The document discusses Node.js, which allows JavaScript to run outside the browser. Node.js has an event-loop approach that makes it easy to build scalable network servers in a non-blocking and asynchronous way. It uses an event model with callbacks to handle parallel input/output operations more efficiently than nested callbacks for serial operations. Modules and the EventEmitter class provide ways to organize code and handle events in Node.js applications.

1. Ian Oxley
Super Mondays, 26 Sept 2011

2. • Hi, I’m Ian
• Web / Application Developer at Sage
• http://ianoxley.com
• @ianoxley

3. What is Node?
“Node is a set of libraries for JavaScript that allows it to run
outside the browser. It is primarily focused on creating simple, easy
to build network clients and servers.”

4. • Event loop approach
• Easy and safe to build scalable
servers
• All I/O should be non-blocking
• Use asynchronous callbacks to handle
events

5. var http = require('http');
http.createServer(function (req, res) {
res.writeHead(200, {'Content-Type': 'text/plain'});
res.end('Hello Worldn');
}).listen(8124, "127.0.0.1");
console.log('Server running at http://
127.0.0.1:8124/');

7. Parallel I/O
• Node assumes all I/O has unbounded
latency
• 0 to infinity time
• Easy to do parallel I/O:
• Use events as placeholders
• Fire callbacks as events happen

8. Serial I/O
• Ordered serial I/O
• Each task completes before the next
one starts
• Nested callbacks
• Pyramid code - hard to understand
and maintain

9. // Nesting callbacks to produce serial requests
server.on('request', function(req, res) {
//get session information from memcached
memcached.getSession(req, function(session) {
//get information from db
db.get(session.user, function(userData) {
//some other web service call
ws.get(req, function(wsData) {
//render page
page = pageRender(req, session, userData, wsData);
//output the response
res.write(page);
});
});
});
});

10. // Using declared functions to seperate out code
var render = function(wsData) {
page = pageRender(req, session, userData, wsData);
};
var getWsInfo = function(userData) {
...
};
var getDbInfo = function(session) {
...
};
var getMemCached = function(req, res) {
...
};

12. Node events
• Potentially infinite based on what’s
happening on the server
• e.g. HTTP server module has a
“request” event

13. EventEmitter
• No DOM in Node so we have the
EventEmitter class
• Main two methods: on and emit
• Inherit from EventEmitter to attach it’s
methods to your class

14. var util = require('util'),
EventEmitter = require('events').EventEmitter;
var Server = function() {
console.log('init');
};
util.inherits(Server, EventEmitter);
var s = new Server();
s.on('wtf', function() {
console.log('wtf');
});
s.emit('wtf');

15. Modules
• Node uses CommonJS module system:
http://www.commonjs.org/
• One module per file approach e.g. the
file foo.js contains the foo module
• Add functions and variables to the
exports object - everything else is

16. // circle.js
var PI = Math.PI;
exports.area = function (r) {
return PI * r * r;
};
exports.circumference = function (r) {
return 2 * PI * r;
};

18. Where to get Node
• Official site: http://nodejs.org
• Download:
• Source code for OS X / Linux
• .exe for Windows
• Wiki: https://github.com/joyent/node/wiki

19. ?