Expected Answer (Koa 3.2.1 / Node.js 18+)

Koa composes async middleware in a cascade. Code before await next() runs while the request moves downstream; code after it runs as the stack unwinds upstream. Middleware registered first is the outermost wrapper. This makes timing, transaction cleanup, response transformation, and global errors natural: wrap await next() and act once downstream middleware has completed.

Calling no next() intentionally short-circuits the request, which is appropriate for authentication rejection or a cached response. Accidentally omitting it prevents later routes from running. Unlike a linear callback pipeline, Koa middleware can reliably perform work both before and after the handler through normal async control flow.

Why It Matters

Understanding the cascade prevents incorrect logging, missing cleanup, and middleware order bugs.

Code Example

import Koa, { Context, Next } from 'koa';

const app = new Koa();
app.use(async (ctx: Context, next: Next) => {
  const start = Date.now();
  await next();
  ctx.set('X-Response-Time', `${Date.now() - start}ms`);
});
app.use((ctx: Context) => { ctx.body = { ok: true }; });
app.listen(3000);

Common Mistakes

• Forgetting await next(): Later middleware and routes never execute.
• Writing response logic before downstream work: A later handler can overwrite the intended result.

Follow-up Questions

• Why register error middleware first? (Answer: It must wrap every downstream middleware.)
• Can middleware end a request early? (Answer: Yes, by setting a response and not calling next().)