Node, Express, domains, uncaught exceptions - still lost

Question

I have been reading for hours on exception handling in Node. I understand the cons of using uncaughtException , I understand that shutting down the process is good for preventing any "unknown state" where "anything can happen". I understand that using domains is the way to go, and I understand how to properly implement domains, specifically Explicit Binding ...

...but I'm still not getting any results for just basic error handling.

I would like to be able to just catch any uncaught exceptions for the purpose of logging. I don't mind killing the process or anything else deemed "undesirable". I just want a log.

I don't feel like I should have to wrap everything in a try/catch or use some library to emit errors... please correct me if I'm wrong and I will change my ways.

I am using Node and Express and I have the following simple code:

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

var serverDomain = domain.create();
serverDomain.on('error', function(err) {
    console.log("SERVER DOMAIN ERROR: " + err.message);
});

serverDomain.run(function() {
    var app = express();
    app.get('/testing', function() {
        app.nonExistent.call(); // this throws an error
    });

    var server = app.listen(8000, function() {
        console.log('Listening on port %d', server.address().port);
    });
});

The error shows up in the console, but the console never receives the "SERVER DOMAIN ERROR..." message. I have also tried wrapping the request/response in their own domain as well, to no avail. Even more disappointing is the fact that using the following does not work either:

process.on('uncaughtException', function(err) {
    console.log('uncaughtException caught the error');
});

Am I doing something wrong? Where do I go from here? How can I catch the above error?

Answer 1

This happens because Express handles by itself the errors that may appear and in this way it simplifies your work. See Express Guide for error handling . You should use the structure below to handle the errors that may appear:

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

Answer 2

You can use connect-domain .

The problem is that the exception happens during Connect's routing, which has both a try/catch block around its execution, as well as a default error handler which prints out stack trace details when running in a non-production mode. Since the exception is handled inside of Express, it never reaches your outer layer for the domains to handle.

Here is an example why to use connect-domain package instead of domain.

http://masashi-k.blogspot.com/2012/12/express3-global-error-handling-domain.html

var express = require('express');
var connectDomain = require('connect-domain');

var app = express();
app.use(connectDomain());
app.get('/testing', function() {
    app.nonExistent.call(); // this throws an error
});

app.use(function(err, req, res, next) {
    res.end(err.message); // this catches the error!!
});

var server = app.listen(8000, function() {
    console.log('Listening on port %d', server.address().port);
});