`app.on("error")`

Registers an application-level error handler that receives all uncaught errors from middleware.

Syntax

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

Parameters

Name	Type	Required	Description
`err`	`Error`	Yes	The error thrown (or rejected) by middleware.
`ctx`	`Context`	No	The Koa context for the request that caused the error. May be `undefined` for errors that occur outside request handling.

Returns

void — No return value; this is an event listener.

Examples

import Koa from 'koa';

const app = new Koa();

app.on('error', (err, ctx) => {
  console.error('Server error:', err.message, {
    url: ctx?.url,
    status: err.status,
  });
  // Report to your monitoring service here
});

app.use(async (ctx) => {
  if (ctx.path === '/boom') throw new Error('Oops!');
  ctx.body = 'ok';
});

app.listen(3000);

Output

Server error: Oops! { url: '/boom', status: undefined }

Notes

Koa silently swallows errors with a `status` of 4xx (client errors) by default to reduce noise; set `app.silent = false` to log all errors. Errors with `expose: true` have their message forwarded to the client; otherwise a generic status text is sent. Always attach an error listener to avoid crashing on unhandled promise rejections in middleware.