Routing, Body Parsing & Static Files

koa-router

const Router = require('@koa/router');
const router = new Router();

router.get('/users/:id', async (ctx) => {
  const user = await findUser(ctx.params.id);
  ctx.body = user;
});

router.post('/users', async (ctx) => {
  const user = await createUser(ctx.request.body);
  ctx.status = 201;
  ctx.body = user;
});

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

Body Parsing & Static Files

const bodyParser = require('koa-bodyparser');
const serve = require('koa-static');

app.use(bodyParser());   // populates ctx.request.body
app.use(serve('./public'));

Both routing and body parsing are separate middleware packages, not core features — the deliberate tradeoff of Koa's minimalist philosophy: more setup decisions upfront, no unused built-in functionality carried by default.

Short-Circuiting

app.use(async (ctx, next) => {
  if (!ctx.headers.authorization) {
    ctx.status = 401;
    ctx.body = { error: 'Unauthorized' };
    return;   // not calling next() — no downstream middleware runs
  }
  await next();
});

Koa vs. Express

Both are middleware-based Node.js frameworks built around composing small functions into a request pipeline — the core mental model transfers between them even though signatures differ ((req, res, next) vs. (ctx, next)) and Koa's minimal-core philosophy trades built-in convenience for flexibility.