koa-lite/docs/error-handling.md

Error Handling

Try-Catch

Using async functions means that you can try-catch next. This example adds a .status to all errors:

app.use(async (ctx, next) => {
  try {
    await next();
  } catch (err) {
    err.status = err.statusCode || err.status || 500;
    throw err;
  }
});

Default Error Handler

The default error handler is essentially a try-catch at the very beginning of the middleware chain. To use a different error handler, simply put another try-catch at the beginning of the middleware chain, and handle the error there. However, the default error handler is good enough for most use cases. It will use a status code of err.status, or by default 500. If err.expose is true, then err.message will be the reply. Otherwise, a message generated from the error code will be used (e.g. for the code 500 the message "Internal Server Error" will be used). All headers will be cleared from the request, but any headers in err.headers will then be set. You can use a try-catch, as specified above, to add a header to this list.

Here is an example of creating your own error handler:

app.use(async (ctx, next) => {
  try {
    await next();
  } catch (err) {
    // will only respond with JSON
    this.status = err.statusCode || err.status || 500;
    this.body = {
      message: err.message
    };
  }
})

The Error Event

Error event listeners can be specified with app.on('error'). If no error listener is specified, a default error listener is used. Error listener receive all errors that make their way back through the middleware chain, if an error is caught and not thrown again, it will not be passed to the error listener. If no error event listener is specified, then app.onerror will be used, which simply log the error if error.expose is true and app.silent is false.