Express.js Guide: The Comprehensive Book on Express.js (2014)

Domains

Contrary to its more popular homonym, domain is a core Node.js module. It aids developers in tracking and isolating errors that could be a juggernaut task. Think of domains as a smarter version of try/catch statements.

When it comes to Express.js, we can apply domains in error-prone routes. Before the routes, we need to define custom handlers to catch errors from domains:

1 var domain = require('domain');

2 var defaultHandler = express.errorHandler();

3 app.use(function (error, req, res, next) {

4 if (domain.active) {

5 console.info('caught with domain')

6 domain.active.emit("error", error);

7 } else {

8 console.info('no domain')

9 defaultHandler(error, req, res, next);

10 }

11 });

Here is a crashy route:

1 app.get("/e", function (req, res, next) {

2 var d = domain.create();

3 d.on("error", function (error) {

4 console.error(error.stack)

5 res.send(500, {"error": error.message});

6 });

7 d.run(function () {

8 //error prone code goes here

9 throw new Error("Database is down.");

10 });

11 });

On the other hand, we can call next with an error object (e.g., when an error variable comes from other nested calls):

1 app.get("/e", function (req, res, next) {

2 var d = domain.create();

3 d.on("error", function (error) {

4 console.error(error.stack)

5 res.send(500, {"error": error.message});

6 });

7 d.run(function () {

8 //error prone code goes here

9 next(new Error("Database is down."));

10 });

11 });

The working (or should we write crashing example) is in the expressjsguide/domains folder and on GitHub.

The full content of expressjsguide/domains/app.js:

1 /**

2 * Module dependencies.

3 */

4

5 var express = require('express');

6 var routes = require('./routes');

7 var user = require('./routes/user');

8 var http = require('http');

9 var path = require('path');

10

11 var app = express();

12

13 // all environments

14 app.set('port', process.env.PORT || 3000);

15 app.set('views', __dirname + '/views');

16 app.set('view engine', 'jade');

17 app.use(express.favicon());

18 app.use(express.logger('dev'));

19 app.use(express.bodyParser());

20 app.use(express.methodOverride());

21 app.use(app.router);

22 app.use(express.static(path.join(__dirname, 'public')));

23

24 var domain = require('domain');

25 var defaultHandler = express.errorHandler();

26 app.use(function (error, req, res, next) {

27 if (domain.active) {

28 console.info('caught with domain', domain.active);

29 domain.active.emit('error', error);

30 } else {

31 console.info('no domain');

32 defaultHandler(error, req, res, next);

33 }

34 });

35 app.get('/e', function (req, res, next) {

36 var d = domain.create();

37 d.on('error', function (error) {

38 console.error(error.stack);

39 res.send(500, {'error': error.message});

40 });

41 d.run(function () {

42 //error prone code goes here

43 throw new Error('Database is down.');

44 // next(new Error('Database is down.'));

45 });

46 });

47

48 app.get('/', routes.index);

49 app.get('/users', user.list);

50

51 http.createServer(app).listen(app.get('port'), function () {

52 console.log('Express server listening on port ' + app.get('port'));

53 });

For more ways to apply domains with Express.js, take a look at this amazing presentation.