express.js error handling on http requests

Question

How to handle errors in http requests?

Here the full error is returned to the client.. How to write the error to the log and return Fatal error to the client?

Express v4.4.4

var express = require('express'),
    app = express(),
    domain = require('domain'),
    port = 3000;

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

app.get('/', function(req, res){
    var d = domain.create();
    d.on('error', function(err){
        console.error('Error', err);

        res.send('Fatal error!', 500);
    });
    d.add(req);
    d.add(res);
    d.run(function(){
        //  a is undefined
        a.ddd();

        res.send('Success!');
    });
})
.listen(port, function(){
    console.log('Express server listening on port '+port);
});

Answer 1

Your .use for the error handler needs to be at the bottom, after the routes/handlers/middlewhare which may cause the errors. Then from your other places, call next(error) instead of returning an error message directly

// this comes first
app.get('/', function(req, res, next){ // note the addition of next
    var d = domain.create();
    d.on('error', function(err){
        next(err); // pass the error on to the next middleware
    });
    // ... 
});

// this comes last
app.use(function(err, req, res, next){
    console.error(err.stack);
    res.send('Fatal error!', 500);
});

Answer 2

Error-handling middleware are defined just like regular middleware, however must be defined with an arity of 4, that is the signature (err, req, res, next):

Express Error Handling

eg code:

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