route-example.js

var app = require('express')();

app.use(function(req, res, next) {
    console.log('\n\nALLWAYS');
    next();
});

app.get('/a', function(req, res) {
    console.log('/a: route terminated');
    res.send('a');
});
app.get('/a', function(req, res) {
    console.log('/a: never called');
});
app.get('/b', function(req, res, next) {
    console.log('/b: route not terminated');
    next();
});
app.use(function(req, res, next) {
    console.log('SOMETIMES');
    next();
});
app.get('/b', function(req, res, next) {
    console.log('/b (part 2): error thrown');
    throw new Error('b failed');
});
app.use('/b', function(err, req, res, next) {
    console.log('/b error detected and passed on');
    next(err);
});
app.get('/c', function(err, req) {
    console.log('/c: error thrown');
    throw new Error('c failed');
});
app.use('/c', function(err, req, res, next) {
    console.log('/c: error deteccted but not passed on');
    next();
});

app.use(function(err, req, res, next) {
    console.log('unhandled error detected: ' + err.message);
    res.send('500 - server error');
});

app.use(function(req, res) {
    console.log('route not handled');
    res.send('404 - not found');
});

app.listen(3001, function() {
    console.log('listening on 3001');
});