How to properly handle errors in Express?

You'll want to check out Express Error Handling. From there:

app.param('userId', function(req, res, next, id) {    User.get(id, function(err, user) {        if (err) return next(err);        if (!user) return next(new Error('failed to find user'));        req.user = user;        next();    });});

The sweetspot that you are missing is the return next(...)

That's because you're doing it wrong: you already threw an Error (which will be processed by Express and return a 500 - Error page for the user or something like that) but you are also trying to send your own response to the client: res.send('event found!');

You should really check out the Express guide about Error Handling here: http://expressjs.com/guide/error-handling.html

What I would do in your example is:

function NotFound(msg){  this.name = 'NotFound';  Error.call(this, msg);  Error.captureStackTrace(this, arguments.callee);} app.get('/event/:id', function(req, res, next){  if (req.params.id != 1) {    throw new NotFound('Cannot find event ' + req.params.id);  } else {    res.send('event found!');  }});app.error(function(err, req, res, next){    if (err instanceof NotFound) {        res.render('404.ejs');    } else {        next(err);    }});

You have a couple of problems in your code:

• When responding to the client, you need to use the response object (res rather than req).

• When sending an error to next, you should return, so the rest of the function doesn't run.

Here's your code after fixing those errors:

app.get('/event/:id', function(req, res, next) {    if (req.params.id != 1) {        return next(new Error('cannot find event ' + req.params.id));    }    res.send('event found!'); // use res.send (NOT req.send)});