Debugging Koa Apps

Most Koa bugs fall into a small set of categories. Knowing which tools to reach for — and which mistakes to look for first — cuts debugging time significantly.

DEBUG=koa*

Koa uses the debug package internally. Setting the DEBUG environment variable tells it to print internal events to stderr.

# Unix / macOS
DEBUG=koa* node server.js

# Windows PowerShell
$env:DEBUG="koa*"; node server.js

You will see lines like:

koa:application use <anonymous> +0ms
koa:application listen +12ms
koa:context undefined +0ms

Add DEBUG=koa*,@koa/router to also trace router matching.

Node —inspect

Attach Chrome DevTools or VS Code for breakpoints and heap inspection:

node --inspect server.js
# or break on first line:
node --inspect-brk server.js

Open chrome://inspect in Chrome and click Open dedicated DevTools for Node. Set breakpoints directly in your middleware files.

In VS Code add a launch configuration:

{
  "type": "node",
  "request": "launch",
  "name": "Koa server",
  "runtimeExecutable": "node",
  "args": ["--inspect", "server.js"],
  "skipFiles": ["<node_internals>/**"]
}

Bug 1 — Missing await next()

Forgetting await before next() causes downstream middleware to run asynchronously while the current middleware continues — usually resulting in an empty response or a race condition.

// Bug: next() is not awaited
app.use(async (ctx, next) => {
  next(); // ← missing await — response may be sent before handler runs
});

// Fix
app.use(async (ctx, next) => {
  await next();
});

Bug 2 — ctx.body Never Set

If no middleware sets ctx.body, Koa responds with 404 Not Found — even if you call ctx.status = 200. This often happens when a router is mounted but the route path does not match.

// Bug: router is not attached
const router = new Router();
router.get("/users", (ctx) => { ctx.body = []; });
// app.use(router.routes()); ← forgotten

// Fix: always attach routes AND allowedMethods
app.use(router.routes());
app.use(router.allowedMethods());

Bug 3 — Unhandled Promise Rejection

Koa catches throw inside async middleware automatically. But if you call an async function without await, the rejection escapes the middleware chain.

// Bug: unawaited async call
app.use(async (ctx, next) => {
  fetchData().then((d) => { ctx.body = d; }); // ← ctx.body set after response!
  await next();
});

// Fix: await everything that touches ctx
app.use(async (ctx, next) => {
  ctx.body = await fetchData();
  await next();
});

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