Error handling

Define error-handling middleware like other middleware, except with four arguments instead of three, specifically with the signature (err, req, res, next)):

app.use(function(err, req, res, next) {
  console.error(err.stack);
  res.status(500).send('Something broke!');
});

You define error-handling middleware last, after other app.use() and routes calls; For example:

var bodyParser = require('body-parser');
var methodOverride = require('method-override');

app.use(bodyParser());
app.use(methodOverride());
app.use(function(err, req, res, next) {
  // logic
});

Responses from within the middleware are completely arbitrary. You may wish to respond with an HTML error page, a simple message, a JSON string, or anything else you prefer.

For organizational (and higher-level framework) purposes, you may define several error-handling middleware, much like you would with regular middleware. For example suppose you wanted to define an error-handler for requests made via XHR, and those without, you might do:

var bodyParser = require('body-parser');
var methodOverride = require('method-override');

app.use(bodyParser());
app.use(methodOverride());
app.use(logErrors);
app.use(clientErrorHandler);
app.use(errorHandler);

Where the more generic logErrors may write request and error information to stderr, loggly, or similar services:

function logErrors(err, req, res, next) {
  console.error(err.stack);
  next(err);
}

Where clientErrorHandler is defined as the following (note that the error is explicitly passed along to the next):

function clientErrorHandler(err, req, res, next) {
  if (req.xhr) {
    res.status(500).send({ error: 'Something blew up!' });
  } else {
    next(err);
  }
}

The following errorHandler “catch-all” implementation may be defined as:

function errorHandler(err, req, res, next) {
  res.status(500);
  res.render('error', { error: err });
}