Koa error middleware that keeps app.on('error') working

[virajrakholiya]

The standard Koa pattern is a try/catch middleware at the top of the stack that turns exceptions into a JSON response. The catch: once you handle the error and set ctx.status yourself, Koa no longer emits 'error' on the app. Anything hooked to app.on('error') (logging, alerting) goes quiet, and all that's left is a 500 status code.

Emit it yourself:

app.use(async (ctx, next) => {
  try {
    await next()
  } catch (err) {
    ctx.status = err.status || 500
    ctx.body = { error: ctx.status < 500 ? err.message : "Internal error" }
    if (ctx.status >= 500) ctx.app.emit("error", err, ctx)
  }
})

app.on("error", (err, ctx) => {
  console.error(ctx.method, ctx.path, err)
})

A few more details:

• Register it first. It only catches errors from middleware registered after it.
• Emit for 5xx only. ctx.throw(404) and validation failures are not incidents.
• Don't send err.message to the client on a 5xx.

I'm building Relia. On Node it starts before your app (relia.start() from relia-sdk/node, or npx relia-sdk init, which detects Koa) and records Koa requests through OpenTelemetry, plus outgoing calls, Postgres/MongoDB/MySQL/Redis queries, and console.error lines as logs. It then tells you in plain words what broke, who it hit and what to check. It suggests fixes but never edits code. Free plan; demo without signup: https://app.tryrelia.com/demo?utm_source=github&utm_campaign=lead-agent

How do you separate expected errors from real ones in Koa: by status, by error class, or something else?