to compose reusable middleware groups.
Question 5
5
Given three middleware A, B, C registered in that order, each calling await next(), and each logging before and after next(): what is the log order?
AA-in, B-in, C-in, C-out, B-out, A-outBA-in, A-out, B-in, B-out, C-in, C-outCC-in, B-in, A-in, A-out, B-out, C-outDA-in, B-in, C-in, A-out, B-out, C-out
Koa's onion model: A starts, calls next → B starts, calls next → C starts (no more next), unwinds → C-out, B-out, A-out.
Question 6
6
koa-mount lets you mount a Koa sub-app or middleware at a specific URL path prefix.
TTrueFFalse
app.use(mount('/api', subApp)) strips the prefix from ctx.url inside the sub-app and restores it on the way out, making path-based composition clean and transparent.
Question 7
7
In Koa error-handling middleware, call ctx.___ = err.status || 500 to forward the HTTP status from a thrown error.
Inside a catch block, ctx.status = err.status || 500 sets the HTTP response code. Without it, Koa defaults to 500 for any unhandled error.
Question 8
8
Which are correct ways to reuse a set of middleware across multiple Koa apps?
Select all that apply.
AExport a koa-composed array and app.use itBCreate a sub-app and mount it with koa-mountCCall app.middleware.push(fn) directlyDWrite a factory function that accepts app and calls app.use for each middleware
koa-compose, koa-mount sub-apps, and factory functions are all idiomatic. Directly mutating app.middleware is not a public API and should be avoided.