Express and Node.js Training from StrongLoop

This document might be outdated relative to the documentation in English. For the latest updates, please refer to the documentation in English.

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 });
}